Human Enzymes Involved in Food Processing
a. I. Disposition Of Cytochrome P4SO Enzymes
The word “enzyme” neither belongs to a vocabulary of a layperson, nor does it quali%r as a household name. This is why before considering the distribution of enzymes inside the human body, and their many functions, we should first understand what the enzymes
Enzymes are very large organic molecules consisting of long chains of amino acids. (Note: a human body contains 20 amino acids with 10 of them being synthesized in vivo, and the rest being introduced through a diet.) They are produced by human cells and are intended to catalyze specific biochemical reactions, at body temperatures. Just imagine a molecular assembly, of a bucket shape, with the walls made of amino acids. The composition — what kind of armno acids are present and in what quantities – and the sequence — in which order the aln_i.no acids are attached to each other – are essential for the structure, and functionality of the enzyme. Most of them contain transition metals, such as iron (Fe), cobalt (Co), or nickel (Ni). Because of the presence of transition metals, usually in their higher oxidation states, the enzymes can interact with different organic molecules and alter their chemical structure. Nature has designed the molecular machinery in such a way that each enzyme has a definitive function to chemically modi&
Guilty Until Proven Innocent
endogenous compounds, i.e., those that are delivered from inside the body (steroids, fatty acids, prostaglandins, leucotrienes). Thus, compound A will initially coordinate with transition metal (TM) located inside the enzymatic assembly 1 (Figure I). The nature of this so-called “coordination” step is not chemical, meaning that both the transition metal and compound A preserve their chemical identities, but they are held together by relatively weak intermolecular forces. A dotted line in assembly a represents a coordination bond that is usually formed by donating electrons from an organic molecule toward a transition metal. Thig interaction, although non-chemical, is critical for the overall process, since an organic molecule positions itself appropriately relative to the transition metal and walls of the en4rne consisting of a plethora of amino acids. An enzymatic reaction is usually a multistep process that converts guest molecule A into a new entity, molecule B. As shown in assembly g, the latter can still be coordinated to the transition metal, or its redox derivative, and be subsequently released into biologcal medium. Enzyme 4 can undergo regeneration in order to be able to host another molecule A.
This is how conceptually enzymes work. Now we should discuss what the enzymes are and how they ape found and identified. Most relevant to the subject matter are enzymes coined Cytochromes P450s,1-7 They belong to the superfalllily – more than 400 – of cell membrane-bound hurnan enzymes (heme proteins containing iron protoporplayrin moiety) that metabolize endogenous (from inside the human body) compounds, such ag fatty acids and steroids. There are a variety ofmethods developed by the generations of biochemsts, enzymatic chemists, and molecular biologists that allow for the identification and isolation of individual enzymes. In each and every case, it is a formidable scientific task! Finding an enzyme inside the Human Enzymes Involved in rood Processing 13
enzyme walls made of arnino acids
transition metal responsible for enzymatic reaction
- chemically modified substrate Compound B
Figure 1. Enzymatic transformation of an endogenous compound
human body is just the first step in identifring the function of a given enzyme in the human body. Isolation of an enzyme in an homogeneous form is usually followed by an intense structural study in order to elucidate its intimate molecular structure, i.e., which amino acids are present, in which order they are linked to each other, and what is the spatial, three-dimensional structure of the enzyme.
The next question the scientists pose is the functionality of the enzyme. What is its biological role? Why is it placed in that particular part of the human body? Enzymatic and organic chemists would like to study, and establish the intimate details Of chemical transformations mediated by a given enzyme. Enzymatic reactions are usually multistep processes catalyzed by the transition metals. For example, an enzyme aromatase responsible for the conversion of testosterone, a male hormone, to ßestradiol, a female hormone,
carries out this conversion in a ligand sphere of an iron atom, by a very complex mechanism. It should be recognized that reactions mediated by natural enzymes usually do not have any comparable analogs in organic chemistry, They are highly emcient, relatively fast and occur at body temperature, under physiological nearly neutral conditions. Analogous reactions, even if they can be accomplished in the chemical laboratory, usually require elevated temperatures and highly acidic, or basic medium. These considerations are Important in order to realize that enqymatic research is an incredibly complex area of science. The major advances usually occur as a result of the systematic and crossdisciplinary studies by biochemists, organic chemists, molecular biologists, structural biologists, and analytical chemists.
The next logical step to make is to recognize that the human enzymes are kllown not only to chemically modi&, or metabolize endogenous compounds, but also those entering the body from the outside, as the components of food, beverages, cosmetics, toiletries, and therapeutics. These alien-to-human-body compounds are called xenobiotics, or exogenous compounds. To what extent the natural enzymatic assemblies are capable of metabolizing compounds from inside and outside the human body is both a scientific and, perhaps, philosophical question. Maybe we should stop worrying about putting too much strain on the natural system because, over the centuries, the metabolizing machinery might have learned how to deal with organic compounds, no matter what the origin is. An informed professional might tell you that the burden has gradually increased over the last few decades due to a massive use ofjunk food, food preservatives, artificial flavors and colors, hormones, growth regulators, and other derivatives of mass production.
Hunan Enzymes Involved in Food Processing 15
Now let ug consider those Cytochrome P450 enzymeg that have been identified in the alimentary canal and human digestive system
(Figure 2). As you can see, the number of enzyrnes identified is very
Their distribution is uneven with the liver being the organ of highest concentration of metabolizing enzymes. It contains 24 Cytochrome P450 enzymes with each of them designated by two numbers and a capital letter. The Erst enzyme in the liver shown is CYP2C9, with the number 2 indicating the family of the enzyme (amino acid sequence identity , letter C showing the subfamily enzyme (amino acid gequence identity >55%), and number 9 identifying the order in which the respective subfamilies have been discovered, The other three areas of relatively high concentration of enzymes are the esophagus (8 enzymes), stomach (9 enzymes), and small intestine (10 enzymes), The lowest number of enzymes is found in the oral cavity (l enzyme), salivary gland (1 enzyme), and rectum (1 enzyme). Between these two groupg are the tongue (6 enzymes), pancreas (5 enzymes), and colon (5 enzymes), Next to each enzyme designation is the percentile called normalized expression. The higher the number is, the higher the concentration of the respective enzyme,
It has to be mentioned that some enzymes are thought to utilize primarily exogenous compounds (CYPI, CYP2 and CY?3 families), while others are assigned a well-demed role in handling native, endogenous compounds (CYP4, CYP5, CYP7, CYP8, CYP11s CYP17, CYP19, CYP20, CYP21, CYP24, CYP26i CY?27, CYP39t CYP46, and CYP51 faanilies). The fist group is less selective and each enzyme can metabolize a wide variety of substrates, They are said to have the “low substrate selectivity.” To the contrary, the representatives of the endogenous group, given their predetermined body function,
Vndl Proven Innocent
can usually recognize a very limited number of substrates, or two (“high substrate selectivity”). AS Figure 2 indicates, their
Figure 2. Cytochrome P450 Enzymes Involved in Exogenous MetatxAism (shown with corresponding normalized expressions).
relative disposition in the body is quite mixed. For example, among six enzymes located on the tongue, three of them belong to the second group: the CYP4F3 enzyme interacts with leukotriene hydroxylating it, the CYP7B1 enzyme “handles” pregxelonone and dehydroepiandrosterone (DHEA), while the function Of me
Ruman Enzymes Involved in Food Processing 17
most abundant (29.67%) enzyme — CYP4F11 — has not been firmly established. Analogously, eight esophagug enzymes are equally split: besides CYP4F3 and CYP4F11, it contains CYP4F12 and CYP51A1. The former is involved with hydroxylation of arachidonic acid, while the latter induces demethylation (14-CH3 position) upon lanosterol. It should be mentioned that dividing body enzymes into two groups is not of absolute value. A molecule of xenobiotics can mimic one of the natural, endogenous molecules and become metabolized by the enzyme from the second group. In fact, this methodologr has long been used, and quite successfully, in drug design for developing synthetic drugs that are capable of inhibiting certain enzymes.
As you can see, the human alimentary canal and the digestive system look like the whole chemical laboratory with multiple benches and fume hoods lined up to perform a variety of chemical functions I Each enzyme ig known to mediate a certain type of chemical transformation when exposed to particular, “matching” chemical substrates. We will now conclude this section with the following question, Why we need all this extensive knowledge on enzymes located not only in the alimentary canal, but also in the lower abdomen, all the way to the rectum? If the exogenous compounds food, beverages, drugg — are introduced through the oral cavity then one could anticipate that only the enzymes in the upper part of the body would be essential to the metabolism of xenobiotics,
Nothing could be further from the truth I The metabolizing system is so delicate that only matching pairs workl It means that unless there is the structural, electronic, conformational, and functional match between an invading molecule and a body enzyme, there will be no chemical interaction. As a result of it, the molecule in question
Tntil Proven Innocent
could bypass a certain number of enzymes, while remaining chemically intact. Phenomenologically, it workg like a matching agency that all have seen on TV: potential candidates are compared by several parameters until they deemed suitable to each other. Inside the body, the chemistry and energetics are at works deciding whiCh enzyme will metabolize which corgpound. It is conceivable that there can be compound that will go through the whole row ofmetabol.izing enzymes unchanged and will escape the human body through the rectum. Another scenario is that in the case of the multicomponent mixture, such as food, while it advances through the alimentary canal, some components could undergo a rapid metabolism by enzymes located in the esophagus, while the others would interact more efficiently With liver enzymes. It can also be enthgioned that; initially metabolized, chemically modified compounds, could umdergo secondary, or even tertiary, transformations while rnoving down the digestive path. These are the reasons why we need to know the enzymatic makeup of the whole body especially that of the alimentary canal and the digestive system.
Besides a gastrointestinal tract, Cytochrome P450 enzymes are also present in the respiratory system that mediates the movement of air in and out ofthe body. For example, nasal mucosa contains gj-x enzymes with Eve of them belonging to the same family (CYP2A6, CYP2A13, CYP2B6, CYP2C, CYP2J2, CYP3A), Some of the same enzymes are also detected in trachea (CYP2A6, CYP2A13, CYP2B6, CYP2S1). The most enzyme-laden part of the respiratory system are the lungs with four different Cytochrome P450 farnilies being present (CYPIAI, CYPIA2, CYPIBII CY?2A6, CYP2A1d, CYP2B6, CYP2C8, CYP2C18, CYP2D6, CYP2E1, CYP2F1, CYP2J2, CYP2S1,
CYP3A4, CYP3A5, CYP4B1). Although food is the main focus of thig
Human Enzymes Involved in Food Processing 19
book, a respiratory channel should be recognized as an alternative channel through which the exogenous compounds could enter the human body. Among them are the vapors of any origin, such as the smell of food, environmental pollutants, pesticides, fertilizers, and also volatile organic compounds present in households because of the very “synthetic” environment that we live in.
Many of us are familiar with the smell of naphthalene, a polynuclear aromatic compound used for decades in households, as a main ingredient of the mothballs. Only recently, it has been firmly established that naphthalene is strongly carcinogenic to humans. While it is easy to detect naphthalene because of its high volatility and distinct smell, we should realize that there is a certain threshold below which human beings cannot sense the organic compounds present. By this criterion, we are not the best in the nature with dogs and butterflies exceeding our sensitivities by many orders of magnitude. The point here is that ifyou do not feel any smell, it does not mean that you are not exposed to certain organic compounds present in your household. Their concentration could simply be below your sensitivity threshold! A low concentration could be as damaging as a large dose, if our bodies are exposed to the same compound over a longer period of time. And this is true for any volatile chemical that can be present in the household, or at the workplace. Finally, there are compounds that do not have any smell, at all. For example, humans cannot, unfortunately, recognize carbon monoxide. So, the very fact that there is no smell in your house does not yet mean that your aging carpet, or wall pamt is not releasing a carcinogen that either does not have any smell, or its concentration is too low to be detected. The message here is that the volatile compounds could enter the body through the respiratory system and cause the same
type of cellular datnage as the components of food, or cosmetics, Since the Cytochrome P450 enzymes are heavily represented in different parts of the respiratory system.
2.2. What Kind of Chemical Can
P4BO Enzymes Induce?
The ability of natural enzymes to chemically transform the invaders, i.e., foreign molecules introduced into the body as food components, cosmetics, or therapeutic agents, is enviable to every organic Synthetic chemist. Summarized below are nine main types of chemical transformations known to be mediated by various Cytochrome P450 enzymes.8-14 While considering them, I will try to avoid excessive scientific terminology, with the main objective in mind being to display, first, a structural disparity of chemical substrates involved and, second, a variety of enzyme-catalyzed reactions which are amazingly distinct by their chemical nature.
Z reaction: Aromatic
Depicted in Scheme I is compound S containing a hexagon-like moiety, a benzene ring. It is also called an “aromatic ring” since all compounds with benzene rings belong to the class of so-called aromatic compounds. All six vertices represent the carbon atoms with the top one being attached to the hydrogen atom (chemical symbol H). Other carbons can have different substituents with wavy lines indicating that any atom, structural unit, or functional group could be attached. me term “aromatic hydroxylation” implies that, when exposed to human enzymes, the carbon-hydrogen bond (shown at the top of the hexagon) can be replaced with a carbonhydroxyl group (OH) moiety. In other words, product 6 is formed by incorporation of an oxygen atom (chemical symbol O) into a
carbon-hydrogen bond (C-H). Understandably, this is a simplistic representation ofthe process. In reality, the mechanism is a complex multistep sequence Of elementary acts occurring in the ligand sphere of a transition metal, such as iron.
carbon-hydrogen bond that undergoes oxidation
An aromatic ring shown in compound 5 is ubiquitous in food products, cosmetics, therapeutics, and polymers, as well as in the environment that we au are exposed to, on a daily basis. In some cases, an aromatic can be a “planned,” or “desired?’ event. For example, a drug can be designed in such a way so that, in the course of the treatment, it would undergo an inside-the-body to convert from an inactive mode into an active, curing mode. This is an aromatic carried out in a conscious manner, as part of the smart drug development strategy, As unexpectedly as it may sound, we in fact count on enzymes to carry out the reaction in question. But this is true only if we face a medical condition to deal with. In other cages, aromatic Pings are introduced inbo the human body as chemical constituents of, say, food or cosmetics. The hydroxylation in question can be carried oub by multiple body enzymes, at different locales (CYPRC9, cypaC18, CYP2C19, CYPREI, CYP2A6). The number of reported enzymatic transformations of this type reaches 222( l) underlining the common nature Of this reaction
until proven Innocent
and an ability of body enzymeg to work with a variety of organic molecules. This kind of massive incorporation of hydroxyl groups into aromatic rings, totally beyond our control and, mostly, outside the confines Of our Imowledge, gan have negative consequences the human body In the following chapters, step-by-step, you will better understand why hydroxylated aromatics are compounds of great concern, and how they can interact, and structurally alter biological molecules, such as DNA, RNA, and proteins.
Type 11, Oxidation reaction: Aliphatic hydrowlation.
Aliphatic hydroxylation is an enzymatic process that is GhernjcaJly related to the previous one, an aromatic hydro.wlaåion. In substrate 7 (Scheme R), you can recognize a carbon-hydrogen bond (C-E), analogous to thatin aromatic compound S (Scheme l). The difference is that now an aromatic ring is not present in the molecule; instead, aliphatic hydrocarbons consist of the carbon chains containmg multiple carbon-carbon single (C-C) bonds, as well as carbon-car’bon double (C—C) and triple (0=0) bonds, The hydroxylation reaction induced by body enzymes is analogoug to that in the aromatic series,
i.e., an oxygen atom incorporates into an oxygen-hydrogen bond (O-H), converting an aliphatic compound 7 into its more oxidized derivative 8 (Scheme 2). These transformations are very relevant to the subject matter because aliphatic compounds are abundant in food components, cosmetics, and therapeutics, as well as in the environment For example, both animad fat and vegetable oil belong to aliphatic compounds. Fatty acids, many of them household names such as “omega are also aliphatic, by the very definition. Biologically important molecules, such as amino acids phenylalanine and tyrosine, contain both aliphatic and aromatic portions. The hydroxylation in question can be carried out by multiple body Food
enzymes, at different locales. Among them are CYPZD6, CYP6A4, CYP3A5, CYP3A7, CYP4F2, and CYP4F3@ Amazingly, the number of reported enzymatic transformations of this type is even higher than that for aromatic hydroxylations (342 vs 222).
newly formed hydroxyl group carbon-hydrogen bond that undergoes oxldation
Type reaction: Functional group modification
(aldehyde to acid)
Aldenvdes are one of the major classes of organic compoundg. The functional group that allows for an easy recognition is called an aldehyde group, as shown in structure 9 (Scheme 6). The central carbon atom is doubly bonded to oxygen and also connected to the hydrogen atom, via a single carbon-hydrogen bond (Scheme 3). A variety of chemical reagents (oxygen, monovalent silver, transition metals in their higher oxidation states) can oxidize aldehydes to the respective carboxylic acids 10, containing a carboxyl group, COOH.
carboxyl aldehyde group
O group enzymatic oxidation
Cytochrome P450 enzymes are knovm to carry out the same type of oxidation inside the body, in vivo, as well as in the test tubes, vitro. The number of reported enzymatic transformations of this type is relatively small (9). Among food products containing the aldehyde
Cuilbr Un HI Proven Innocent
group are acycli0 forms Of sugars, or monosaccharides called aldoses. The best-known example is gucose, a cornpound affiliated with diabetes and also being a structural unit of table sugar (sucrose), What are the consequences of oxidizing an aldehyde to carboxylic acid? The very nature of the compound changes drastically, that, in turn, affects its ability to interact with body systems. For example, carboxylic acids are known to be more acidic than aldehydes, by many orders of magnitude, and capable of forming relatively stable hydrogen bonds (H-bondg) w-ith a variety of functionalities, such as carboxylic (COOR), hydroxyl (OH), thiol (SH), and amino (NEZ) groups. These functional groups are the constituent parts Of amino acids and proteins (aspartio acid, glutamic acid, serine, threoninet tyrosine, cysteine, lysine, asparagine, glutamine, argilline), as well as DNA and FNA bases (cytosine, adenine, guanine). Because of this, if a foreign compound (xenobfotfc) enters the body and undergoeg an enzymatic oxidation to the carboxylic acid, then the pharmacokinetic profile and the nature of its Interaction with natural assemblies, such as enzymes, proteins, DNA, and RNA, also change. The presence of the carboxylic acid in the molecule vrill also affect its absorption by body tissues, its distribution among different organs, its secondary metabolism, as well as its excretion from the body.
Oxidation reaction: Functional group modification
The enzymatic 0 tion ofalcohols il to ketones 12 (Scheme 4) is analogous to the aldehyde- -carboxylic acid conversion (Scheme
Z). In the chemical practice, the dard oxidizing agents can convert structurally diverse alcohols to keto and, to the contrary, a variety of reducing agents Till effect the reverse tion. What is so remarkable about enzymatic transformations – num a total
f49 cages — is that the oxidations ooour in aqueous medium, at body te erature, and with high efficiency and selectivity. These types of transfo tions are important Given the fact that hydroxyl groups are abundan one food convonents, natural compoundg extracted from the animal d plant kingdoms, as well as environmental pollutants.
The enzymatic oxidation of alcohols to ketones changes eir p ical and chemical properties. For example, ketones are expec to be less polar, more soluble in hydrophobic fatty tissues, and less ca ble of forming hydrogen bonds. Chemical properties of ketones are o quite distinct from those of precursor alcohols. A double bond b n carbon and oxygen atoms (carbonyl group) is a highly polar moi and, contrary to alcohols, can receive the attack of a nucleophil such as an amino acid or DNA base. The point here is that alcoho when coming into contact with a variety of oxidizing agents inside human body, can drastically change their physical and chemical p erties. And if a particular component in, say, food or plant extract demonstrated some valuable properties in test tubes (in vitro), we ed to check if the same compound, inside the body, preserves its che cal integrity, or not. Unless it has been convincingly demonstrated, referably, by the qualified independent parties, we should not p to conclusions and advise everybody, on TV and through print me a,
to some particular food, or plant extract, and expect miraculous results. , to give some false sense Of security to the consumers that the adve ed products have disease-preventive powers. Thig task can be relati simple if the compound in question has only one hydroxyl group. Th ality iS that many natural products have not one, but several hy 1 groups, each of them with its own chemic al environment and uni ability to get oxidized by body enzymes. It is conceivable that nt enzymes would produce different metabolites from the same co d, and each newly formed structure would react differently, an ometimes unpredictably; with body organics.
Type V. Oxidation reaction; Oxygen atom transfer (alkene to
The conversion of alkenes 13 to epoxides 14 (Scheme 5) iS the reaction of utmost importance to organic synthetic chemistry. Both alkenes and epoxldes represent two maill classes of organic compounds and have been thoroughly studied, over the decades, by organic, organometallic, and polymer scientists. By its type, the conversion belongs to the oxidation reactions in which an ozqrgen atom ig transferred, by an oxidizing agent, toward the carbon-carbon double bond in alkene IS. As the structure of epoxide 14 shows, the newly formed carbon-oxygen bonds are pointed in the same direction, whereas hydrogen atoms are pointed in the opposite direction. The significance of this reaction – also called an “epoxidation” reaction — is so hi<h that it has unrecognizably modified the very landscape of chemical sciences, spanning mom synthetic organic to medicinal chemistry, from total synthesis to the dye industry, and from materials science to polymers.
Human Enzymes Involved in Food Processing 27
carbon-carbon double bond
enzymatic epoxidation epoxide moiety
The biological relevance of the epoxidation reaction is wen recognized given the ability Of the Cytochrome P450 enzymes to oxidize double bonds which are introduced, in any capacity, into the human body. The number of reported enzymatic transformations of this type reaches 75, highlighting the feasibility of the reaction and a high level of enzyme tolerance toward organic substrates. Double bonds are present in many food components, cosmetics, preservatives, vitamins, steroids, fatty acids, therapeutics, and environmental pollutants. The mere description of even a tiny fraction of organic molecules with double bonds would require significant space allocation in this book. Given the constraints, it suffices to say that, throughout the day, our bodies are exposed to a variety of structurally diverse compounds with double bonds, and each of them is susceptible to the epoxidation reaction.
At this point, you might want to ask, why should we, as consumers, be concerned about the formation of epoxides? The reason is that, in the past decade, scientists have convincingly demonstrated that an interaction Of epoxide ring-containing molecules with body systems can be detrimental to human health. An epoxy ring, Aven its highly Strained nature, can interact with DNA and RNA bases, such as cytosine, adenine, or guanine, and form adducts some of which were isolated and structurally characterized. It can also interact with
amino acids that have nucleophiIiC moieties in the Side chain; among them are lysine, arginine (amino group, NHz), cysteine (thiol group, SH), serine and threonine (hydroxyl group, OH). The overall result is that, despite its peaceful appearance, an epoxy ring, formed by an oxidation of alkenes by body enzymes, can structurally modi& the vital body systems — DNA, RNA, genes, proteins — them dysfunctional! For example, a DNA strand with bases structurally altered won’t be able to participate in the key process called a bage pajrjng. This is the very process that holds a double-stranded DNA together! Thus, when exposed to epoxides, a double-stranded DNA can lose its “inner strength,” start uncoiling, and surrender its famous helical three-dimensional structure. Subsequent chapters provide ample evidence for you to become more knowledgeable, and better informed about consequences of exposure to epoxides. And this newly acquired knowledge will be quite relevant the next time you do a barbecue in your backyard, consume charred meat products at the restaurant, or use cosmetics and beverages in which preservatives, coloring, and flavoring agents are those potentially forming epoxides inside the human body.
Type reaction: atom transfer (amines and N-heterocycles to N-oxides).
The next two enzymatic transformations are analogous to each other by the type of atom undergoing an oxidation, i.e., the centrd atom in both cases is nitrogen (chemical symbol N; Schemes 6, 7). The difference is that, in the first case, the nitrogen atom is a part of the aliphatic molecule, while, in the second case, it belongs to the class of heterocycles, or heteroaromatåcs, wherein the nitrogen atom is an integral part of the aromatic ring. The enzymatic oxidation converts trivalent nitrogen in structure IS into its N-oxide derivative 16
Hunan Enzymes Involved in Food Processing 29
(Scheme 6). The latter ig recognized in chemistry ag an oxidizing agent due to the presence of a gmgly bonded, and transferable oxygen atom. For example, in transition metal chemistry, trimethylIT-oxide — [(CHe)aNO] — ig used for the oxidative removal of the metal core and release of organic molecules. The oxidizing power is quite remarkable and comparable to that of high-valent metals, such as trivalent iron, FeCIII), or tetravalent cerium, Ce(IV). It is worthy to mention that the latter is reported to cleave a DNA strand at room temperature I Because of this, we ghould be concerned that N-oxides, while formed in the human body, could interact with vital biological molecules by oxidizing them analogous to high-valent transition metals. We all still remember the hexavalent chromium (CrV%) story from the “Erin Brockovich” movie, don’t we?
It is worthy to mention that compounds with trivaAent nitrogen, such as IS, belong to amines, one of the main classes of organic compounds. At this point, you might what to ask yourself, “Are we, as consumers, exposed, to any considerable extent, to amines, throughout our daily lives?” The answer flat out Yesl Structural analogs known as primary, secondary; and tertiary amines are well represented in practically every chemistry field, due to their diverse chemical properties and remarkable synthetic potential. A careful analysis Of multicomponent formulationg used in the cosmetics industry, food products, pharmaceutical and polymer chemistries will allow us to easily identi%r the “compounds of interest.” The specific function played by a given compound is not relevant. It can be an active ingredient in a cosmetic formulation, or it can be an inactive component of a polymeric material used in your household. What matters is our exposure to these compounds. What matters is the fate of N-oxides enzymatically formed inside the human body. To
30 Guilty Until
the same extent, we should be concerned with N-oxides formed from nitrogen-containing heterocycles, such as pyridine (17, Scheme 7). The latter is known to be oxidized by Cytochrome P450 enzymes to form the respective N-oxide 18 that could have the same undesirable ability to oxidize the vital body systems, such as a DNA strand. The oxidation of amines and nitrogen-containing heterocycles is relatively well studied with the number of reported enzymatic transformations totaling 60.
Type Oxidation reaction: Dehydrogenation (alkanes to alkenes).
The dehydrogenation reaction introduces a carbon-carbon double bond into the saturated chain. Schematically, it is represented in Scheme 8 with the generic saturated compound 19 losing two hydrogen atoms which are disposed vicinally to each other. Product 20 belongs to alkenes, a class of organic compounds having a doubly bonded carbon-carbon moiety. To carry out this reaction in the industrial setting, chemists have to apply high temperatures (up to
Ruman Enzymes Involved in rood Processing 31
750 0 C), special equipment called an autoclave, and algo a catalyst made Of transition metals, such as chromium or zinc. The process is the cornerstone of the mammoth petroleum industry producing vast quantities of ethylene, propylene, styrene, and butadiene. Because of the severity of the experimental conditions, this reaction is not used in the laboratory setting for the synthesis of alkenes. In comparison, body enzymes can dehydrogenate a variety Of organic molecules at body temperature! And the reaction seems to be quite common since the number of reported enzymatic transformations Of this type reaches 41. One of them is the conversion of succinic acid to fumaric acid carried out by the succinate dehydrogenage enzyme.
to be eliminated carbon-carbon by an enzyme double bond
The problem is that dehydrogenase enzymes are capable of not only “working” with natural, endogenous molecules present in the body, but also with those introduced from the outside, i.e., with foods, beverages, and cosmetics, as well as airborne mixtures. The reason is that because of the inherent flexibility of body enzymes, they can “accommodate” organic molecules that resemble natural ones by their volume, shape, functionalities, and top010Ü. You might then wonder, what kind of problem may arise if a ‘foreiÜ1″ molecule enters the human body and gives off two hydrogen atoms? The formation of the double bond drastically changes the very nature of the molecule, to the extent that a completely harmless compound can do harm to the human body.
TO conceptually demonstrate such possibility, eyolohexane 21, a saturated molecule with no double bonds, is chosen as a model compound (Scheme 9). Because of the absence of so-called unsaturated units, it cannot react with oxidizing body enzymes called oxygenases and, most importantly, remains benign to the body. Ag soon as the demydrogenation occurs, because of the presence of the double bond, cyclohexene (22) can now be oxidized to form the epoxide 23. The latter belongs to the class of organic compounds known to be carcinogenic because of the presence of the strained, three-membered ring and its intrinsic ability to react with DNA bases, or amino acids, structurally altering them.
Cyclohexane cannot torm carcinogenic torm carcinogenic epoxides epoxides
Another case of dehydrogenation that changes the nature of organic compounds from “benign” to “malign” is shown in Scheme
10. Cyclohexanone (a4) does not contain critical double-bond functionality and does not present any obvious danger to the human body. As goon as dehydrogenation takes place, a newly formed double bond in enone 2S enters into a conjugation with a carbonyl group (0=0) that makes it a recipient of the nucleophilic attack. In organic chemistry, this kind of transformation is called Michael addition reaction. What are the nucleophilic species in the human body? The DNA and RNA bases (cytosine, adenine, guanine), and amino acids having nucleophilic moieties in the side chaj-n, such as lysine,
Human Enzymes Involved in rood Procegging 33 ar4_njne (amino group, NH2), cysteine (thiol group, SH), serine and monine (Ivdroxyl group, OH). The addition to the double bond will form adducts, such as 86, either with a DNA base, or an ammo acid. In both cases, the structural changes will render natural molecules dysfunctional. For example, a DNA strand with structurally modiåed bases will lose its ability to base pairing, a process that holds a doublestranded DNA together! Subsequently, a double-stranded DNA will lose its structural integrity and won’t be able to maintain its famous helical three-dimensional structure. Analogously, amino acids with amino (NH2), thiol (SH), or hydroxyl (OH) groups being attached to the forei@ molecule won’t be able to perform their biological “duties.” For exar»ple, the ability to form H-bonds, an essential Step ill enzymatic reactions, will be severely handicapped, It is wortlv to mention that enone as belongs to the same class of a,ß-ungaturated carbonyl compounds as derivatives of acrylic acid — acrylam.ide, etnvl acrylate, acrylonitrile – which are present in the list of carcinogens compiled by the International Agency for Research on Cancer
(IARC). Other representatives are acrolein (27), crotonaldehyde
(28), and 4-hydroxJr-2E-nonena1 (29), the endogenous by-products Of lipid peroxidation caused by an oxidative stress (Figure 3). All three compounds belong to a,ß-unsaturated aldehydes which are known for their DNA damaging properties.
C’ßohgrnone cannot 1Meract with ONA bases and amino acids by %chae addition mechanism
Cyclohexen can interact with DNA bases arid amino acids addition mechanisrn
acrolein (27) crotonaldehyde (28) 4-hydroxy-2E-nonenal (ä)
e Cleavage of carbon-oxygen (C-O) bonds:
One o most common types of enzymatic reactions ig the cleavage of a arbon-oxygen bond (O-deaRyIation). The number of reported trans rmations reaches 164, underlining the ability of enzymes (CYPIA CYPIA2, CYP2B6) to “work” with a variety of organic molecules. its chemical nature , O is different from previously co ered processes, such as dehydrogenation, epoxidation, and ation. In the previous cases, the carbon skeleton of the substrates mained intact, with only functional groups — double bond, alde e or hydroxyl groups — undergoing an oxidation. In the case of the dealkylation reaction, the carbonoxygen bond undergoes cleavage, ectively bisecting the molecule into two unequal parts. Thus, ether 30, ontainingarequisite carbonoxygen single bond, can convert to alcoh 31, in which anallgngroup is replaced with a hydrogen atom (Scheme 1). As insignificant as it might seem, the conversion of ether to alcoho substantially changes both physical and chemical properties ofthe co ound. For example, contrary to ether 30, alcohol 31, and the likes, form relatively strong hydrogen bonds (H-bonds), undergo oxidatio to ketones, or, if a hydroxyl group is attached to an aromatic ring, p duce orthoquinones endowed with carcinogenic properties,
Human Enzymes Involved in rood Processing 36
enz tically Cleavable ca n-oxygen hydroxyl group sin bond enzymatic
the drug discov -dealkylation is used to develop so-called prodrugs that contain ethe al groups with relatively Short alkyl moieties (methyl, CH3•, ethyl, B). The trick is that compounds without an OH group can be better lerated by the patients because of their inherent resistance to oxi ion. At some point, while traveling inside the body, a prodrug can dergo an reform a hydroxyl group, and exhibit its properties, such as an ability to kill cancer cells.
When an O-dealkylation occurs in a controlled er, as pro by its very design, we should not be concerne out lt. It is expe d that a prodrug will convert into an active mode, the right time an the right place, and exert a positive effect without violating the act nt healthy cells, tissues, and organs. Another thing is that when we nsume a variety of organic compounds, as food components, or ingre nts of the cosmetic formulations, they can convert into their respecti active modes practically all over the body; at the wrong time and in wrongplace.
It h as to be mentioned that there is a ry fine line between an ability of, say, ortho-quinones, to cure by cancer cells (cytotoxicity) and their ability to cause cancer damaging healthy cells (carcinogenicity). In an Ideal case sc we, as
COnsurners, should know what kind of dealkylation reaction uld
in which part of the alimentary canal, and what will be the damag xerted upon adjacent healthy cells. Only then, if there is a certain vel of confidence that the metabolites would not cause an irreparab damage to our vital organs, we could comfortably let these compoun in.
Type rx. Clea age of carbon-nitrogen (C-N) bonds:
An enzymatic cleavag f carbon-nitrogen bonds is analogous to O-dealkylation, although the umber of reported transformations is even higher, reaching a s ring total of 2261 An alkyl group attached to the nitrogen (N) atom an be cleaved in amine 32 and replaced with a hydrogen (H) atom ( heme 12). Compound 33 also belongs to the class of amines, but witha werdegree of substitution due to the removal of an alkyl group. The resence of an N-H bond ompletely modify both physical and che cal properties of the molec . In particular, amine 33 should be ab to form relatively strong en bonds (H-bonds), exhibit a er solubility in the aqueous um because of the removal of hydrophobic alkyl groups, and c d also become more susceptible to enzymatic oxidation. This is not d news because in the presence of the aromatic ring directly at d to the nitrogen atom, respective oxidation products could have c ogenic properties.
enzymatically cleavable carbon-nitrogen nitrogen-hydrogen single bond enzymatic bond
Human Enzymes Involved in Food Processing 37
demonstrated that the body enzyrnes caninitiate avariety of chemic actions (dealkylation, hydroxylation, epoxidation, dehydrogenation, dation), we will now 100k at the enzymatiO transformations under a t angle. Let us ask ourselves, “What do all these reactions have in co on?” The answer is that for each type ofenzymatic transformation, the on(v one product formed (Schemes 1-12). Is it possible that a given could enter the human body, interact With a battery of enzymes, form more than one product? Unfortunately, the answer is YES I
Let us choose tamoxifen 34 – an antiestrogen widely used in clinical practice to treat breast cancer — and identi& all those chemical transformations which are known to occur in the human body (Scheme 13). In Other words, the discussion will now focus on so-called metabolic path vvqvs. First, why tamoxifen? The compound was introduced in the early 1970s as an antiestrogen, a mimic for natural estrogen that could compete, with ß-estradiol, for an enzymatic site.16 Since it is well known that breast tumors are rich in estradiol and estrogen receptors, the strategr has long been to mimic an estrogen with a synthetic molecule that could block an estrogen receptor, without producing the sarne type of biological response, such as a cancer cell proliferation. In other words, an ideal anticancer drug will be an antåestrogen, but it will not have any estrogenicity.
Another reason why tamoxifen 34 was chosen is the level and degree of its metabolic exploration. Given its status in medicinal Chemistry, as a widely prescribed therapy against breast cancer, tamoxifen 34 was thoroughly studied both in vitro and in vivo. You should be aware that the understanding of what exactly happens to
the chemical compound inside the human body is not an easy task. It requires significant resource allocation, pertinent biochemical assays, and the most advanced analytical and spectroscopic equipment. The main experimental difficulty ig the very amount Of the metabolite produced from the parent compound and the degree of conversion. The reality is that not the whole amount Of a drug ig converted to the secondary products, but only a small part Of it. Sometimes, it can be a very tiny fraction of the initial dose (<1%) introduced into the human body by alternative means, i.e., orally, intramuscularly, subcutaneously, or via patoheg.
Another difficulty is that the body enzymes can cauge not only a single chemical reaction, but a cascade of transformations, making it much more difficult to isolate the products initially formed, The complexity of this kind of research is the reason why very often a given drug is used in clinical practice for a long time, but not much is knovm about its metabolic products. Tamoxifen 34 is one of those compounds for which not just a single derivative, but multiple metabolites have been isolated and structurally characterized
The molecule of tamoxifen 34 contains several functional groups, i.e., phenyl group (regular hexagon with three double bonds), ether linkage CO-atom with two carbon atoms attached to it), carbon-carbon double bond (in the center of the molecule), and tertiary amino group (N-atom tmth three carbon atoms attached to it (Scheme 13). Amazingly, out of three aromatic rings present, only one of them is known to undergo an enzymatic reaction caused by Cytochrome P450 enzymes (CYPRC9, CYP2C18, CYP2C19, CYP2E1,
CYPRA6). The hydroxylation product, 4-hydroxytamoxifen 3B, also
Human Enzymes Involved in rood procegging 39
known as metabolite B, contains a hydroxyl group, OH, in position 4 of the aromatic nucleus. A newly formed moiety with an aromatic ring and OH-group is called a phenol unit, a moiety to the topic of this book (Chapter 3). The same aromatic ring can undergo the second enzymatic hydroOation introducing another hydroxyl group, OH, to the position 3 of the aromatic moiety (36, metabolite
D). Moving clockwise in Scheme 13, we will arrive at metabolite E (37) formed by an enzymatic O-deanvlation reaction. As the structure indicates, the nitrogen-containing moiety is removed from
metabolite X. or rnetabolito Z (O)
Guilty Until Innocent
the top of the molecule exposing another phenol unit. A side chain can also be oxidized by a.n O-atom transfer from the enzyme, onto a nitrogen atom, forming IT-oxide 38. Sequential N-dealkylation reactions will convert tamoxifen 34 to metabolites X and Z (39 and
40, respectively). The last compound in Scheme 13 is metabolite Y (41) formed by a sequence of reactions including N-dealkylation and hydrolysis. The message here is that a single molecule, like tamoüen 34 can form more than one organic product when interacting with a variety of body enzymes, at alternative locations throughout the human body.
To further emphasize the power of the chemical machinery at work, we now look at the substrate specificity, i.e., if a given enzyme is capable of interacting with only one substrate, an endobiotic compound designed, and fine tuned, in the process of evolution, or it can also “handle” the synthetic compounds, or xenobiotics. The answer is unfortunately YES I The natural enzymes can metabolize not only compounds native to the body (endobiotics), but also those entering from the outside, In other words, the so-called substrate selectivltyof body enzymes is very low. Some statistics here will be relevant to further prove this point, For example, the cytochrorne P450 enzyme CYPIAI found in the esophagus, stomach, and liver (Figure 2), is shown to interact with 165 substrates and form products. Another body enzyme — CYP2D6 – has even a lower substrate specificity, interacting with a larger number of organic compounds (217) and forming even more products (289). This enzymae can be found in three different locations in the body, i.e., on the tongue, in the small intestine, and in the liver (Figure 2). Another family of enzymes is represented by the CYP3A4 species which are located on the tongue, in the stomach, and in the small intestine,
Ruman Involved in rood Procegi.ng 41
and are capable Of metabolizing 423 substrates forming 624 products! One of the most abundant enzymes — CYP3A6 — that can be found in the esophagus, stomach, small intestine, pancreas, liver, and colon, has a somewhat legs impressive profile: 61 substrates consumed and 87 products formed.
DO We Know What Happens to rood During a Digesåon Process?
Having learned the basics of chemistry and the
major types of chemical reactions that can be catalyzed by enzymes, let us now pose a more general question, i.e., do know what happens to each and every food component entering the human body? In other words, how comforuble are we With mere& hundreds of organic compounds perpetually penetrating our bodies, disguised as food? Or cosmetics? Are we dealing with not one, but multiple Trojan horses?
Let us consider food from the standpoint of an organic chemist. Any regular item in the consumer basket represents a mixture of hundreds of compounds. And the structures of of them either completely unknown, or only chæ•uterized. This is what we consume, on a daily basis, throughout our lives! TNnk about cooking, baking, smoking, and other processes
used in food preparation, as the chemical alteration of hundreds of unknown compounds into a much larger group of compounds with their structures equally unknown. And the number of compounds present in the processed food can increue because of the chemical interaction between food components, their oxidation with when cooked in the open in the oven, or fried on a pan. Well, you can avoid these oxidations by creating a
completely anaerobic, oxygen-free environment in your kitchem but you might need to wear a gas mask and carry an oxygen tank around while cooking. I am wondering if people who like to cook will, in fag enthusiastically embrace this idea in order to avoid an unwanted oxidation of food components. Maybe we’re talking about the kitchen setting of the late 21 st, or early 22nd century?
Ask any chemist if he/she will do a chemical reaction with a reagent of an unknown structure. Ask another one if hundreds of reagents of unknown structures could be, or has ever been, added into the reaction vessel. Most probably, you will hear that no one in his/her sound mind would do it simply because it does not make any sense, either from the scientific, or the practical point of view. How can we achieve a desired synthetic outcome if we add the unlmowns into the reaction vessel? Understandably, no one will do it in the chemical laboratory, unless the person is heavily intoxicated and needs to be removed from the university premises, by campus security, in the most urgent manner.
From the viewpoint of an organic chemist, the human body represents a chemical vessel equipped with powerful enzymatic machinery: the one that is capable of inducing multiple organic transformations, or the one that can form seven different metabolites from a molecule of tamoxifen. When food components enter the human body, through the alimentary canal, hundreds of foreign compounds (xenobiotics) present will interact with Cytoch.rome P450 enzymes and form multiple metabolites, in different locales, throughout the body. If the structure of a given food component iS not known, understandably, we cannot establish the structures Of multiple metabolites potentially formed. And the main question here
Ruman Involved in Food Processing 43
is not just the sheer number of metabolites formed, either primary or secondary, but their properties, their compatibility with the vital body systems, such ag DNA, RNA, and proteins, and their impact on cellular processes.
It is also important to understand that an enzymatic make-up is not necessarily the same for different individuals. Fingerprints or genetic make-up are good ana104es here. It means that the food component being introduced into the human bodies might yield different products (metabolites) because the concentration of enzyme (expression) inducing the chemical change could vary for different genders, age groups, and ethnic groups. Consequentially, the same food can be harmless for one individual, and, on the harmful and toxic for the other. The reason is that the latter might have a high concentration of a given enzyme that converts one of the components present into, say, a carcinogenic derivative. Or, to the contrary, the presence of a certain enzyme might be good news if it could detoxi%T, due to metabolism, a harmful compound entering the body.
Various enzymatic make-ups could also explain why the same toxic compound does not affect different the same
For example, we all have seen on TV, or heard on the radio, or read in the newspapers, about a group of people who consumed, say, tainted alcohol, or spoiled fish, and only some of them fell ill, or even died. The reason is that their bodies, because of the unique enzymatic make-up, processed the food, and the toxic compounds present, in different ways. Some ofthem were able to convert toxic components into the harmless metabolites, and some of them were not.
Let me provide you with another example that further support the notion that the metabolism, by its very nature, is a unique process. You have definitely heard of allergies. Millions of people suffer from them, and it is not always easy for the medical professional identiÜ the true source of an allergic reaction. Usually, by excluding the exposure to certain types of food, or cosmetics, in some c. it becomes possible to pinpoint the culprit. The problem could not only be food, but also a household item. For example, a carpet or wallpaint. The allergr symptoms can be so bad that it might be necessary to scrape off the old paint, or remove the carpet. Or; some people cannot “process” the milk Of mammalian origin. mey are said to have allergies toward lactose, a chemical compound (disaccharide) present in the milk. So-called lactose-intolerant people lack an enzyme called ß-galactosidase, or simply lactase. What is especially relevant to this context is that the presence Of this vital enzyme is age and geography dependent. Normal infants are capable of digesting their mothers’ milk because the enzyme in question is present in their digestive tracts. If the people stop using milk after an early Ohildhood , and this is true for certain parts ofthe world, then the formation ofthe enzyme does not occur any more and the ability to digest the lactose is gradually lost. When we, the most generous nation in the world, help victims of natural disasters, ill the remote areas of the world, sometimes halfway around the globe, it needs to be verified if the locals are in fact able to digest the food products provided. In practical terms, if it is known that the target group is lacking the ability to consume the lactose, then they should be provided with lactose-free formulas, such as soybean milk.
Enzymatic make-up can also be dependent upon ethnicity. FOT example, certain ethnic groups, when migrating to new countries’ Enzymes Involved in rood Procegin’ 45
start developing diseases that are less frequent, or non-existent, in their native countries. Such a statiStiC exists, in cancer patients: South-Asian women relocating to the United suffer from disease in larger proportions than their countrywomen. While cancer, as a disease, ig an incredibly complicated subject, urd cancer incidence can be affected by multiple factors, one contributor could be a disparity in distribution that does not allow to “correctly” metabolize a myriad of xenobi0tics that Qie migrants are exposed to in a new environment.
Another hint is commg from the drug industry where the talk of the day is about switching to the personalized medicine that is based on the genetic make-up of an individual. People working in the drug industry, or medical professionals working in the trenches, with the patients, long know that drugs work different& on different individuals. There has never been a singe drug that was “right” for every patient. This is why to treat the same disease, scientists are developing a battery of potential drug candidates. We are all aware that if the given drug does not provide a relief from painful symptoms, then a family doctor would try something else in an attempt to find the “right” medicine for you. If the personalized approach is correct for drugs, then It should be correct for food also, because the basic metabolic pathwQvs are the same for every xenobiotic, no matter what the orlgn IS. Why should the label food vs drug — matter that much? Why should we apply a higher standard for drugs and much lower standard for food? In fact, it should be the other way around because we all are exposed to Øod much more frequently than to drugs/
I understand that this discussion can be quite unsettling for you.
But my civic duty, as a scientist, is not to comfort, but to alert you. The truth is that the level of knowledge about the metabolism of xenobiotics, and their impact on the human body, is inadequate. The common sense is that we should consider every component present in the food as potentially dangerous, unless its safety is proven by the qualified, independent parties. The same line ofthinking, and the same scientific standards, should be applied to cosmetics, toiletries, and environmental pollutants. Every new addition to the consumer basket, be it a fruit or a spicy ingredient, should be thoroughly studied for their long-term effects on human health before advertisements are allowed on national TV or radio waves. If the Food and Drug Administration (FDA) careftzlly studies every new drug before it is put on the pharmacy shelf, then every component present in the food should be equally scrutinized and allowed for use, by the FDA. The passing score should only be given if the comprehensive scientific review (toxicologr, metabolomics, pharmacomnetics, etc.) is carried out and the long-term impact on the human body is fully understood.
Question 2 (4 points)
Anemia, a well commonly known condition is caused by Iron deficiency. What are some of the dangers of having too much Iron?
Question 2 options:
You’re visiting old Auntie Harriet for the weekend in the woods, for lunch she has collected some celery and fennel from what you presumed was her garden – but Aunt Harriet has always been what the family called a herbalist. A few bites into your salad and you find it too bitter to eat any more – you simply smile and slip it under the table to her vegetarian dog, “scout.”
Half an hour later Auntie starts to mention that her throat is burning and she has a funny feeling as if her hands were made of “fur.” You look around to see if she slipped some funny mushrooms into the salad and find none.
She begins sweating and shivering and complains that her blood feels like ice – you call poison control and the ambulance immediately – informing them that she has been poisoned by
what plant and what poison?
Question 3 options:
What are some of the chemical components of enzymes – be as specific as possible?
Question 4 options:
Do substrates have to perfectly match an enzyme for it to carry out a chemical transformation? (if not give an example of an enzyme that is promiscuous)
Question 5 options:
Melikyan suggests we should have a higher standard for regulation of food than we do for drugs. Argue the opposite – that our current system for regulating drugs but not foods is a better system. (Make a convincing argument 4-5 sentences)
Question 6 options:
Melkyan suggests in chapter 2 (enzymes) that aromatics (Benzene ring) that have OH groups (together we call this a phenol) are of concern for consumption. What are some reasons for his concern?
Question 7 options:
Describe cancer in your own words. Describe features and traits, knowledge you posses. (3-5 sentences)
Question 8 options:
When are our cells most susceptible to genetic damage and cancer onset?
Question 9 options:
When looking at a cumulative graph – how do we detect change that might not be immediately apparent – or that an author might be trying to conceal from us?
Question 10 options:
“Do the number of films Nicolas Cage appears in cause an increase in the number of people who drown?”
explain how these two things MIGHT be related using a “third factor x.”
Question 11 options:
Reporting statistics is a difficult proposition. Errors or outright incompetence can be introduced into the data at many points! Experiment design, data collection, analysis or interpretation! Which of these 4 things are easy for a reader to decipher or ponder or question. Which of these things are usually difficult for a newspaper reader to understand?
Question 12 options:
Give a real world example of CLASSIC PROBABILITY
Question 13 options:
What are some features of a virus?
Question 14 options:
What are some features of a bacteria?
Question 15 options:
Define in your terms electromagnetic radiation and separately ionizing radiation.
Question 16 options:
Define what a free radical is.
Question 17 options:
How does a coal power plant expose humans to more radiation than a nuclear power plant?
Question 18 options:
Acid rain is caused by the formation of strong acids in the atmosphere when certain chemical species react with water. What are some of the sources (natural or not) of SO3 and SO2 species? Where does the nitrogen in NO and NO2 species come from?
Question 19 options:
What are some specific green houses gases that you learned about (list 2) and describe their role as completely as you can in global warming (and cooling) processes. List sources – roles – effects, etc.
Question 20 options:
Chemoreceiption is a process in which our body interacts with chemicals outside our bodies – such as tastes and smells. Describe how taste works – in your own words!
how does smell differ from taste?
Question 21 options:
Describe the two pain systems our body has evolved.
Question 22 options:
We have access to how many tastes? and how many odors?
Question 23 options:
Define the concept of antioxidant in YOUR OWN TERMS.
Question 24 options:
Why are we so obsessed with free radicals and anti-oxidants? (any number of possible reasons you could provide, I can think of 3 entirely different approaches to answering this question).
Question 25 options:
You’re eating at veggie shack, having exhausted every entertaining thing online – you start taking notes about who orders what and whether the waitstaff brings ketchup without being asked to.
Are you more likely to get ketchup if you order a veggie burger than the average customer? How much more likely? (report as a percent). (hint: find the difference between the two probabilities)