Biochemical Oxidation

Transcription

1 iochemical xidation An important part of metabolism is oxidation. An estimated 75% of oxidative reactions are performed by a family of enzymes called cytochrom P-450. ombustion reactions are necessary to our lives. ot only do you put gasoline in your car s gas tank, you also put fats, carbohydrates and proteins in your body. When you breath out, some of the same combustion products are expelled from your lungs as from your car s exhaust pipe: carbon dioxide and water. You body also uses oxidation to make less polar molecules more polar, which allows them to better dissolve in blood (water) and get excreted from the body or get transported from one place in your body to another place. ow does your body do it? Do you have a big cylinder, piston and spark plug deep in your abdomen, where you explode your body s fuel? Your body can also attack - bonds with free radicals, though in a much more controlled manner. Instead of wasting much of the energy as heat in a single gigantic explosion, the body captures a little bit of energy at a time in many small, single bond oxidations using highly specific enzymes, which can selectively oxidize specific sites that provide messages that tell the body to do things: become a man (testosterone) or become a woman (estrogen). We will simplify the complicated biomolecules to take a brief look at a biochemical oxidation reaction and add it to our bio-organic game. We do need oxygen, which we breathe with every breath, so let s start there. An oxygen molecule is picked up by an iron atom in myoglobin in the lungs, where an oxygen atom steals an electron from an iron atom bound in myoglobin. Myoglobin transports oxygen through the blood, transferring it to hemoglobin molecules in the cells of the body. The iron atoms in myoglobin and hemoglobin are supported inside huge prophyrin ring structures. Four porphyrin nitrogen atoms surround the iron in a square planar shape, and a sulfur atom from a protein molecule is connected from the bottom to keep the whole complex in the correct location on the enzyme. There are actually six bonding positions at the iron atom, so there is one open position. This is the position that will bond to oxygen to begin the oxidative attack on reactive sites in the body. The iron atom changes its oxidation state as the oxygen reacts, ranging anywhere from +2 to +5. The simplistic schematic below shows how iron could pick up an oxygen molecule and how that oxygen molecule could attack and oxidize an sp 3 - in the body or a = or the lone pair of a sulfur or nitrogen. There s lots of free radical chemistry going on inside you, so make sure you eat your fruits and vegetables to keep yourself well stocked up on free radical protecting anti-oxidants. protoporphyrin (heme) found in cytochrom P-450 oxidative enzymes, commonintheliver +3 Fe simplif ied structure, binds to enzyme through a sulfur enzyme, cysteine Enz 2 Enz 2 possible oxidative mechanisms on the next pages y:\files\classes\iochem Examples\biochem cycles\free radical oxidation.d

2 xidation in the ody 2 Enz +3 Fe 2 simplified structure Fe simplified structure heme, protoporphyrin IX, found in cytochrom P-450 oxidative enzymes Enz simplified structures +4 Fe 2 The business end is here simplified structures P P AD+ 2 AD+ AD 1 electron reduction 1 electron oxidation 1 electron oxidation 1e- 1e- FAD FAD then FAD 2 Flavin (oxidized) Flavin (partly reduced) Flavin (completely reduced) 1 electron reduction implified mechanism of action for reduction of = by FAD 2 FAD: oa oa FAD 2 FAD FAD 2 AD+ FAD AD y:\files\classes\iochem Examples\biochem cycles\free radical oxidation.d

3 Possible sequence of events, more complicated than we will show it (next page). y:\files\classes\iochem Examples\biochem cycles\free radical oxidation.d

4 Possible mechanism for heme-dependent hydroxylation reactions sp 3 - bonds 8 The free radical-like oxygen atom abstracts a hydrogen atom from a - bond in the enzyme cavity, forming an - bond and a carbon free radical. 9 alcohols The carbon free radical abstracts hydroxyl () from iron, making an - bond where a - bond had been. The iron is reduced back at to begin the process all over again. Possible mechanism for heme-dependent epoxidation reactions The free radical-like oxygen atom adds to a = bond (alkene or aromatic) in the enzyme cavity, forming a - bond and a carbon free radical. Possible mechanism for heme-dependent sulfur and nitrogen oxidation reactions The carbon free radical abstracts the oxygen atom from the iron, making an epoxide ring. The iron is reduced back at to begin the process all over again. eactive epoxides can be opened up to diols (more water soluble). epoxides sulfur substrate (1e-) sulfur substrate sulfoxides, further oxidation is possible, all the way to sulfate, 4-2 / nitrogen substrate (1e-) / Fe nitrogen +3 / substrate -oxides, further oxidation is -1 possible, all the way to nitrate, 3 y:\files\classes\iochem Examples\biochem cycles\free radical oxidation.d

5 ommon steroid transformations while similar in appearance, everyone of these steroids has a specific signal to transmit, such as turning on a genetic switch. They are extremely powerful chemicals, some operating at the nanomolar level. V2 2, p. 703, p lanosterol - the compound from which all steroids are derived. 3 3 about 19 steps AD+ and isomerization cholesterol - essential component of cell walls, precursor of steroid hormones, bile acids and vit D 3 3? steps pregnenolone - neuroactive steroid AD+ and isomerization hydroxypregnenolone - higher levels are found at end of puberty and in pregnacy.? steps hydroxyprogesterone - sometimes given to prevent premature birth progesterone - involved in menstrual cycle and pregnacy 2 dehydroisoandrosterone - most abundant 11-deoxycortisol (cortodoxone) circulating steroid, many uses glucocorticoid (similar to cortosol) AD reduced = is dihydrotestosterone (maybe with FAD 2 ) cortisol - glucocorticoid, increases testosterone - produced (more potent by 3x) blood sugar, decreases immune in all vertebrates, 20x more system (hydrocortisone) in males, male sex characteristics. three well known members of the estrogen family see 3 3 reactions 3 below 3 3 and AD+ oxidation 11-deoxycorticosterone intermediate to aldosterone corticosterone - stress hormone, immune system -estradiol - prominant estrogen in reproductive years, more potent than estriol (10x) and estrone (80x) estriol - higher levels in pregnancy, marker of fetal health AD+ oxidation estrone - known female carcinogen, causes breast tenderness, in men causes anorexia 5 categories of steroid hormones 1. progestins - overies and placenta- mediate menstrual cycle and maintains pregnacy 2. glococorticoids - adrenal cortex - affect metabolism in diverse ways, decrease inflamation,incease resistance to stress 3. mineralocorticoids - adrenal cortex - maintain salt and water balance 4. androgens - gonads - maturation and function of secondary sex organs, particularly in males, male sexual differentiation 5. estrogens - gonads - maturation and function of secondary sex organs, particularly in females aldosterone - a+,k+ balance, blood pressure reg. y:\files\classes\iochem Examples\biochem cycles\free radical oxidation.d

6 There are some crucial transformation in the above steroids. learly oxidation is an important part of changes that are occurring, but some of this is happening in places that look only like an alkane and this seems very puzzling. The body has many oxidative enzymes that are capable of doing this. The cyctochrom P450 family of enzymes is the major method (=75%) for metabolizing lipids, steroids and xenobiotics in the body (foreign chemicals). We will try to simplify a few of these strategies below. We will use some of the intermediate forms in the steroid paths to speculate how this could occur. The very first changes shown involve lanosterol going to cholesterol, which is said to involve 19 steps. elow we have shown some of those changes with only a portion of the actual molecules so we don t have to do so much work. The arrows show 3 places that lanosterol loses methyl groups. This appears to occur by two different strategies. ince I could not find any reference that showed all the steps I had to speculate from partial intermediates that were indicated. The isolated methyl (3) appears to be lost (as formate) by a completely different mechanism from the 2 methyls next to the (1 and 2, lost as carbon dioxide), but all occur by logical organic chemistry lanosterol 3 about 19 steps cholesterol Proposed steps for loss of methyl #3. a. First free radical oxidation of an alkane - is needed to allow other biochemistry to occur. b. nce the is present AD+ can oxidize the primary alcohol to an aldehyde. nce the aldehyde is made it can hydrate and the adjacent 2 needs to oxidize in a manner similar to step a. y:\files\classes\iochem Examples\biochem cycles\free radical oxidation.d

7 c. The alcohol is made into a good leaving group and probably spontaneously loses formate to form an alkene with loss of phosphate. The = has to be hydrogenated, maybe with a co-factor like FAD 2 used in fatty acid synthesis. P ATP -2 P 3 Make into a good leaving group using ATP and spontaneously lose 2. formate P 3-2 FAD The methyl is gone, with no hint as to how it happened. FAD 2 Transfers 2e-s and 2, possibly as hydride and a proton. Proposed steps for loss of methyl #1 and #2 are slightly different. The methyl groups are oxidized to carboxylic acids (one at a time) and the alcohol is oxidized to a -ketone, which spontaneously loses carbon dioxide (instead of formate) as in loss of the #3 methyl. a. First free radical oxidation of an alkane - to an alcohol - is needed to allow other biochemistry to occur. y:\files\classes\iochem Examples\biochem cycles\free radical oxidation.d

8 b. nce the is present, AD+ can oxidize the primary alcohol to an aldehyde. nce the aldehyde is made it can hydrate and oxidize again with another AD+ to a carboxylic acid (or possibly a sulfur hemiacetal). c. The adjacent alcohol needs to oxidize to a ketone using AD+, which likely spontaneously decarboxylates (- 2 ). When the ketone gets reduced back to an alcohol it looks like nothing happened to it. Many of the steroid transformation shown in the page of steroid structures involve reactions similar to these. Yet another variation showing testosterone to estradiol is on the next page. The above oxidations can also lead to leakage of free radicals (like hydroxyl radicals) which are extremely dangerous to living organisms. There are abundant antioxidizing strategies used to prevent such damage (vitamins, E, glutathione and other phytochemicals). The process of photosynthesis in plants posses major free radical problems for the plants and many of their chemicals help protect against free radical damage (polyphenolics, carotenoids, etc.) n the other hand, it appears that our immune system makes hydroxyl radicals on purpose to attack and kill foreign cells invading our bodies (a 2 edged sword). y:\files\classes\iochem Examples\biochem cycles\free radical oxidation.d

9 The oxidation of testosterone to make estradiol shows yet another way of losing a methyl group through a conjugated = bond. Methyl is removed and an aromatic ring is formed testosterone a male sex hormone -estradiol a female sex hormone 2 sp 3 - oxidation 2 Fe 2 AD+ AD+ AD FAD 2 tautomers FAD dehydrogenation decarboxylation xidation of an aromatic ring using an epoxide (phenylalanine tyrosine), which opens to form a phenol. imilar reactions oxidize PA compounds (and other xenobiotics) in the body (forms carcinogins, shown in aromatic topic). y:\files\classes\iochem Examples\biochem cycles\free radical oxidation.d

10 Another mysterious transformation (cholesterol pregnenolone) ?steps 3 cholesterol pregnenolone ?steps 3 17-hydroxypregnenolone dehydroisoandrosterone impler model reactions Fe+4 Fe+4 AD+ new ketone site Fe+4 y:\files\classes\iochem Examples\biochem cycles\free radical oxidation.d