The Coenzymes

Folic acid and its derivatives (mostly the tri-and heptaglutamyl peptides) are widespread in nature. It is a specific growth ctor for certain micro-organisms, but in animals the intestinal bacteria provide the small quantities needed for growth. The coenzyme forms are actually... 

Figure 10.3-19. Representation of the reaction shown fn Figure 10.3-16, indicating all the atoms and bonds of the chemical structures as well as the reaction center. For the sake of clarity, the coenzyme A has been abbreviated.

This poster indicates the structures of the compounds involved in a reaction, the enzymes catalyzing a reaction, the coenzymes and regulators involved, and whether such a reaction is a general pathway occurring in all species, or a pathway specific to higher plants, animals, or unicellular organisms. 

This is exactly what we have done [21], For each reaction the constitution and stereochemistry of the reaction partners, the coenzymes, and regulators were stored as connection tables (as far as they were known), and the enzymes by name and EC number. 

FIGURE 15 5 Structure of NAD the oxidized form of the coenzyme nicotinamide adenine dinucleotide The functional part of the coen zyme is framed in red... 

According to the proposed mechanism for biological 0x1 dation of ethanol the hydrogen that is transferred to the coenzyme comes from C 1 of ethanol Therefore the dihydropyridme ring will bear no deuterium atoms when CD3CH2OH IS oxidized because all the deuterium atoms of the alcohol are attached to C 2... 

Although a variety of oxidizing agents are available for this transformation it occurs so readily that thiols are slowly converted to disulfides by the oxygen m the air Dithiols give cyclic disulfides by intramolecular sulfur-sulfur bond formation An example of a cyclic disulfide is the coenzyme a lipoic acid The last step m the laboratory synthesis of a lipoic acid IS an iron(III) catalyzed oxidation of the dithiol shown... 

Many naturally occurring substances are epoxides You have seen two examples of such compounds already m disparlure the sex attractant of the gypsy moth (Section 6 18) and m the carcinogenic epoxydiol formed from benzo[a]pyrene (Section 118) In most cases epoxides are biosynthesized by the enzyme catalyzed transfer of one of the oxy gen atoms of an O2 molecule to an alkene Because only one of the atoms of O2 is trans ferred to the substrate the enzymes that catalyze such transfers are classified as monooxy genases A biological reducing agent usually the coenzyme NADH (Section 15 11) is required as well... 

An example of a biologically important aide hyde is pyridoxal phosphate which is the active form of vitamin Bg and a coenzyme for many of the reac tions of a ammo acids In these reactions the ammo acid binds to the coenzyme by reacting with it to form an imine of the kind shown in the equation Re actions then take place at the ammo acid portion of the imine modifying the ammo acid In the last step enzyme catalyzed hydrolysis cleaves the imme to pyridoxal and the modified ammo acid... 

The enzyme is a single enantiomer of a chiral molecule and binds the coenzyme and substrate m such a way that hydride is transferred exclusively to the face of the carbonyl group that leads to (5) (+) lactic acid Reduction of pyruvic acid m the absence of an enzyme however say with sodium borohydride also gives lactic acid but as a racemic mixture containing equal quantities of the R and S enantiomers... 

NAD and NADP are required as redox coen2ymes by a large number of enzymes and ia particular dehydrogenases (Fig. 6). NAD" is utilized ia the catabohe oxidations of carbohydrates, proteins, and fats, whereas NADPH2 is the coenzyme for anaboHc reactions and is used ia fats and steroid biosynthesis. NADP+ is also used ia the cataboHsm of carbohydrates (2). 

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

There is one exception to the rule that requires bulky hydrophobic residues to fill the interior of eight-stranded a/p barrels in order to form a tightly packed hydrophobic core. The coenzyme Biz-dependent enzyme methylmalonyl-coenzyme A mutase, the x-ray structure of which was determined by Phil Evans and colleagues at the MRC Laboratory of Molecular... 

The first structure, flavodoxin (Figure 4.14a), has one such position, between strands 1 and 3. The connection from strand 1 goes to the right and that from strand 3 to the left. In the schematic diagram in Figure 4.14a we can see that the corresponding a helices are on opposite sides of the p sheet. The loops from these two p strands, 1 and 3, to their respective a helices form the major part of the binding cleft for the coenzyme FMN (flavin mononucleotide). 

Ohlsson, I., Nordstrom, B., Branden, C.-I. Structural and functional similarities within the coenzyme binding domains of dehydrogenases. /. Mol. Biol. 89 339-354, 1974. 

Several classes of vitamins are related to, or are precursors of, coenzymes that contain adenine nucleotides as part of their structure. These coenzymes include the flavin dinucleotides, the pyridine dinucleotides, and coenzyme A. The adenine nucleotide portion of these coenzymes does not participate actively in the reactions of these coenzymes rather, it enables the proper enzymes to recognize the coenzyme. Specifically, the adenine nucleotide greatly increases both the affinity and the speeifieity of the coenzyme for its site on the enzyme, owing to its numerous sites for hydrogen bonding, and also the hydrophobic and ionic bonding possibilities it brings to the coenzyme structure. 

Riboflavin was first isolated from whey in 1879 by Blyth, and the structure was determined by Kuhn and coworkers in 1933. For the structure determination, this group isolated 30 mg of pure riboflavin from the whites of about 10,000 eggs. The discovery of the actions of riboflavin in biological systems arose from the work of Otto Warburg in Germany and Hugo Theorell in Sweden, both of whom identified yellow substances bound to a yeast enzyme involved in the oxidation of pyridine nucleotides. Theorell showed that riboflavin 5 -phosphate was the source of the yellow color in this old yellow enzyme. By 1938, Warburg had identified FAD, the second common form of riboflavin, as the coenzyme in D-amino acid oxidase, another yellow protein. Riboflavin deficiencies are not at all common. Humans require only about 2 mg per day, and the vitamin is prevalent in many foods. This vitamin... 

It has been said that God created an organism especially adapted to help the biologist find an answer to every question about the physiology of living systems if this is so it must be concluded that pyridoxal phosphate was created to provide satisfaction and enlightenment to those enzymologists and chemists who enjoy pushing electrons, for no other coenzyme is involved in such a wide variety of reactions, in both enzyme and model systems, which can be reasonably interpreted in terms of the chemical properties of the coenzyme. Most of... 

As we have seen, the metabolic energy from oxidation of food materials—sugars, fats, and amino acids—is funneled into formation of reduced coenzymes (NADH) and reduced flavoproteins ([FADHg]). The electron transport chain reoxidizes the coenzymes, and channels the free energy obtained from these reactions into the synthesis of ATP. This reoxidation process involves the removal of both protons and electrons from the coenzymes. Electrons move from NADH and [FADHg] to molecular oxygen, Og, which is the terminal acceptor of electrons in the chain. The reoxidation of NADH,... 

FIGURE 24.21 A mechanism for the methylmalonyl-CoA mntase reaction. In the first step, Co is rednced to Co dne to homolytic cleavage of the Co —C bond in cobalamin. Hydrogen atom transfer from methylmalonyl-CoA yields a methylmalonyl-CoA radical that can undergo rearrangement to form a snccinyl-CoA radical. Transfer of an H atom regenerates the coenzyme and yields snccinyl-CoA. 

The coenzyme for the oxidation-reduction reactions of fatty acid synthesis is NADP /NADPH, whereas degradation involves the NAD /NADH couple. 

Carboxylic acids with labile a-methylene protons react with isatin in the presence of strong aqueous base. In the total synthesis of methoxatin, the coenzyme of methanol dehydrogenase and glucose dehydrogenase, Weinreb employs a Pfitzinger condensation of an isatin 37 and pyruvic acid as a key step to provide the 4-quinolinic acid 38 in 50% yield under the standard basic conditions. ... 

Dihydropyridines not only are intermediates for the synthesis of pyridines, but also are themselves an important class of N-heterocycles an example is the coenzyme NADH. Studies on the function of NADH led to increased interest in the synthesis of dihydropyridines as model compounds. Aryl-substituted dihy-dropyridines have been shown to be physiologically active as calcium antagonists. Some derivatives have found application in the therapy of high blood pressure and angina pectoris. For that reason the synthesis of 1,4-dihydropyridines has been the subject of intensive research and industrial use. The Hantzsch synthesis has thus become an important reaction. 

Divalent sulfur compounds are achiral, but trivalent sulfur compounds called sulfonium stilts (R3S+) can be chiral. Like phosphines, sulfonium salts undergo relatively slow inversion, so chiral sulfonium salts are configurationally stable and can be isolated. The best known example is the coenzyme 5-adenosylmethionine, the so-called biological methyl donor, which is involved in many metabolic pathways as a source of CH3 groups. (The S" in the name S-adenosylmethionine stands for sulfur and means that the adeno-syl group is attached to the sulfur atom of methionine.) The molecule has S stereochemistry at sulfur ana is configurationally stable for several days at room temperature. Jts R enantiomer is also known but has no biological activity. 

As another example, studies with deuterium-labeled substrates have shown that the reaction of ethanol with the coenzyme NAD+ catalyzed by yeast alcohol dehydrogenase occurs with exclusive removal of the pro-R hydrogen from ethanol and with addition only to the Re face of NAD+. 

Direct hydroxylation of an aromatic ring to yield a hydroxybenzene (a phenol) is difficult and rarely done in the laboratory., but occurs much more frequently in biological pathways. An example is the hydroxylation of p-hydroxyphenyl acetate to give 3,4-dihydroxyphenyl acetate. The reaction is catalyzed by p-hydroxyphenylacctate-3-hydroxylase and requires molecular oxygen plus the coenzyme reduced flavin adenine dinucleotide, abbreviated FADH2. 

In living organisms, aldehyde and ketone reductions are carried out by either of the coenzymes NADH (reduced nicotinamide adenine dinucleotide) or NADPH (reduced nicotinamide adenine dinucleotide phosphate). Although... 

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