Coenzyme oxidative

The two forms of these coenzymes (oxidized and reduced) have different spectral characteristics (Figure 8.17), which permit the reduced form to be detected at 340 nm in the presence of the oxidized form. 

Proenzymes and other precursor proteins are often almost totally inactive until they are activated by some alteration that occurs when they reach their destination in a cell or in the body. Cleavage of the polypeptide chains of proenzymes, covalent attachment of coenzymes, oxidation (Eq. 16-57) or halogenation (Eq. 25-6) of fyrosine or tryptophan side chains, and oxidation of cysteine in the sequence CTPSR to formylglycine (in sulfatase formation Eq. 

Oxidation-reduction coenzymes follow the same principles as activation-transfer coenzymes, except that they do not form covalent bonds with the substrate. Each coenzyme has a unique functional group that accepts and donates electrons and is specific for the form of electrons it transfers (e.g., hydride ions, hydrogen atoms, oxygen). A different portion of the coenzyme binds the enzyme. Like activation-transfer coenzymes, oxidation-reduction coenzymes are not good catalysts without participation from amino acid side chains on the enzyme. 

Nicotinamide adenine coenzymes Oxidation-reduction Niacin 15.9, 17.3, 19.3... 

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... 

IS the oxidation of lactic acid to pyruvic acid by NAD and the enzyme lactic acid coenzyme NAD ... 

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... 

Section 15 11 Oxidation of alcohols to aldehydes and ketones is a common biological reaction Most require a coenzyme such as the oxidized form of nicotin amide adenine dmucleotide (NAD" )... 

The product (6 hexanohde) is a cyclic ester or lactone (Section 19 15) Like the Baeyer-Vilhger oxidation an oxygen atom is inserted between the carbonyl group and a carbon attached to it But peroxy acids are not involved m any way the oxidation of cyclohexanone is catalyzed by an enzyme called cyclohexanone monooxygenase with the aid of certain coenzymes... 

The ready reversibility of this reaction is essential to the role that qumones play in cellular respiration the process by which an organism uses molecular oxygen to convert Its food to carbon dioxide water and energy Electrons are not transferred directly from the substrate molecule to oxygen but instead are transferred by way of an electron trans port chain involving a succession of oxidation-reduction reactions A key component of this electron transport chain is the substance known as ubiquinone or coenzyme Q... 

All the individual steps are catalyzed by enzymes NAD" (Section 15 11) is required as an oxidizing agent and coenzyme A (Figure 26 16) is the acetyl group acceptor Coen zyme A is a thiol its chain terminates m a sulfhydryl (—SH) group Acetylation of the sulfhydryl group of coenzyme A gives acetyl coenzyme A... 

Acyl earner Oxidized form Water protein of coenzyme... 

Two and twelve moles of ATP are produced, respectively, per mole of glucose consumed in the glycolytic pathway and each turn of the Krebs (citrate) cycle. In fat metaboHsm, many high energy bonds are produced per mole of fatty ester oxidized. Eor example, 129 high energy phosphate bonds are produced per mole of palmitate. Oxidative phosphorylation has a remarkable 75% efficiency. Three moles of ATP are utilized per transfer of two electrons, compared to the theoretical four. The process occurs via a series of reactions involving flavoproteins, quinones such as coenzyme Q, and cytochromes. 

The preparation of coenzyme Q usually iavolves either 2,3-dimethoxy-5-methylbenzoquinone or hydroquiaone as the starting material. Treatment of the hydroquiaone with geranyl bromide followed by oxidation affords (61, n = 2) (214). A facile and efficient preparation of ubiquiaone-10 (61, n = 10) has been developed (215). 

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). 

In oiological systems, the most frequent mechanism of oxidation is the remov of hydrogen, and conversely, the addition of hydrogen is the common method of reduc tion. Nicotinamide-adenine dinucleotide (NAD) and nicotinamide-adenine dinucleotide phosphate (NADP) are two coenzymes that assist in oxidation and reduction. These cofactors can shuttle between biochemical reac tions so that one drives another, or their oxidation can be coupled to the formation of ATP. However, stepwise release or consumption of energy requires driving forces and losses at each step such that overall efficiency suffers. 

All known eight-stranded a/p-barrel domains have enzymatic functions that include isomerization of small sugar molecules, oxidation by flavin coenzymes, phosphate transfer, and degradation of sugar polymers. In some of these enzymes the barrel domain comprises the whole subunit of the protein in others the polypeptide chain is longer and forms several additional domains. An enzymatic function in these multidomain subunits, however, is always associated with the barrel domain. 

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