Coenzymes hydrogen-transferring

Model Reaction of Asymmetric Inter-Coenzyme Hydrogen Transfer... 

Ribonucleotide reductase differs from the other 5 -deoxyadenosyl-cobalamin requiring enzymes in a number of respects. Hydrogen is transferred from coenzyme to the C2-position of the ribose moiety without inversion of configuration. Also since lipoic acid functions in hydrogen transfer, exchange with solvent protons takes place. Furthermore, exchange between free and bound 5 -deoxyadenosylcobalamin occurs rapidly during catalysis. Evidence for a Co(I)-corrin as an intermediate for this reduction is presented in our section on electron spin resonance. 

Mechanism of Hydrogen Transfer by Pyridine Nucleotide Coenzymes 239... 

Flavin Coenzymes.—5-Deazaflavin-adenine dinucleotide (2) can be prepared from the 5-deazaFMN,21 using a FAD pyrophosphorylase from rat liver.22 When the apoprotein of D-amino-acid oxidase from pig kidney is reconstituted with (2), no oxidation of D-alanine is observed, although the flavin chromophore in the reconstituted enzyme is reduced on the addition of DL-amino-acids.22 This has been interpreted as indicating that hydrogen transfer from the amino-acid to (2) can still... 

Beedle, A. S., Munday, K. A., Wilton, D. C. The stereochemistry of hydrogen transfer from NADPH catalyzed by 3-hydroxy-3-methylglutaryl-coenzyme A reductase from rat liver. European J. Biochem. 28, 151—155 (1972). 

Biellmann, J.-F., Branlant, G., Olomucki, A. Stereochemistry of the hydrogen transfer to the coenzyme by octopine dehydrogenase. FEBS Letters 32, 254— 256 (1973). 

Fisher, H. F., Adija, D. L., Cross, D. G. Dehydrogenase-reduced coenzyme difference spectra, their resolution and relationship to the stereospecificity of hydrogen transfer. Biochemistry 8, 4424—4430 (1969). 

Most compounds oxidized by the electron transport chain donate hydrogen to NAD+, and then NADH is reoxidized in a reaction coupled to reduction of a flavoprotein. During this transformation, sufficient energy is released to enable synthesis of ATP from ADP. The reduced flavoprotein is reoxidized via reduction of coenzyme Q subsequent redox reactions then involve cytochromes and electron transfer processes rather than hydrogen transfer. In two of these cytochrome redox reactions, there is sufficient energy release to allow ATP synthesis. In... 

The adrenal gland and adrenergic neurons continue the synthesis by hydroxylating dopamine into norepinephrine (noradrenaline). Ascorbic acid (vitamin C see p.368) acts as a hydrogen-transferring coenzyme here. 

Why are there four major hydrogen transfer coenzymes, NAD+, NADP+, FAD, and riboflavin phosphate (FMN), instead of just one Part of the answer is that the reduced pyridine nucleotides NADPH and NADH are more powerful reducing agents than are reduced flavins (Table 6-7). Conversely, flavin coenzymes are more powerful oxidizing agents than are... 

Hydrogen Transfer and Dependent upon a Vitamin B12 Coenzyme... 

Three types of enzymatic reactions depend upon alkyl corrin coenzymes. The first is the reduction of ribonucleotide triphosphates by cobalamin-dependent ribonucleotide reductase, a process involving intermo-lecular hydrogen transfer (Eq. 16-21). The second type of reaction encompasses the series of isomerizations shown in Table 16-1. These can all be depicted as in Eq. 16-34. Some group X, which may be attached by a C-C, C-O, or C-N bond, is transferred to an adjacent carbon atom bearing a hydrogen. At the same time,... 

Dehydrogenases responsible for hydrogen transfer between substrates possess NAD+ (nicotinamide) as the coenzyme, owing to which chiral alcohols are formed from ketones and aldehydes. 

Equation 21 depicts the reaction of the simplest NAD+/NADH-dependent enzyme, alcohol dehydrogenase153. This enzyme performs the last reaction in ethanol fermentation. Indicated in the equation is the fundamental finding on such reactions first reported by Westheimer, Vennesland and coworkers154-157. The hydrogen transfer is not only direct in both directions (as opposed to solvent-mediated), but is totally stereospecific in both directions for both the coenzyme and the substrate. The pro-R hydrogen is transferred from both the coenzyme and the substrate ethanol. This was an observation of enormous implications for NAD+-dependent reactions, in particular, and enzyme catalyzed reactions, in general, that has been borne out by literally hundreds of examples. 

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