Solely nicotinamide coenzymes

Both NAD-dependent and NADP-dependent dehydrogenases were found to be stereospecific. Correlation of the sides of NAD and NADP [38,39] established the absolute configuration of NADP. 

It is now generally held that all dehydrogenases requiring nicotinamide coenzymes utilize only the 4-position of the nicotinamide ring, that the specificity for 4-pro-R or 4-pro-S is essentially complete, and that the mechanism is equivalent to hydride transfer. 

In non-enzymic reactions, hydride transfer between carbon atoms is unusual. Examples are the Cannizzaro reaction [40], and the Meerwein-Ponndorf-Verley-Oppenhauer reaction [41], in which a preferred steric course is discernible in some cases [42]. 

Reductions of carbonyl groups with lithium aluminium hydride or sodium borohydride occur by hydride transfer to carbon from aluminium or boron, respectively. The course of reaction is subject to steric approach control and product development control [43-45]. Enzymic reactions may or may not form the epimer favoured in the chemical reduction. This has been discussed elsewhere [46]. It is quite clear that the steric course of a dehydrogenase reaction is determined by the structure of the enzyme. 

Because of the nature of the priority rules [32], substrates sometimes differ in formal chirality, but nevertheless have essentially the same handedness [51,52]. Such reactions are homofacial rather than homochiral. The problem is of nomenclature interpretation for the enzymologist, not of stereospecific recognition by the enzyme. 

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The most important coenzymes in synthetic organic chemistry [14] and industrially applied biotransformations [15] are the nicotinamide cofactors NAD/ H (3a/8a, Scheme 43.1) and NAD(P)/H (3b/8b, Scheme 43.1). These pyridine nucleotides are essential components of the cell [16]. In all the reactions where they are involved, they serve solely as hydride donors or acceptors. The oxidized and reduced form of the molecules are shown in Scheme 43.1, the redox reaction taking place at the C-4 atom of the nicotinamide moiety. 

The presence of two coenzyme-binding sites is unexpected since they cannot be inferred solely from the crystal structure of CPR. Kinetic studies with wild t) e and W676H CPR at different concentrations of NADPH have, however, provided further support for the existence of two sites The rate of flavin reduction in the isolated FAD domain and CPR increases as NADPH is decreased from molar excess to stoichiometric concentrations. At stoichiometric concentration, the second noncatalytic site is predominantly vacant and the partial inhibition on the rate of flavin reduction from the catalytic site is therefore relieved (Figure 4.9). Occupation of the noncatalytic site occurs at NADPH concentrations in excess of the enzyme concentration, and impairs NADP" " release from the catalytic site. This in turn partially inhibits flavin reduction, the rate of which is gated by NADP release. Preincubation of the enzyme with a stoichiometric amount of adenosine 2, 5 -diphosphate does not lead to inhibition of the flavin reduction rate. We infer that the binding of adenosine 2, 5 -diphosphate prevents NADPH from binding to the noncatalytic site. This observation also suggests that it is the nicotinamide-ribose-phosphate portion of NADPH bound at the second site that hinders NADP" release from the catalytic site. Clearly, these new... 

From a nutritional standpoint, it is significant that five of the B-complex vitamins (riboflavin, nicotinamide, thiamine, vitamin Be, and pantothenic acid) have been shown to be constituents of the coenzymes. The nutritional requirement of these vitamins is explained on the basis of their coenzyme function. In all cases the coenzyme form appears to be the sole bound form of the vitamin, and this then becomes the only metabolically active form for these particular vitamins. 

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