Applications to Enzyme-Catalyzed Reactions

Once one finds out which of two stereoheterotopic ligands or faces of a substrate is involved in an enzyme-catalyzed reaction, one is in a position to make a meaningful statement as to the location of the substrate in relation to the active site of the enzyme. While considerations of prostereoisomerism are thus useful in helping elucidate the enzyme-substrate relationship in the activated complex of an enzyme-mediated reaction, it must also be stressed that such considerations in themselves are insufficient to provide the complete picture and that they must necessarily be supplemented by many other techniques in enzyme chemistry. 

The literature in the area of prostereoisomerism in enzyme reactions is vast and we must confine ourselves in this section to the discussion of a few representative examples. For more detailed information the reader is referred to a number of review articles 19-32-m u8 and two books 119 120 which have appeared in the last dozen years. 

We shall start the discussion with a classical experiment related to the stereochemistry of oxidation of ethanol and reduction of acetaldehyde mediated by the enzyme yeast alcohol dehydrogenase in the presence of the oxidized (NAD+) and reduced (NADH) forms, respectively, of the coenzyme nicotinamide adenine dinucleotide (Fig. 54). The stereochemically interesting feature of this reaction stems from the fact that the methylene hydrogens in CH3CH2OH and the faces of the carbonyl in CH3CH = 0 are enantiotopic. The question thus arises which of the CH2-hydrogens 

When the configuration of the ethanol- 1-d is inverted by conversion to the tosylate followed by treatment with hydroxide and the inverted ethanol-l-d is then oxidized with yeast alcohol dehydrogenase and NAD+, the deuterium (which has taken the stereochemical position of the original hydrogen) is now removed and the product is unlabeled CH3CH=0.28 The sequence of events121 s is summarized in Fig. 55. 

28 The sequence is not entirely clean in that some CH3CDO is also obtained. Probably this is due to incomplete inversion in the tosylate — hydroxide reaction resulting from O—S cleavage (with retention). 

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Since X + In X is a transcendental function, Eq. (2-67) cannot be solved for [A], Two methods are usually used. The method of initial rates is the more common one, since it converts the differential equation into an algebraic one. Values of v(, determined as a function of [A]o, are fit to the equation given for v. This application to enzyme-catalyzed reactions will be taken up in Chapter 4. The other method regularly used relies on numerical integration these techniques are given in Chapter 5. 

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