The Stereochemistry of Enzyme-Catalyzed Reactions

NADH. These experiments were pioneering with respect to contemporary enzymology, especially with regard to early recognition that coenzymes are held within enzyme active sites in stereochemically preferred ways. One typically utilizes NADH that contains a tritium or deuterium atom in the 4R or 45 position, and the success or failure of substrate deuteration/tritiation indicates the stereochemistry. Westheimer has tabulated the known examples of dehydrogenases that exhibit specificity for a particular face of NADH. Creighton and Murthy have reproduced this tabulation in their comprehensive review on the stereochemistry of enzyme-catalyzed reactions at carbon. 

John Warcup Cornforth (1917-), Australian-English chemist, receives the Nobel Prize for his work on the stereochemistry of enzyme-catalyzed reaction. Vladimir Prelog (1906- ), Croatian-Swiss chemist, also receives the Nobel Prize in chemistry for his research into the stereochemistry of organic molecules and reactions. 

For studies on the stereochemistry of enzyme-catalyzed reactions. Sir John Comforth received the Nobel Prize in chemistry in 1975 (sharing it with Vladimir Prelog, page 188). Bom in Australia in 1917, he studied at the University of Sydney and received a Ph.D. from Oxford. His major research was carried out in laboratories at Britain s Medical Research Council and in laboratories at Shell Research Ltd. He was knighted in 1977. 

Fundamental work on the stereochemistry of enzyme-catalyzed reactions was done by Frank H. Westheimer. Westheimer was born in Baltimore in 1912 and received his graduate training at Harvard University. He served on the faculty of the University of Chicago and subsequently returned to Harvard as a professor of chemistry. 

In this review, we shall concentrate on the stereochemistry of enzymic reactions of amino acids, many of which involve transformations at prochiral centers. We shall use the nomenclature of Hanson (8) to specify the stereochemistry of prochiral atoms and groups as pro-R (Hjj) and pro-S (Hj) and of prochiral faces as Re and Si and the nomenclature of Mislow and Raban (2) to describe prochiral groups as having enantiotopic or diastereotopic relationships. Reviews on the stereochemistry of enzymic reactions of amino acids were published in 1978 (9,10), and since the seminal review by Dunathan in 1971 (11), several reviews comparing the stereochemistry of pyridoxal phosphate-catalyzed enzymic reactions have appeared (12-15). 

Before leaving this biosynthetic scheme notice that PGE2 has four chirality cen ters Even though arachidomc acid is achiral only the stereoisomer shown m the equa tion IS formed Moreover it is formed as a single enantiomer The stereochemistry is controlled by the interaction of the substrate with the enzymes that act on it Enzymes offer a chiral environment m which biochemical transformations occur and enzyme catalyzed reactions almost always lead to a single stereoisomer Many more examples will be seen m this chapter... 

Gold and Linder (17) studied the esterase catalyzed hydrolysis of A-(-)-acetoxymethyl-(l-phenylethyl)nitrosamine. They found that the stereochemistry of 1-phenylethanol produced in the reaction was the same as that observed in the base catalyzed hydrolysis of the nitrosamine and also of N-(l-phenylethyl)nitrosocarbamate. These results indicated that the same diazotate was produced in all three reactions. The fact that no irreversible inhibition of the enzymatic hydrolysis of the nitrosamine was observed, while extensive irreversible inhibition was obtained with the nitrosocarba-mate, led these workers to conclude that the a-hydroxynitro-samine produced by the hydrolysis had sufficient stability to diffuse away from the active site of the enzyme. 

Williams, G.J., Domann, S., Nelson, A. and Berry, A. (2003) Modifying the stereochemistry of an enzyme-catalyzed reaction by directed evolution. Proceedings of the National Academy of Sciences of the United States of America, 100, 3143-3148. 

Hydride Transfer in NAD+- and NADP -Dependent Enzymes. The transfer of the hydride ion in redox reaction of NAD+- and NADP+-dependent enzymes can occur either to the re- or the xi-face of the pyridine ring of the coenzyme . Such stereochemistry is crucial in the characterization of these enzymes. The same enzymes from different sources can express different stereospecificities. For example, E. coli NAD(P)+ transhydrogenase expressed one form of stereospecificity whereas the Pseudomonas aeruginosa enzyme catalyzes the identical reaction with the other NAD form . 

An advantage of these enzymes is that they are stereocomplementary, in that they can synthesize the four possible diastereoisomers of vicinal diols from achiral aldehyde acceptors and DHAP (Scheme 4.2). Although this statement is generally used and accepted, it is not completely true since tagatose-l,6-bisphosphate aldolase (TBPA) from Escherichia coli-the only TBPA that has been investigated in terms of its use in synthesis-does not seems to control the stereochemistry of the aldol reaction when aldehydes different from the natural substrate were used as acceptors [7]. However, this situation could be modified soon since it has been demonstrated that the stereochemical course of TBPA-catalyzed C—C bond formation may be modified by enzyme-directed evolution [8]. 

The stereochemistry of pyridoxal phosphate-catalyzed reactions was last summarized comprehensively in 1971 by Dunathan [2], who outlined many of the basic concepts in this field. Aspects of PLP catalysis have been discussed in other reviews on enzyme reaction stereochemistry (e.g., [9]), and a brief review, emphasizing their own work, has recently been published by the present authors [ 10]. Much work has been done in this field during the past ten years, most of it supporting the concepts laid out in Dunathan s review, often refining the picture and sometimes modifying the original ideas. 

Recently, the group of Soda [76] examined the stereochemistry of the reaction catalyzed by ineso-a, c-diaminopimelate decarboxylase from Bacillus sphaericus. This enzyme, which they purified to homogeneity [70], shows similarly unique substrate... 

The stereochemistry of the acetoacetate decarboxylase catalyzed reaction was also investigated. It was first demonstrated that the enzyme selectively exchanged one of two methylene protons of butanone with deuterons . Subsequently, it was shown using substrates 17 and 18 that the reaction proceeds with retention of configuration giving 19 and (equation 12), as appears to be the case in the pyridoxal catalyzed... 

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