Stereochemistry enzyme-reaction

The high degree of stereoselectivity observed with enzyme reactions provides further evidence as to the importance of dmg stereochemistry for pharmaceutical activity. 

Rose, I. A. Stereochemistry of pyruvate kinase, pyruvate carboxylase, and malate enzyme reactions. J. Biol. Chem. 245, 6052—6056 (1970). 

The COX reaction is quite remarkable. Arachidonic acid has no chiral centers, but PGE2 and PGI2 have four and TxB2 has five The immediate product of the COX reaction, known as PGH2, also has five chiral centers. This is an elegant example of the ability of enzymes to control the stereochemistry of reactions. 

The stereochemistry of reactions at zinc atoms has been studied in small molecules (Auf der Heyde and Nassimbeni, 1984) and in proteins (Holmes and Matthews, 1981 Vallee and Auld, 1990a,b). Zinc enzymes include carboxypeptidase A (Quiocho and Lipscomb, 1971 Rees et al., 1983), in which the zinc is coordinated to two histidine nitrogen atoms, two glutamate oxygen atoms, and water (involved in hydrolysis) (Fig. 26). 

The conversion includes at least three enzymic reactions.169-171 In the first stage, which requires pyridoxamine 5 -phosphate as a cofactor,171,172 dehydration of 7b occurs through intermediate formation of the Schiff base.173 Reduction of the resulting, unsaturated derivative with NADPH, the mechanism of which is not completely clear,174 leads to CDP-3,6-dideoxy-D-eryf/iro-hexos-4-ulose,169 and, in the third stage, further reduction of the latter at C-4 of the hexosyl group produces the derivatives of paratose or abequose the stereochemistry of the reaction is determined by the source of the enzyme.168 The tyvelose derivative is formed as a result of enzymic epimerization at C-2 of the hexosyl group in CDP-paratose.175... 

Tabic 8.1 Stereochemistry of enzyme-reactions" catalyzed phosphoryl transfer ... 

The following Pd-catalysed stereoselective transformations of 142 and 143 are possible. The Pd-catalysed reaction of the cis product 143 with malonate gives the coproduct 148 with retention of the stereochemistry. However, reaction of 143 without the Pd catalyst affords the trans product 149. The cis product 142 is a mesa form and can be converted to the chiral half ester 150 by enzyme-catalysed partial hydrolysis. 

Overton and coworkers discovered a leucine 2,3-aminomutase in plant tissue cultures of Andrographis paniculata that converts (S)-leucine in (R)-f-leucine [43] (Scheme 1.6.10). The enzyme activity was investigated in cell free extracts by incubation with (S)-[U-14C]leucine and by measuring the radioactivity of the methyl ester camphanamide derivatives of the reaction mixtures by radio-GC. The stereochemistry of the /i-am i no acid was determined by radio-GC comparison of the enzyme reaction product as methyl ester camphanamide derivative with an authentic sample. The enzyme is not dependent on cobalamin, because addition of intrinsic factor does not induce its inhibition. 

Biotechnology currently emphasizes microscale aspects that were not visible or envisioned until about 150 years ago, including (1) microbes (2) stereochemistry of reactions, molecular biology including transcription of genetic codes and translation for protein production and cell multiplication and (3) enzymes as catalysts. Humans came to only partially understand the nature of systems (animals, plants, insects, and soil) they handled in agriculture. Modern biotechnology must address needs of these systems, as well as those of people. Animals and plants must be healthy and adequately nourished, the pests (weeds and insects) controllable, the soil kept productive, and all must function in acceptable harmony if the quality of human life is to be sustained or improved as the population increases. 

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. 

In the following discussion of more recent results on the stereochemistry of PLP enzyme reactions, some of the newer data will be analyzed in terms of these early concepts. 

Maturation of the petro-chemical industry, environmental pressures for "clean chemistry" and the explosive development of biotechnology have increased interest in the application of enzymatic processes to organic synthesis. Enzymatic processes play an increasing role in the generation of chiral pharmaceutical intermediates, water-soluble materials and biopolymers. One problem in the development of enzymatic reactions for organic synthesis is the prediction of the stereochemistry of reaction. Reliable models for prediction of stereochemistry are needed to broaden the application of enzymes to organic synthesis. 

For enzymic reactions, it was proposed that the carbonyl oxygen of the substrate points toward the dihydropyridine ring nitrogen of NAD(P)H in the transition state( ). Based on the same assumption the stereochemistry of the product in the mimetic reduction can be predicted as shown in... 

Fig. 8. Stereochemistry of reactions catalyzed by maltose phosphorylase with inversion of configuration. (A) a-Maltose synthesis from /3-D-glucopyranosyl phosphate and the reverse phosphorolysis of a-maltose (B) a-maltose synthesis from /3-D-glucopyranosyl fluoride plus a-D-glucose. X represents a protein component whose interaction with the axial 1-OH of a-D-glucopyranose is required to activate all reactions promoted by the enzyme. Reproduced from Tsumuraya et al., Arch. Biochem. Biophys., 281 (1990) 58-65, with permission of Academic Press.

The stereochemistry of reactions at the C-4 Schiff base faces in the binary (coenzyme-enzyme) and ternary (substrate-coenzyme) complexes... 

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). 

Vederas, J. C., Floss, H. G. (1980). Stereochemistry of pyridoxal phosphate catalyzed enzyme reactions, Acc. Chem. Res., 13 455. 

Stereospecificity of enzyme reactions , in Progress in Stereochemistry (Butterworths Scientific Publications, London), 1954, p. 318. 

The conversion of lysine into piperidine alkaloids involves retention of hydrogen isotope at C-2/° The sequence is suggested to be that shown in Scheme 1, and catalysis of the reaction may be attributed to L-lysine decarboxylase. This enzyme, from the micro-organism Bacillus cadaveris, has been found to carry out the conversion of L-lysine into cadaverine with retention of configuration. Decarboxylation of L-[2- H]lysine by this enzyme then affords [15- H]-cadaverine. When this material is converted into alkaloids, e.g. iV-methyl-pelletierine (4 R = Me), the tritium attached to what becomes C-2 is lost cf. refs. 5 and 6. On the other hand, conversion of lysine into sedamine (27) in Sedum acre results in retention of the tritium originally present at C-2. The simplest explanation is that protonation of (26) in the micro-organism and plant proceeds with opposite stereochemistry. This is at variance, however, with current ideas on the stereochemistry of reactions that are catalysed by pyridoxal phosphate. ... 

Incomplete substrate specificity or stereoselectivity of enzyme reactions appears to be responsible for several spur products, examples of which are shown in Scheme 10. Although different chanoclavine isomers are produced 10), the stereochemistry of the tetracyclic clavines and 1 implies that chanoclavine I (Id), but not chanoclavine II (30), is an intermediate. Also, 22 can isomerize spontaneously to 1 or isolysergic acid (31). Typically, both C(8) diastereomers of ergopeptines are obtained in preparations, and those with an isolysergic acid moiety - which are... 

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