Enzymic aspects

The versatility of pyridoxal-P for providing stereoelectronically assisted bond cleavages is further illustrated by y replacement and y elimination-deamination reactions where mechanistic and stereochemical interest now focusses on the changes which occur at the three centres (C , and C ) in the a-amino acid substrate. Representative examples of this class of enzyme catalysed reactions are shown in Fig. 35. 

Cystathionine-y-synthase isolated from Salmonella typhimurium is a tetramer (molecular weight 160000) and catalyses, in vivo, the y-replacement of O-suc-cinylhomoserine with cysteine [79] to yield cystathionine. The latter, by way of homocysteine, is involved in the biosynthesis of methionine. In other species of bacteria and plants the succinyl moiety may be replaced by acetyl, phosphoryl, or malonyl moieties [80]. In the absence of cysteine the enzyme catalyses an abnormal reaction resulting in the formation of a-oxobutyrate. The latter reaction has been utilised for mechanistic investigations pertinent to the y-eUmination-deamination process (vide infra). 

Threonine synthetase has been purified from N. crassa by Flavin [83] and catalyses the synthesis of l-threonine from O-phosphohomoserine and H2O via a y- replacement process. 

A general mechanism encompassing each of these processes can be formulated which involves the formation at the active site of the common intermediate (Fig. 36, 

1) which arises via a multistep process discussed later (cf. Fig. 39). 

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Hokfelt T, Fuxe K, Goldstein M, Joh TH (1973b) Immunohistochemical localization of three catecholamine synthesizing enzymes aspects on methodology. Histochemie 33 231-254. 

Eisen, V., Enzymic Aspects of Plasma Kinin Formation, Proc. Royal Soc. 

In recent years sulfohydrolases have attracted considerable attention because of their involvement in several human genetic disorders (Table 7). We will now discuss the enzymic aspects of diseases associated with the abnormal activities of sulfohydrolases. 

Priest, F.G. (1984) Extracellular Enzymes Aspects of Microbiology 9. Van Nostrand Reinhold, Wokingham, UK, 79 pp. 

CoRi, O., Enzymic aspects of the biosynthesis of monoterpenes in plants, Phytochemistry, 22, 331-341 (1983). 

Tschudy, D.P. Enzyme aspects of acute intermittent porphyria. Molec. Cell. Biochem. 2, 63-70 (1973)... 

This chapter is concerned primarily with the enzymic systems involved in the transformation or transfer of the amino, amine, and amide nitrogen moiety of amino acids, amino acid amides, and amines. Reactions involved in purine biosynthesis and degradation will be discussed in Chapter 18. Complete coverage of the literature since the publication of the first edition of this chapter (I) has not been attempted. Of necessity there has been a selection of the literatme references dictated for the most part by the interests of the authors. In general, preference has been ven to studies which emphaaze the enzymic aspects of chemical transformations. 

Support for this enzymic desulfhydration came from the work of Metzler and Snell (72), who proposed a detailed chemical mechanism of desulfhydration in model experiments in the presence of pyridoxine and a metal ion. Similar enzyme models were proposed by others (73). The enzymic aspect of this reaction is not entirely satisfactory however, ce relatively little is known of the desulfhydrase enzyme, either derived from the liver (74) or from bacteria (75). In addition several pyridoxal phosphate-protein enzymes can catalyze cysteine desulfhydration (see Section IV). 

The literature on the biosynthesis of triterpenes, steroids, and carotenoids for 1970 and 1971 has been reviewed in Volunne I. The reader is, therefore, referred to this for much basic information regarding biosynthetic pathways. This Report attempts to survey material related to the biosynthesis of triterpenes, steroids, and carotenoids published during 197Z Since much of the work which has been reported during that period has centred on enzymic aspects, this chapter of necessity reflects such emphasis. It is inevitable that there will be some overlap with chapters in another volume of the Specialist Periodical Reports series. Reviews centred on enzymic as well as stereochemical and mechanistic aspects of sterol biosynthesis have appeared. 

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