O-Acetyl-L-serine

In a series of papers, Cook et al.60-63 presented results of the 31P NMR studies of pyridoxal 5 -phosphate dependent enzyme. O-acetylserine sulf-hydrylase is the enzyme which catalyses the final step of biosynthesis of l-cysteine, the replacement of p-acetoxy group of O-acetyl-L-serine by thiol [30] in bacteria and plants. 

This enzyme ]EC 4.2.99.13], also known as lupinic acid synthase, catalyzes the reaction of O-acetyl-L-serine with zeatin to produce lupinate and acetate. Zeatin is N -(4-hydroxy-3-methyl-butyl-iran5 -2-enylamino)purine. A number of other A -substituted purines can function as substrates for this enzyme. 

National Reference System for the Clinical Laboratory O-acetyl-L-serine sulfhydrylase... 

In the oxidative deamination reaction, the enzyme was active toward N-[l-D-(carboxyl)ethyl]-L-methionine, N-[l-D-(carboxyl)ethyl]-L-phenylalanine, etc. The substrate specificity for amino donors of ODH in the reductive secondary amine-forming reaction was examined with pyruvate as a fixed amino acceptor [15,24]. The enzyme utilized L-norvaline, L-2-aminobutyric acid, L-norleucine, P-chloro-L-alanine, o-acetyl-L-serine, L-methionine, L-isoleucine, L-valine, L-phenylalanine, L-homophenylalanine, L-leucine, L-alanine, etc. 3-Aminobutyric acid and L-phenylalaninol also acted as substrates for the enzyme. Other amino compounds, such as P-amino acids, amino acid esters and amides, amino alcohols, organic amines, hydroxylamines, and hydrazines, were inactive as substrates. Pyruvate, oxaloacetate, glyoxylate, and a-ketobutyrate were good amino acceptors. We named the enzyme as opine... 

Figure 12.17 Important targets for the generation of an L-cysteine production strain. Mutant proteins are marked with an asterisk. Exp. cysteine export proteins. SerA 3-phosphoglycerate dehydrogenase. CysE serine transacetylase. CysB transcriptional activator of the cystein regulon. CysK O-acetyl-L-serine (thiol)-lyase A. CysM O-acetyl-L-serine (thiol)-lyase B.

O-acetyl-L-serine (thiol)-lyase A (CysK), O-acetyl-L-serine (thiol)-lyase B (CysM) and MalY. Thus in order to engineer a potent L-cysteine over-producer excessive degradation of the amino acid must be prevented by inactivation of the genes encoding the major L-cysteine desulfhydrase activities. 

S.a. is regulated by enzyme induction and repression, and by feedback inhibition (Fig. 2). In E. coli, l-cysteine represses all steps of S.a. (assuming S.a. to proceed via free intermediates) starting with the uptake of sulfate by the cell. O-Acetyl-t-serine induces uptake of sulfate, synthesis of APS and PAPS and synthesis of sulfite reductase, but not of sulfate reductase. Acetylation of L-serine to O-acetyl-L-serine is feedback-inhibited by L-cysteine. 

Selenium is present in biological materials in a number of compounds that correspond in their structure to sulfur compounds. The amino acid L-selenocysteine (see Section 2.2.1.1.1) bound in proteins arises from selane and O-acetyl-L-serine, methyla-tion yields Se-methyl-L-selenocysteine, reaction with O-succinyl-L-homoserine leads to L,L-selenocystathionine, hydrolysis of the last compound provides i-selenohomocysteine and methylation... 

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