Enzymatic Synthesis of Cystathionine by Microbial Enzymes

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L-Cystathionine is an important intermediate in transsulfuration in a living thinsg. As it has two asymmetric carbons in the molecule, it is not easy to synthesize L-cystathionine chemically. In addition, L-cystathionine is very expensive amino acid at present. L-Cystathionine is markedly deficient in the brains of untreated homocystinuric patients and fetal brains. Thus, it is meaningful to establish the simple and economical ways of synthesizing L-cystathionine for use in future medical research. We, therefore, investigated enzymatic synthesis of L-cystathionine using microbial cystathionine <gamma> -lyase and cystathionine <gamma> -synthase.Cystathionine <gamma> -lyase occurring in Streptomyces phaeochromogenes can catalyze the <gamma> -replacement reaction of L-homoserine in the presence of L-cysteine to form L-cystathionine. Optimal reaction conditions for the enzymatic synthesis of L-cystathionine were examined. Since concentrations of greater than 10 mM L-cysteine inhibit the <g … More amma> -replacement reaction, it is necessary to keep the concentration of L-cysteine around 10 mM for a high yield of L-cystathionine. Under optimal reaction conditions, defined as the conditions producing the highest yield, cystathionine (3 g) was synthesized enzymatically in 1L of reaction mixture. O-Succinyl-L-homoserine and DL-vinylglycine were better substrates than L-homoserine. Under the optimal conditions, L-cystathionine (11 g/L) was synthesized from O-succinyl-L- homoserine with a substrate conversion ratio of 100%. Next, the reaction conditions for the enzymatic production of L-cystathionine were optimized, using cystathionine <gamma> -synthase, which is abundant in the cells of Erwinia carotovora and Bacillus sphaericus. Under optimal conditions, L-cystathionine (41g/L) was synthesized with conversion ratio of about 90% with the Erwinia and Bacillus enzymes.It can be concluded that cystathionine <gamma> -synthase is more promising and suitable for the enzymatic production of L-cystathionine than cystathionine <gamma> -lyase based on the following reasons;(1) Cystathionine <gamma> -synthase does not catalyze the degradation reaction of L-cystathionine formed.(2) Cystathionine <gamma> -synthase is not sensitive to the allosteric inhibitor , L-cysteine.(3) Specific activity of cystathionine <gamma> -synthase is much higher than cystathionine <gamma> -lyase.In addition, it was also possible to synthesize various kinds of L-homocysteine related amino acids using the cystathionine <gamma> -lyase and cystathionine <gamma> -synthase. Less