Enzymatic Synthesis of Optically-active Selenium and Tellurium Containing Amino Acids and the Reaction Mechanism

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Various selenium-containing amino acids occur in nature, and most of them are physiologically active. The present study has been undertaken to prepare optically active selenium-containing amino acids, which are synthesized chemically with great difficulty, with multi-functional pyridoxal-P enzymes, and also to clarify the mechanism of the enzymatic reactions. The results that we have obtained are as follows.(1) We have synthesized tellurium-containing amino acids, which are expected to be useful as a specific imaging agent for pancreas. Te-Phenyl-L-tellurohomocysteine and Te-phenyl-L-tellurocysteine have been synthesized.(2) Various optically-active selenium-containing amino acids have been obtained from selenols and cheaply-available amino acids with bacterial L-methionine <gamma> -lyase and yeast O-acetylhomoserine(thiol)-lyase. As both the enzymes catalyze not only <beta> but also <gamma> -replacement reactions, we could synthesize not only Se-substituted-L-selenocysteine but Se-substituted-L-selenohomocysteine. O-Acetylhomoserine(thiol)-lyase catalyzes only replacement reactions. Therefore, the synthetic reactions proceed efficiently in contrast with that with L-methionine <gamma> -lyase which catalyzes elimination reactions in addition to replacement reactions.(3) However, we have found that L-methionine <gamma> -lyase entrapped in polyurethane catalyzes only replacement reactions, and that the modified enzyme works actually as an efficient biocatalyst for the synthesis of selenium amino acids.(4) We have also developed a novel synthetic procedure for optically-active selenium-containing amino acids with bacterial tryptophan synthase, which inherently functions in the tryptophan synthesis.