Reactions of O-substituted L-homoserines catalyzed by L-methionine .GAMMA.-lyase and their mechanism.

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Abstract
L-Methionine y-lyase (EC 4.4. 1. 1 1) catalyzes α, γ-elimination of O-substituted L-homoserines (i.e., ROCH2CH2CH(NH2)COOH; R = acetyl, succinyl, or ethyl) to produce a-ketobutyrate, ammonia, and the corresponding carboxylate or alcohol, and also their γ-replacement reactions with various thiols to produce the corresponding 5-substituted L-homocysteines. The reactivities of Osubstituted L-homoserines in α, γ-elimination relative to that of L-methionine were as follows: Oacetyl, 140%; Osuccinyl, 17%; and O-ethyl-L-homoserine, 99%. However, the enzyme does not catalyze the synthesis of O-substituted L-homoserines from alcohol or carboxylic acids in a γ-replacement reaction. Wehave analyzed the α, γ-elimination of O-acetyl-L-homoserine in deuterium oxide by 1H-NMR. The [β-2H, γ-2H]-species of α-ketobutyrate was exclusively formed from 0-acetyl-L-homoserine. The enzyme catalyzes deamination of L-vinylglycine to give the identically labeled α-ketobutyrate species. Incubation of the enzymewith O-acetyl-L-homoserine resulted in the appearance of a new absorption band at 480 nm, which was observed also with L-vinylglycine. These results strongly suggest that the α, γ-elimination and γ-replacement reactions of O-acetyl-Lhomoserine proceed through the stabilized α-carbanion of a SchifF base between pyridoxal 5'-phosphate and vinylglycine, which has been suggested as the key intermediate of L-methionine γ-lyase-caralyzed reactions of S-substituted L-homocysteines [N. Esaki, T. Suzuki, H. Tanaka, K. Soda and R. R. Rando, FEBS Lett., 84, 309 (1977).

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