Reactions of 0-substituted l-homoserines catalyzed by l-methionine γ-lyase and their mechanism

L-Methionine γ-lyase (EC 4.4.1.11) 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 y-replacement reactions with various thiols to produce the corresponding S-substituted L-homocysteines. The reactivities of O- substituted L-homoserines in a, y-elimination relative to that of L-methionine were as follows: O- acetyl, 140%; 0-succinyl, 17%; and 0-ethyl-L-homoserine, 99%. However, the enzyme does not catalyze the synthesis of O-substituted L-homoserines from alcohol or carboxylic acids in α γ- replacement reaction. We have analyzed the α,γ-elimination of O-acetyl-L-homoserine in deuterium oxide by XH-NMR. The [β-2H, γ-2H]-species of a-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 enzyme with 0-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 y-replacement reactions of 0-acetyl-L- homoserine 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 β-substituted L-homocysteines [N. Esaki, T. Suzuki, H. Tanaka, K. Soda and R. R. Rando, FEBS Lett., 84, 309 (1977).