1H NMR Studies of Substrate Hydrogen Exchange Reactions Catalyzed by L-Methionine γ-Lyase

Hydrogen exchange reactions of various l-amino acids catalyzed by l-methionine γ-lyase (EC 4.4.1.11) have been studied. The enzyme catalyzes the rapid exchange of the α- and β-hydrogens of l-methionine and S-methyl-l-cysteine with deuterium from the solvent. The rate of a-hydrogen exchange was about 40 times faster than that of the enzymatic elimination reaction of the sulfur-containing amino acids. The enzyme also catalyzes the exchange reaction of α- and β-hydrogens of the following straight-chain l-amino acids which are not susceptible to elimination: norleucine, norvaline, α-aminobutyrate, and alanine. The exchange rates of the α-hydrogen and the total β-hydrogens of l-alanine and l-α-aminobutyrate with deuterium followed first-order kinetics. For l-norvaline, l-norleucine, S-methyl-l-cysteine, and l-methionine, the rate of α-hydrogen exchange followed first-order kinetics, but the rate of total β-hydrogen exchange decreased due to a primary isotope effect at the α-position. One β-hydrogen of S-methyl-l-cysteine was exchanged faster than the other, although both the β-hydrogens were exchanged completely with deuterium ultimately. l-Phenylalanine and l-tryptophan slowly underwent α-hydrogen exchange. The pro-R hydrogen of glycine was deuterated stereospecifically. None of the following amino acids were susceptible to the enzymatic hydrogen exchange: d isomers of the above amino acids, branched chain l-amino acids, acidic l-amino acids, and basic l-amino acids. l-Methionine derivatives such as methionine sulfoxide having a good leaving group at the γ-carbon, which are substrates in α, γ-elimination, were also not susceptible to the hydrogen exchange. In this case, the β-2H,γ-2H species of α-ketobutyrate was exclusively formed. The enzyme catalyzes deamination of l-vinylglycine in 2H20 to give also the same a-ketobutyrate species. These results are consistent with the proposed mechanism that the α, γ-elimination proceeds through a Schiff base of vinylglycine with pyridoxal 5'-phosphate [Davis, L., & Metzler, E. (1972) Enzymes, 3rd Ed. 7, 33].