Methionine γ-lyase: Mechanistic deductions from the kinetic pH-effects The role of the ionic state of a substrate in the enzymatic activity

We have studied and compared the pH-dependencies of the main kinetic parameters for the alpha,gamma-elimination reactions of methionine gamma-lyase (MGL) of Citrobacter inter-medius with natural substrate, L-methionine, with its phosphinic analogue, and for alpha,beta-elimination reaction with S-methyl-L-cysteine. From the pH-dependency of k(cat)/K-m for the reaction with L-methionine we have concluded that MGL is selective with respect to the zwitterionic form of its natural substrate. For the reaction of MGL with 1-amino-3-methylthiopropylphosphinic acid the pK(a), of the substrate's amino group, equal to 7.55, is not reflected in the pH-profile of k(cat)/K-m. Consequently, the enzyme does not manifest well-defined selectivity with respect to the zwitterion and anion ionic forms of the substrate. The ascending limbs of pH-dependencies of k(cat)/K-m for reactions with L-methionine and S-methyl-L-Cysteine are controlled by a single pK(a) equal to 7.1-7.2, while for the reaction with 1-amino-3-methylthiopropylphosphinic acid two equal pK(a)s of 6.2 were found in the respective pH-profile. The descending limbs of pH-dependencies of k(cat)/K-m for the reactions with S-methyl-L-cysteine and racemic 1-amino-3-methylthiopropylphosphinic acid are very similar and are controlled by two acidic groups having average pK(a), values of 8.7. On the basis of these results we suggest a mechanism of catalytic action of MGL According to this mechanism Tyr 113, in its conjugated base form, acts as an acceptor of the proton from the amino group of the substrate upon its binding in the active site. Elimination of the leaving thiol groups during both alpha,gamma- and alpha,beta-elimination reactions is assisted by the acidic groups of Tyr 113 and Tyr 58. Both tyrosyl residues are able to fulfill this catalytic function with different efficiencies depending on the type of elimination reaction. Tyr 113 residue plays the determining role in the alpha,gamma-elimination, and Tyr 58 - in the alpha,beta-elimination process. (C) 2009 Elsevier B.V. All rights reserved.