Characterization and application of l-methionine γ-lyase Q349S mutant enzyme with an enhanced activity toward l-homocysteine

l-Methionine γ-lyse (MGL), a pyridoxal 5'-phosphate-dependent enzyme, catalyzes the α,γ-elimination of l-methionine (l-Met) and l-homocysteine (l-Hcy) to produce α-keto acids, thiols, and ammonia. Previously, various mutant enzymes of Pseudomonas putida MGL (PpMGL) were prepared to identify a homocysteine (Hcy)-specific enzyme that would assist the diagnosis of homocystinuria. Among the mutat enzymes the Q349S mutant exhibited high degradation activity toward l-Hcy. In the present study, PpMGL Q349S was characterized; the results suggested that it could be applied to determine the amount of l-Hcy. Compared to the wild-type PpMGL, specific activities of the Q349S mutant with l-Hcy and l-Met were 1.5 and 0.7 times, respectively. Additionally, we confirmed that l-Hcy in plasma samples could be accurately detected using the Q349S mutant by preincubating it with cysteine desulfurase (CsdA). Furthermore, we determined the X-ray crystal structure of PpMGL Q349S l-Met or l-Hcy complexes Michaelis complex, germinal diamine, and external aldimine at 2.25-2.40 Å. These 3D structures showed that the interaction partner of the β-hydroxyl group of Thr355 in the wild-type PpMGL was changed to the carboxyl group of the Hcy-PLP external aldimine in the Q349S mutant. The interaction of Ser349 and Arg375 was different between l-Met and l-Hcy recognition, indicating that it was important for the recognition of the carboxyl group of the substrate.

The role of amino acid residues in the active site of L-methionine γ-lyase from Pseudomonas putida

Cys116, Lys240*, and Asp241* (asterisks indicate residues from the second subunit of the active dimer) at the active site of L-methionine γ-lyase of Pseudomonas putida (MGL_Pp) are highly conserved among heterologous MGLs. In a previous study, we found that substitution of Cys116 for His led to a drastic increase in activity toward L-cysteine and a decrease in that toward L-methionine. In this study, we examined some properties of the C116H mutant by kinetic analysis and 3D structural analysis. We assumed that substitution of Cys116 for His broke the original hydrogen-bond network and that this induced a significant effect of Tyr114 as a general acid catalyst, possibly due to the narrow space in the active site. The C116H mutant acquired a novel β-elimination activity and lead a drastic conformation change in the histidine residue at position 116 by binding the substrate, suggesting that this His residue affects the reaction specificity of C116H. Furthermore, we suggest that Lys240* is important for substrate recognition and structural stability and that Asp241* is also involved in substrate specificity in the elimination reaction. Based on this, we suggest that the hydrogen-bond network among Cys116, Lys240*, and Asp241* contributes to substrate specificity that is, to L-methionine recognition at the active site in MGL_Pp.