Quantum Chemical Study of Dual-Substrate Recognition in ω-Transaminase

摘要

ω-Transaminases are attractive biocatalysts for the production of chiral amines. These enzymes usually have a broad substrate range. Their substrates include hydrophobic amines as well as amino acids, a feature referred to as dual-substrate recognition. In the present study, the reaction mechanism for the half-transamination of l-alanine to pyruvate in (S )-selective Chromobacterium violaceum ω-transaminase is investigated using density functional theory calculations. The role of a flexible arginine residue, Arg416, in the dual-substrate recognition is investigated by employing two active-site models, one including this residue and one lacking it. The results of this study are compared to those of the mechanism of the conversion of (S )-1-phenylethylamine to acetophenone. The calculations suggest that the deaminations of amino acids and hydrophobic amines follow essentially the same mechanism, but the energetics of the reactions differ significantly. It is shown that the amine is kinetically favored in the half-transamination of l-alanine/pyruvate, whereas the ketone is kinetically favored in the half-transamination of (S )-1-phenylethylamine/acetophenone. The calculations further support the proposal that the arginine residue facilitates the dual-substrate recognition by functioning as an arginine switch, where the side chain is positioned inside or outside of the active site depending on the substrate. Arg416 participates in the binding of l-alanine by forming a salt bridge to the carboxylate moiety, whereas the conversion of (S )-1-phenylethylamine is feasible in the absence of Arg416, which here represents the case in which the side chain of Arg416 is positioned outside of the active site.