Structural basis of substrate recognition by a novel thermostable (S)-enantioselective ω-transaminase from Thermomicrobium roseum

Transaminases catalyze the reversible transfer reaction of an amino group between a primary amine and an alpha-keto acid, utilizing pyridoxal 5'-phosphate as a cofactor. omega-transaminases (omega TAs) recognize an amino group linked to a non-alpha carbon of amine substrates. Recently, a novel (S)-enantioselective omega TA from Thermomicrobium roseum (Tr-omega TA) was identified and its enzymatic activity reported. However, the detailed mechanism of (S)-enantioselective substrate recognition remained unclear. In this study, we determined the crystal structure of Tr-omega TA at 1.8 angstrom resolution to elucidate the mechanism underlying Tr-omega TA substrate (S)-enantioselectivity. A structural analysis of Tr-omega TA along with molecular docking simulations revealed that two pockets at the active site tightly restrict the size and orientation of functional groups of substrate candidates. Based on the structural information and docking simulation results, we propose a comprehensive catalytic mechanism of Tr-omega TA. The present study thus provides structural and functional insights into the (S)-enantioselectivity of Tr-omega TA.