Structural insights into the enzyme specificity of a novel omega-transaminase from the thermophilic bacterium Sphaerobacter thermophilus

Abstract

Transaminases are pyridoxal 5'-phosphate-dependent enzymes that reversibly catalyze transamination reactions from an amino group donor substrate to an amino group acceptor substrate. omega-Transaminases (omega TAs) utilize compounds with an amino group not at alpha-carbon position as their amino group donor substrates. Recently, a novel omega TA with broad substrate specificity and high thermostability from the thermophilic bacterium Sphaerobacter thermophilus (St-omega TA) has been reported. Although St-omega TA has been biochemically characterized, little is known about its determinants of substrate specificity. In the present study, we determined the crystal structure of St-omega TA at 1.9 angstrom resolution to clarify in detail its mechanism of substrate recognition. The structure of St-omega TA revealed that it has a voluminous active site resulting from the unique spatial arrangement of residues comprising its active site. In addition, our molecular docking simulation results suggest that substrate compounds may bind to active site residues via electrostatic interactions or hydrophobic interactions that can be induced by subtle rearrangements of active site residues. On the basis of these structural analyses, we propose a plausible working model of the enzymatic mechanism of St-omega TA. Our results provide profound structural insights into the substrate specificity of St-omega TA and extend the boundaries of knowledge of TAs.