Structural Determinants of the beta-Selectivity of a Bacterial Aminotransferase

Chiral beta-amino acids occur as constituents of various natural and synthetic compounds with potentially useful bioactivities. The pyridoxal 5'-phosphate (PLP)-dependent S-selective transaminase from Mesorhizobium sp. strain LUK (MesAT) is a fold type I aminotransferase that can be used for the preparation of enantiopure beta-Phe and derivatives thereof. Using x-ray crystallography, we solved structures of MesAT in complex with (S)-beta-Phe, (R)-3-amino-5-methylhexanoic acid, 2-oxoglutarate, and the inhibitor 2-aminooxyacetic acid, which allowed us to unveil the molecular basis of the amino acid specificity and enantioselectivity of this enzyme. The binding pocket of the side chain of a beta-amino acid is located on the 3'-oxygen side of the PLP cofactor. The same binding pocket is utilized by MesAT to bind the alpha-carboxylate group of an alpha-amino acid. A beta-amino acid thus binds in a reverse orientation in the active site of MesAT compared with an alpha-amino acid. Such a binding mode has not been reported before for any PLP-dependent aminotransferase and shows that the active site of MesAT has specifically evolved to accommodate both beta- and alpha-amino acids.