Crystal structure of serine dehydrogenase from Escherichia coli: important role of the C-terminal region for closed-complex formation.

Serine dehydrogenase from Escherichia coli is a homotetrameric enzyme belonging to the short-chain dehydrogenase/reductase (SDR) family. This enzyme catalyses the NADP(+)-dependent oxidation of serine to 2-aminomalonate semialdehyde. The enzyme shows a stereospecificity for β-(3S)-hydroxy acid as a substrate; however, no stereospecificity was observed at the α-carbon. The structures of the ligand-free SerDH and SerDH-NADP(+)-phosphate complex were determined at 1.9 and 2.7 Å resolutions, respectively. The overall structure, including the catalytic tetrad of Asn106, Ser134, Tyr147 and Lys151, shows obvious relationships with other members of the SDR family. The structure of the substrate-binding loop and that of the C-terminal region were disordered in the ligand-free enzyme, whereas these structures were clearly defined in the SerDH-NADP(+) complex as a closed form. Interestingly, the C-terminal region was protruded from the main body and it formed an anti-parallel β-sheet with another C-terminal region on the subunit that is diagonally opposite to that in the tetramer. It is revealed that the C-terminal region possesses the important roles in substrate binding through the stabilization of the substrate-binding loop in the closed form complex. The roles of the C-terminal region along with those of the residues involved in substrate recognition were studied by site-directed mutagenesis.