Identification of amino acid residues important for recognition of O-phospho-l-serine substrates by cysteine synthase.

Pyridoxal-5'-phosphate-dependent cysteine synthases synthesize l-cysteine from their primary substrates, O-acetyl-l-serine (OAS) and O-phospho-l-serine (OPS), and their secondary substrate, sulfide. The mechanism by which cysteine synthases recognize OPS remains unclear; hence, we investigated the OPS recognition mechanism of the OPS sulfhydrylase obtained from Aeropyrum pernix K1 (ApOPSS) and the OAS sulfhydrylase-B obtained from Escherichia coli (EcOASS-B), using protein engineering methods. From the amino acid sequence alignment data, we found that some OPS sulfhydrylases (OPSSs) had a Tyr corresponding to the Phe225 and Phe141 residues in ApOPSS and EcOASS-B, respectively, and that the Tyr residue could facilitate OPS recognition.