Role of F225 in O-phosphoserine sulfhydrylase from Aeropyrum pernix K1.

O-Phosphoserine sulfhydrylase (OPSS) synthesizes cysteine from O-phospho-L-serine (OPS) and sulfide. We have determined the three-dimensional structures of OPSS from hyperthermophilic archaeon Aeropyrum pernix K1 (ApOPSS) in complex with aminoacrylate intermediate (AA) formed from pyridoxal 5'-phosphate with OPS or in complex with cysteine and compared them with that of ApOPSS. We found an orientational change of F225 at the active-site entrance and constructed an F225A mutant to examine its activities and AA stability and clarify the role of F225 in ApOPSS. The OPS and O-acetyl-L-serine (OAS) sulfhydrylase activities of the F225A mutant decreased by 4.2- and 15-fold compared to those of the wild-type (wt) ApOPSS, respectively. The ability of OPS and OAS to form AA also decreased by 12- and 27-fold, respectively. AA was less stable in the F225A mutant than in the wt ApOPSS. Simulated docking showed that leaving groups, such as phosphate and acetate, were oriented to the inside of the active site in the F225A mutant, whereas they were oriented to the entrance in the wt ApOPSS. These results suggest that F225 in ApOPSS plays important roles in maintaining the hydrophobic environment of AA from solvent water and in controlling the orientation of leaving groups.