Structural features of cephalosporin acylase reveal the basis of autocatalytic activation

Cephalosporin acylase (CA), a member of the N-terminal nucleophile hydrolase family, is activated through two steps of intramolecular autoproteolysis, the first mediated by a serine residue, and the second by a glutamate, which releases the pro-segment and produces an active enzyme. In this study, we have determined the crystal structures of mutants which could affect primary or secondary auto-cleavage and of sequential intermediates of a slow-processing mutant at 2.0-2.5A resolutions. The pro-segments of the mutants undergo dynamic conformational changes during activation and adopt surprisingly different loop conformations from one another. However, the autoproteolytic site was found to form a catalytically competent conformation with a solvent water molecule, which was essentially conserved in the CA mutants.