A PROTEIN CATALYTIC FRAMEWORK WITH AN N-TERMINAL NUCLEOPHILE IS CAPABLE OF SELF-ACTIVATION

THE crystal structures of three amidohydrolases have been determined r ecently(1-3): glutamine PRPP amidotransferase (GAT), penicillin acylas e, and the proteasome. These enzymes use the side chain of the amino-t erminal residue, incorporated in a beta-sheet, as the nucleophile in t he catalytic attack at the carbonyl carbon. The nucleophile is cystein e in GAT, serine in penicillin acylase, and threonine in the proteasom e. Here we show that all three enzymes share an unusual fold in which the nucleophile and other catalytic groups occupy equivalent sites. Th is fold provides both the capacity for nucleophilic attack and the pos sibility of autocatalytic processing. We suggest the name Ntn (N-termi nal nucleophile) hydrolases for this structural superfamily of enzymes which appear to be evolutionarily related but which have diverged bey ond any recognizable sequence similarity.