STRUCTURE OF THE COBALT-DEPENDENT METHIONINE AMINOPEPTIDASE FROM ESCHERICHIA-COLI - A NEW TYPE OF PROTEOLYTIC-ENZYME

The X-ray structure of Escherichia coli methionine aminopeptidase (MAP ) has been determined to 2.4-angstrom resolution and refined to a crys tallographic R-factor of 18.2%. The fold is novel and displays interna l pseudo-2-fold symmetry which structurally relates the first and seco nd halves of the polypeptide chain. The topology consists of a central antiparallel beta-sheet covered on one side by two pairs of alpha-hel ices and by a C-terminal loop. The other face of the beta-sheet, toget her with some irregular loops, forms the active site, which contains t wo cobalt ions 2.9 angstrom apart. These metal ions are liganded by th e side chains of Asp 97, Asp 108, Glu 204, Glu 235, and His 171 with a pproximate octahedral coordination. In terms of both the novel backbon e fold and the constitution of the active site, MAP appears to represe nt a new class of proteolytic enzyme.