STRUCTURE AND MECHANISM OF A PROLINE-SPECIFIC AMINOPEPTIDASE FROM ESCHERICHIA-COLI

The structure of the praline-specific amino-peptidase (EC 3.4.11.9) fr om Escherichia coli has been solved and refined for crystals of the na tive enzyme at a 2.0-Angstrom resolution, for a dipeptide inhibited co mplex at 2.3-Angstrom resolution, and for a low-pH inactive form at 2. 7-Angstrom resolution, The protein crystallizes as a tetramer, more co rrectly a dimer of dimers, at both high and low pH, consistent with ob servations from analytical ultracentrifuge studies that show that the protein is a tetramer under physiological conditions, The monomer fold s into two domains, The active site, in the larger C-terminal domain, contains a dinuclear manganese center in which a bridging water molecu le or hydroxide ion appears poised to act as the nucleophile in the at tack on the scissile peptide bond of Xaa-Pro, The metal-binding residu es are located in a single subunit, but the residues surrounding the a ctive site are contributed by three subunits, The fold of the protein resembles that of creatine amidinohydrolase (creatinase, not a metallo enzyme). The C-terminal catalytic domain is also similar to the single -domain enzyme methionine aminopeptidase that has a dinuclear cobalt c enter.