The YPO3991 gene product from Yersinia pestis is predicted to be an enzyme of the insulinase family of proteases which are involved in degradation of insulin, glucagon and other polypeptides. Insulinases are metallopeptidases belonging to the MEROPS peptidase family M16 (clan ME). They include proteins, which are classified as non-peptidase homologues that have been found either to be without peptidase activity, or lack amino acid residues that are believed to be essential for catalytic activity. Our protein does not have a common HEXXH amino-acid motif found in about half of metalloproteases. The structure shows a putative substrate binding site in a region where the N-terminal part of the protein is placed outside the main body of the molecule and buried in a cavity of a neighbor molecule in the crystal. There are 6 zinc ions in the structure. Zinc 604 possibly represents a metal site necessary for protein activity.