The X-ray crystal structures of two zmc endopeptldases. astacin from crayfish. and adamalysm II from snake venom, reveal a strong overall lopologlcal equivalence and virtually Identical extended HEXXHXXGXXH zinc-binding segments, but m addition a methlomne-containing turn of similar conformatlon (the 'Met-turn'). which forms a hydrophobic basis for the zmc ion and the three ligandmg hlstidine residues. These two features are also present in a similar arrangement m the matrix metalloproteinases (matrixins) and in the large bacterial Serratra proteinase-like peptidases (serralysins).We suggest that these four proteinases represent members of distinct subfamilies which can be grouped together in a family, for which we propose the designation, metzmcins.Astacm; Snake venom metalloproteinase; Matrix metalloprotemase; Thermolysm; Zmc endopeptidase family: Protein crystal structure