Peptidase M16, N-terminal <p>In the MEROPS database peptidases and peptidase homologues are grouped into clans and families. Clans are groups of families for which there is evidence of common ancestry based on a common structural fold:</p><ul> <li>Each clan is identified with two letters, the first representing the catalytic type of the families included in the clan (with the letter 'P' being used for a clan containing families of more than one of the catalytic types serine, threonine and cysteine). Some families cannot yet be assigned to clans, and when a formal assignment is required, such a family is described as belonging to clan A-, C-, M-, N-, S-, T- or U-, according to the catalytic type. Some clans are divided into subclans because there is evidence of a very ancient divergence within the clan, for example MA(E), the gluzincins, and MA(M), the metzincins.</li><li>Peptidase families are grouped by their catalytic type, the first character representing the catalytic type: A, aspartic; C, cysteine; G, glutamic acid; M, metallo; N, asparagine; S, serine; T, threonine; and U, unknown. The serine, threonine and cysteine peptidases utilise the amino acid as a nucleophile and form an acyl intermediate - these peptidases can also readily act as transferases. In the case of aspartic, glutamic and metallopeptidases, the nucleophile is an activated water molecule. In the case of the asparagine endopeptidases, the nucleophile is asparagine and all are self-processing endopeptidases. </li></ul><p>In many instances the structural protein fold that characterises the clan or family may have lost its catalytic activity, yet retain its function in protein recognition and binding. </p><p>Metalloproteases are the most diverse of the four main types of protease, with more than 50 families identified to date. In these enzymes, a divalent cation, usually zinc, activates the water molecule. The metal ion is held in place by amino acid ligands, usually three in number. The known metal ligands are His, Glu, Asp or Lys and at least one other residue is required for catalysis, which may play an electrophillic role. Of the known metalloproteases, around half contain an HEXXH motif, which has been shown in crystallographic studies to form part of the metal-binding site [<cite idref="PUB00003579"/>]. The HEXXH motif is relatively common, but can be more stringently defined for metalloproteases as 'abXHEbbHbc', where 'a' is most often valine or threonine and forms part of the S1' subsite in thermolysin and neprilysin, 'b' is an uncharged residue, and 'c' a hydrophobic residue. Proline is never found in this site, possibly because it would break the helical structure adopted by this motif in metalloproteases [<cite idref="PUB00003579"/>].</p><p>The majority of the sequences in this entry are metallopeptidases and non-peptidase homologs belong to MEROPS peptidase family M16 (clan ME), subfamilies M16A, M16B and M16C; they include:</p><ul><li>Insulinase, insulin-degrading enzyme (<db_xref db="EC" dbkey="3.4.24.56"/>)</li><li>Mitochondrial processing peptidase alpha subunit, (Alpha-MPP, <db_xref db="EC" dbkey="3.4.24.64"/>)</li><li>Pitrlysin, Protease III precursor (<db_xref db="EC" dbkey="3.4.24.55"/>)</li><li>Nardilysin, (<db_xref db="EC" dbkey="3.4.24.61"/>)</li><li>Ubiquinol-cytochrome C reductase complex core protein I,mitochondrial precursor (<db_xref db="EC" dbkey="1.10.2.2"/>)</li><li>Coenzyme PQQ synthesis protein F (<db_xref db="EC" dbkey="3.4.99"/>)</li> </ul><p>These proteins do not share many regions of sequence similarity; the most noticeable is in the N-terminal section. This region includes a conserved histidine followed, two residues later by a glutamate and another histidine. In pitrilysin, it has been shown [<cite idref="PUB00004194"/>] that this H-x-x-E-H motif is involved in enzymatic activity; the two histidines bind zinc and the glutamate is necessary for catalytic activity.The proteins classified as non-peptidase homologues either have been found experimentally to be without peptidase activity, or lack amino acid residues that are believed to be essential for the catalytic activity. </p>