<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>This group of metallopeptidases belong to the MEROPS peptidase family M1 (clan MA(E)), the type example being aminopeptidase N from <taxon tax_id="9606">Homo sapiens</taxon> (Human). The protein fold of the peptidase domain for members of this family resembles that of thermolysin, the type example for clan MA.</p> <p>Membrane alanine aminopeptidase (<db_xref db="EC" dbkey="3.4.11.2"/>) is part of the HEXXH<sup>+</sup>Egroup; it consists entirely of aminopeptidases, spread across a widevariety of species [<cite idref="PUB00003579"/>]. Functional studies show that CD13/APN catalyzes the removal of single amino acids from the amino terminus of small peptides and probably plays a role in their final digestion; one family member (leukotriene-A4 hydrolase) is known to hydrolyse the epoxide leukotriene-A4to form an inflammatory mediator [<cite idref="PUB00003579"/>]. This hydrolase has been shown tohave aminopeptidase activity [<cite idref="PUB00000193"/>], and the zinc ligands of the M1 familywere identified by site-directed mutagenesis on this enzyme [<cite idref="PUB00003579"/>] CD13 participates in trimming peptides bound to MHC class II molecules [<cite idref="PUB00008035"/>] and cleaves MIP-1 chemokine, which alters target cell specificity from basophils to eosinophils [<cite idref="PUB00008036"/>]. CD13 acts as a receptor for specific strains of RNA viruses (coronaviruses) which cause a relatively large percentage of upper respiratory trace infections.</p><p>CD molecules are leucocyte antigens on cell surfaces. CD antigens nomenclature is updated at Protein Reviews On The Web (http://prow.nci.nih.gov/).</p>