<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>Cysteine peptidases have characteristic molecular topologies, which can be seen not only in their three-dimensional structures, but commonly also in the two-dimensional structures. These are peptidases in which the nucleophile is the sulphydryl group of a cysteine residue. Cysteine proteases are divided into clans (proteins which are evolutionary related), and further sub-divided into families, on the basis of the architecture of their catalytic dyad or triad [<cite idref="PUB00011704"/>]. </p><p>This group of cysteine peptidases belong to the MEROPS peptidase family C19 (ubiquitin-specific protease family, clan CA). Families within the CA clan are loosely termed papain-like as protein fold of the peptidase unit resembles that of papain, the type example for clan CA. Predicted active site residues for members of this family and family C1 occur in the same order in the sequence: N/Q, C, H. The type example is human ubiquitin-specific protease 14.</p><p>Ubiquitin is highly conserved, commonly found conjugated to proteins ineukaryotic cells, where it may act as a marker for rapid degradation, orit may have a chaperone function in protein assembly [<cite idref="PUB00003577"/>]. The ubiquitin is released by cleavage from the bound protein by a protease [<cite idref="PUB00003577"/>]. A number ofdeubiquitinising proteases are known: all are activated by thiol compounds[<cite idref="PUB00003577"/>, <cite idref="PUB00002419"/>], and inhibited by thiol-blocking agents and ubiquitin aldehyde [<cite idref="PUB00003577"/>, <cite idref="PUB00004632"/>], and as such have the properties of cysteine proteases [<cite idref="PUB00003577"/>].</p><p>The deubiquitinsing proteases can be split into 2 size ranges (20-30 kDa, <db_xref db="INTERPRO" dbkey="IPR001578"/>,and 100-200 kDa) [<cite idref="PUB00003577"/>]: this family are the 100-200 kDa peptides which includes the Ubp1 ubiquitin peptidase from yeast. Only one conserved cysteine can be identified, along with two conserved histidines. The spacing between the cysteine and the second histidine is thought to be more representative of the cysteine/histidine spacing of a cysteine protease catalytic dyad [<cite idref="PUB00003577"/>].</p> Peptidase C19, ubiquitin carboxyl-terminal hydrolase 2