Peptidase C1A, cathepsin B <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 MEROPS peptidase family C1, sub-family C1A (papain family, clan CA). It includes proteins classed as non-peptidase homologs. These are have either been shown experimentally to lack peptidase activity or lack one or more of the active site residues. This entry icludes cathepsin B and the CPC peptidases (cathepsin B-like) from <taxon tax_id="5664">Leishmania major</taxon>, <taxon tax_id="5665">Leishmania mexicana</taxon> and <taxon tax_id="5693">Trypanosoma cruzi</taxon>. </p><p>Cathepsin B cysteine protease enzymes have been shown to be involved in a variety of different biological processes in eukaryotes. The isolation and characterisation of four distinct cathepsin B-like genes from <taxon tax_id="6239">Caenorhabditis elegans</taxon> has been reported [<cite idref="PUB00034822"/>].</p>