<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>This group of cysteine peptidases correspond to MEROPS peptidase family C30 (clan PA(C)). These peptidases are related to serine endopeptidases of family S1 and are restricted to RNA viruses, where they are involved in viral polyprotein processing during replication [<cite idref="PUB00011621"/>, <cite idref="PUB00011622"/>, <cite idref="PUB00011623"/>].<p>This entry represents the CoV M-pro which comprises three domains. Domains I and II are six-stranded antiparallel beta barrels and together resemble the architecture of chymotrypsin and of picornaviruses 3C proteinases. The substrate-binding site is located in a cleft between these two domains. The catalytic site is situated at the centre of the cleft. A long loop connects domain II to the C-terminal domain (domain III). This latter domain, a globular cluster of five helices, has been implicated in the proteolytic activity of M-pro. In the active site of M-pro, Cys and His form a catalytic dyad. In contrast to serine proteinases and other cysteine proteinases, which have a catalytic triad, there is no third catalytic residue present [<cite idref="PUB00011621"/>, <cite idref="PUB00022402"/>, <cite idref="PUB00048878"/>, <cite idref="PUB00051210"/>].</p>