Peptidases are grouped into clans and families. Clans are groups of families for which there is evidence of common ancestry. 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-, 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.

Families are grouped by their catalytic type, the first character representing the catalytic type: A, aspartic; C, cysteine; G, glutamic acid; M, metallo; S, serine; T, threonine; and U, unknown. The serine, threonine and cysteine peptidases utilise the catalytic part of an 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.

Peptidases are grouped into clans and families. Clans are groups of families for which there is evidence of common ancestry. Families are grouped by their catalytic type, the first character representing the catalytic type: A, aspartic; C, cysteine; G, glutamic acid; M, metallo; S, serine; T, threonine; and U, unknown. A clan that contains families of more than one type is described as being of type P. The serine, threonine and cysteine peptidases utilise the catalytic part of an 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.

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 [PMID:7674922]. 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 [PMID:7674922].

Metalloproteases are the most diverse of the four main types of protease, with more than 30 families identified to date [PMID:7674922]. 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 [PMID:7674922]. 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 [PMID:7674922].

This group of metallopeptidases belong to the MEROPS peptidase family M1 (clan MA(E)), the type example being aminopeptidase N from Homo sapiens. The protein fold of the peptidase domain for members of this family resembles that of thermolysin, the type example for clan MA.

Membrane alanine aminopeptidase (EC: 3.4.11.2) is part of the HEXXH⁺E group; it consists entirely of aminopeptidases, spread across a wide variety of species [PMID:7674922]. 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-A4 to form an inflammatory mediator [PMID:7674922]. This hydrolase has been shown to have aminopeptidase activity [PMID:2244921], and the zinc ligands of the M1 family were identified by site-directed mutagenesis on this enzyme [PMID:7674922] CD13 participates in trimming peptides bound to MHC class II molecules [PMID:8691132] and cleaves MIP-1 chemokine, which alters target cell specificity from basophils to eosinophils [PMID:8627182]. CD13 acts as a receptor for specific strains of RNA viruses (coronaviruses) which cause a relatively large percentage of upper respiratory trace infections.

CD molecules are leucocyte antigens on cell surfaces. CD antigens nomenclature is updated at Protein Reviews On The Web