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The proteins that inhibit peptidases are of great importance in medicine and biotechnology, but there has never been a comprehensive system of classification for them. Some of the terminology currently in use is potentially confusing. In the hope of facilitating the exchange, storage and retrieval of information about this important group of proteins, we now describe a system wherein the inhibitor units of the peptidase inhibitors are assigned to 48 families on the basis of similarities detectable at the level of amino acid sequence. Then, on the basis of three-dimensional structures, 31 of the families are assigned to 26 clans. A simple system of nomenclature is introduced for reference to each clan, family and inhibitor. We briefly discuss the specificities and mechanisms of the interactions of the inhibitors in the various families with their target enzymes. The system of families and clans of inhibitors described has been implemented in the MEROPS peptidase database (http://merops.sanger.ac.uk/), and this will provide a mechanism for updating it as new information becomes available.

Peptidases (often termed proteases) are of great relevance to biology, medicine, and biotechnology. This practical importance creates a need for an integrated source of information about peptidases. In the MEROPSdatabase (www.merops.ac.uk), peptidases are classified by structural similarities in the parts of the molecules responsible for their enzymatic activity. They are grouped into families on the basis of amino acid sequence homology, and the families are assembled into clans in light of evidence that they share common ancestry. The evidence for clan-level relationships usually comes from similarities in tertiary structure, but we suggest that secondary structure profiles may also be useful in the future. The classification forms a framework around which a wealth of supplementary information about the peptidases is organized. This includes images of three-dimensional structures, alignments of matching human and mouse ESTs, comments on biomedical relevance, human and other gene symbols, and literature references linked to PubMed. For each family, there is an amino acid sequence alignment and a dendrogram. There is a list of all peptidases known from each of over 1000 species, together with summary data for the distributions of the families and clans throughout the major groups of organisms. A set of online searches provides access to information about the location of peptidases on human chromosomes and peptidase substrate specificity.

Zinc metallopeptidases that contain the His‐Glu‐Xaa‐Xaa‐His (HEXXH) motif generally have a third ligand of the metal ion that may be either a Glu residue (in clan MA) or a His residue (in clan MB) (Rawlings and Barrett (1995) Methods Enzymol. 248, 183–228). Thimet oligopeptidase has not yet been assigned to either clan, and both Glu and His residues have been proposed as the third ligand. We mutated candidate ligand residues in the recombinant enzyme and identified Glu, His and Asp residues that are important for catalytic activity and/or stability of the protein. However, neither of the Glu and His residues close to the HEXXH motif that have previously been suggested to be ligands is required for the binding of zinc. We conclude that thimet oligopeptidase is not a member of clan MA or clan MB and it is likely that the enzyme possesses a catalytic site and protein fold different from those identified in any metallopeptidase to date. The definitive identification of the third zinc ligand may well require the determination of the crystallographic structure of thimet oligopeptidase or one of its homologues.

The known cysteine peptidases have been classified into 35 sequence families. We argue that these have arisen from at least five separate evolutionary origins, each of which is represented by a set of one or more modern-day families, termed a clan. Clan CA is the largest, containing the papain family, C1, and others with the Cys/His catalytic dyad. Clan CB (His/Cys dyad) contains enzymes from RNA viruses that are distantly related to chymotrypsin. The peptidases of clan CC are also from RNA viruses, but have papain-like Cys/His catalytic sites. Clans CD and CE contain only one family each, those of interleukin-1β-converting enzyme and adenovirus L3 proteinase, respectively. A few families cannot yet be assigned to clans. In view of the number of separate origins of enzymes of this type, one should be cautious in generalising about the catalytic mechanisms and other properties of cysteine peptidases as a whole. In contrast, it may be safer to generalise for enzymes within a single family or clan.

Summary We have examined the amino acid sequences of a number of proteins that have been suggested to be related to chicken cystatin, a protein from chicken egg white that inhibits cysteine proteinases. On the basis of statistical analysis, the following proteins were found to be members of the cystatin superfamily: human cystatin A, rat cystatin A(α), human cystatin B, rat cystatin B(β), rice cystatin, human cystatin C, ox colostrum cystatin, human cystatin S, human cystatin SA, human cystatin SN, chicken cystatin, puff adder cystatin, human kininogen, ox kininogen, rat kininogen, rat T-kininogens 1 and 2, human α2HS-glycoprotein, and human histidine-rich glycoprotein. Fibronectin is shown not to be a member of this superfamily, and the c-Ha-ras oncogene protein p21(Val-12) probably is not a member also. It was convenient to divide members of the superfamily into four types on the basis of the presence of one, two, or three copies of cystatin-like segments and the presence or absence of disulfide bonds. Evolutionary dendrograms were calculated by three methods, and from these we have constructed a scheme depicting the sequence of events in the evolution of these proteins. We suggest that about 1000 million years ago a precursor containing disulfide loops appeared, and that all disulfide-containing cystatins are derived from this. We follow the evolution of the proteins of the superfamily along four main lineages, with special attention to the part that duplication of segments has played in the development of the more complex molecules.

The amino acid sequence of papaya proteinase IV (PPIV), a major proteinase from the latex of Carica papaya[(1989) Biochem. J. 261, 469-476] is described. The enzyme has a high degree of sequence identity with papaya proteinase III, chymopapain and papain (81, 70 and 67%, respectively), and is clearly a member of the papain superfamily of cysteine proteinases. Nevertheless, the sequence shows substitution of certain residues conserved in all other known members of the superfamily. It is suggested that some of these substitutions may account for the unusual specificity of PPIV.

The amino acid sequence of stem bromelain, the major cysteine proteinase from pineapple stem is described. It shows that the enzyme is a member of the papain superfamily of cysteine proteinases, but is not very closely related to any other known member of this group. The sequence shows mutation or deletion of several residues that have been conserved in cysteine proteinases examined previously, including Asn-175 (papain). We suggest that some of these changes have the effect of altering the active-site geometry of stem bromelain, and that this accounts for the resistance of the enzyme to inhibition by cystatins and E-64 [L-3-carboxy-2,3- trans-epoxypropionylleucylamido(4-guanidino)butane].

We show by site‐directed mutagenesis that the catalytic residues of mammalian legumain, a recently discovered lysosomal asparaginycysteine endopeptidase, form a catalytic dyad in the motif His‐Gly‐spacer‐Ala‐Cys. We note that the same motif is present in the caspases, aspartate‐specific endopeptidases central to the process of apoptosis in animal cells, and also in the families of clostripain and gingipain which are arginyl/lysyl endopeptidases of pathogenic bacteria. We propose that the four families have similar protein folds, are evolutionarily related in clan CD, and have common characteristics including substrate specificities dominated by the interactions of the S1 subsite.

Legumain is a cysteine endopeptidase that shows strict specificity for hydrolysis of asparaginyl bonds. The enzyme belongs to peptidase family C13, and is thus unrelated to the better known cysteine peptidases of the papain family, C1 (Rawlings, N. D., and Barrett, A. J. (1994) Methods Enzymol.244, 461-486). To date, legumain has been described only from plants and a blood fluke, Schistosoma mansoniWe now show that legumain is present in mammals. We have cloned and sequenced human legumain and part of pig legumain. We have also purified legumain to homogeneity (2200-fold, 8% yield) from pig kidney. The mammalian sequences are clearly homologous with legumains from non-mammalian species. Pig legumain is a glycoprotein of about 34 kDa, decreasing to 31 kDa on deglycosylation. It is an asparaginyl endopeptidase, hydrolyzing Z-Ala-Ala-Asn-7-(4-methyl)coumarylamide and benzoyl-Asn-p-nitroanilide. Maximal activity is seen at pH 5.8 under normal assay conditions, and the enzyme is irreversibly denatured at pH 7 and above. Mammalian legumain is a cysteine endopeptidase, inhibited by iodoacetamide and maleimides, but unaffected by compound E64 (trans-epoxysuccinyl-L-leucylamido-(4-guanidino)butane). It is inhibited by ovocystatin (cystatin from chicken egg white) and human cystatin C with Kivalues < 5 nM. We discuss the significance of the discovery of a cysteine endopeptidase of a new family and distinctive specificity in man and other mammals.

MEROPS is a database of proteolytic enzymes as well as their inhibitors and substrates. Proteolytic enzymes and protein inhibitors are organized into protein domain families. In turn, families are organized into clans. Each peptidase, inhibitor, family, and clan has associated annotation, a multiple sequence alignment, a phylogenetic tree, literature references, and links to other databases. Interactions between proteolytic enzymes and inhibitors and between proteolytic enzymes and substrates are also presented. The entries in MEROPS are available via the World Wide Web. This unit contains detailed information on how to access and utilize the information present in the MEROPS database. Details on running MEROPS both remotely and locally are presented. © 2014 by John Wiley & Sons, Inc.