Name | Peptidase family C13 (legumain family) |
Family type peptidase | C13.001 - legumain (plant beta form) (Canavalia ensiformis), MEROPS Accession MER0000845 (peptidase unit: 36-312) |
Content of family | Peptidase family C13 contains asparaginyl endopeptidases and glycosylphosphatidylinositol:protein transamidase. |
History |
Identifier created: Biochem.J. 290:205-218 (1993) An asparginyl endopeptidase was first discovered in beans, and the sequencing of homologues from the blood fluke Schistosoma mansoni led to the discovery of expressed sequence tags from humans and the completion of the first mammalian sequence (Chen et al., 1997). Mammalian legumain (C13.004) was localized to the lysosome, an organelle already known to be rich in peptidases. A second mammalian enzyme, glycosylphosphatidylinositol:protein transamidase (C13.005), has been discovered. |
Catalytic type | Cysteine |
Active site residues | H158 C200 |
Active site | The active site residues are in the order His, Cys in the protein sequence. The histidine occurs within the motif Xaa-Xaa-Xaa-Xbb-Xbb-His-Gly-Xbb, in which Xaa is a hydrophobic amino acid and Xbb is a small amino acid (Ala, Asp, Glu, Gly, Ser or Thr). The cysteine occurs in a similar motif, Xaa-Xaa-Xaa-Xbb-Xbb-Cys. |
Activities and specificities | Most members of the family are endopeptidases with a restricted specificity for asparaginyl bonds, although only a minority of such bonds are cleaved in proteins (Dando et al., 1999). One of the best synthetic substrates for mammalian legumain is Z-Ala-Ala-Asn-NHMec. Maximal activity is seen at pH 4-6; at higher pH legumain is denatured. At pH 4.0, cleavage at aspartyl bonds has been observed. The strict specificity for the P1 residue of the substrate is a characteristic shown also by other peptidases in clan CD. |
Inhibitors | Peptidases in family C13 are inhibited by thiol-blocking reagents such as iodoacetamide and N-ethylmaleimide, but are unaffected by E-64 and leupeptin, potent inhibitors of peptidases in family C1. Members of family C13 react more rapidly with iodoacetamide than with iodoacetate, the reverse of what is found for peptidases in family C1. Aza-peptide epoxides are also potent inhibitors of legumains (James et al., 2003). Like many peptidases in family C1, legumain is inhibited by ovocystatin (I25.001), but by a different reactive site (Alvarez-Fernandez et al., 1999). |
Molecular structure | In keeping with its lysosomal location, mammalian legumain is glycosylated. Some plant legumains also are glycosylated, although not that of castor bean. No tertiary structure has been reported, but similarities in sequence and predicted secondary structure around the active site residues lead Chen et al., (1998) to assign family C13 to clan CD, and to suggest that the fold is similar to that of the caspases in family C14. |
Clan | CD |
Basis of clan assignment | Active site residues for members of this family and family C14 occur in the same order in the sequence: H, C. |