Peptidase C1B subfamily (MEROPS database nomenclature); composed of eukaryotic bleomycin hydrolases (BH) and bacterial aminopeptidases C (pepC). The proteins of this subfamily contain a large insert relative to the C1A peptidase (papain) subfamily. BH is a cysteine peptidase that detoxifies bleomycin by hydrolysis of an amide group. It acts as a carboxypeptidase on its C-terminus to convert itself into an aminopeptidase and peptide ligase. BH is found in all tissues in mammals as well as in many other eukaryotes. Bleomycin, a glycopeptide derived from the fungus Streptomyces verticullus, is an effective anticancer drug due to its ability to induce DNA strand breaks. Human BH is the major cause of tumor cell resistance to bleomycin chemotherapy, and is also genetically linked to Alzheimer's disease. In addition to its peptidase activity, the yeast BH (Gal6) binds DNA and acts as a repressor in the Gal4 regulatory system. BH forms a hexameric ring barrel structure with the active sites imbedded in the central channel. The bacterial homolog of BH, called pepC, is a cysteine aminopeptidase possessing broad specificity. Although its crystal structure has not been solved, biochemical analysis shows that pepC also forms a hexamer.
Comment:The active sites of bleomycin hydrolase are imbedded in a channel, as in the proteasome, within the hexameric ring barrel structure. They are accessible only by substrates through the central channel.
Comment:The substrate specificity of bleomycin hydrolase is modulated by the positioning of its own C-terminus in the active site cleft, not by the features of the substrate.
Jiyao W et al.(2023). "The conserved domain database in 2023.",