Cathepsin B group; composed of cathepsin B and similar proteins, including tubulointerstitial nephritis antigen (TIN-Ag). Cathepsin B is a lysosomal papain-like cysteine peptidase which is expressed in all tissues and functions primarily as an exopeptidase through its carboxydipeptidyl activity. Together with other cathepsins, it is involved in the degradation of proteins, proenzyme activation, Ag processing, metabolism and apoptosis. Cathepsin B has been implicated in a number of human diseases such as cancer, rheumatoid arthritis, osteoporosis and Alzheimer's disease. The unique carboxydipeptidyl activity of cathepsin B is attributed to the presence of an occluding loop in its active site which favors the binding of the C-termini of substrate proteins. Some members of this group do not possess the occluding loop. TIN-Ag is an extracellular matrix basement protein which was originally identified as a target Ag involved in anti-tubular basement membrane antibody-mediated interstitial nephritis. It plays a role in renal tubulogenesis and is defective in hereditary tubulointerstitial disorders. TIN-Ag is exclusively expressed in kidney tissues.
Comment:The catalytic residues of C1 family peptidases are Cys and His, forming a catalytic dyad. Two other residues play an important role in catalysis: a Gln preceding the catalytic Cys, believed to help in the formation of the oxyanion hole; and an Asn residue which orients the imidazolium ring of the catalytic His.
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