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Introduction

Cathepsin B is one of approximately a dozen members of the Cathepsin (Greek kata- "down" and hepsein "boil”) family, a group of proteases widely distributed in animal cells with vital roles in a variety of processes, including intracellular protein turnover, immune response triggered apoptosis, and extracellular bone resorption. Because most of these enzymes are activated in low pH environments, their functions are generally localized to the interior of lysosomes. One notable exception to this rule is Cathepsin K, which is secreted extracellularly by osteoclasts during bone resorption. They share the general function of degrading polypeptides, and are differentiated by their respective structures, catalytic mechanisms, and substrate specificities ^{[1] [2]}. Cathepsin B is coded for by the gene CTSB (homo sapiens) located on chromosome 8 ^[3], and is conserved in chimpanzee, dog, cow, mouse, rat, chicken, zebrafish, fruit fly, and several other organisms ^[4]. Its general function in normal cells is autophagy; the lysosome-mediate catabolism and recycling of dysfunctional cytoplasmic proteins and damaged organelles. Aside from helping to maintain normal cellular metabolism, it plays a major role in apoptosis of stressed cells, as in starvation ^[5]. It has also been implicated in lipopolysaccharide induced apoptosis ^[6], as well asTNF-α mediated apoptosis in hepatocytes ^[7]. Cathepsin B is normally synthesized as an inactive zymogen, and is processed by other proteases in the low pH environment of the lysosome. In certain cases however, activation can be achieved in neutral pH when bound to certain negatively charged molecules, like heparin ^[8]. One important implication of this is that Cathepsin B is capable of extracellular peptidase activity targeting extracellular matrix proteins ^{[9] [10]}. Observation of this type of activity, as well as evidence of over-expression in cancer cells has made Cathepsin B a suspect in studies of factors affecting tumor invasion and cancer metastasis ^{[11] [12]}.

Structure

Mechanism

Medical Implications

References