Structural requirements for the collagenase and elastase activity of cathepsin K and its selective inhibition by an exosite inhibitor

Abstract

Human cathepsin K (CatK) is a major drug target for the treatment of osteoporosis. Although its collagenase activity is unique, CatK also exerts a potent elastolytic activity that is shared with human cathepsins V and S. Other members of the cysteine cathepsin family, which are structurally similar, do not exhibit significant collagen and elastin degrading activities. This raises the question of the presence of specific structural elements, exosites, that are required for these activities. CatK has two exosites that control its collagenolytic and elastolytic activity. Modifications of exosites 1 and 2 block the elastase activity of CatK, whereas only exosite-1 alterations prevent collagenolysis. Neither exosite affects the catalytic activity, protease stability, subsite specificity of CatK or the degradation of other biological substrates by this protease. A low-molecular-mass inhibitor that docks into exosite-1 inhibits the elastase and collagenase activity of CatK without interfering with the degradation of other protein substrates. The identification of CatK exosites opens up the prospect of designing highly potent inhibitors that selectively inhibit the degradation of therapeutically relevant substrates by this multifunctional protease.

Structural basis of collagen fiber degradation by cathepsin K

“…We have recently demonstrated this concept by showing that the binding of dihydrotanshinone at exosite 1 of CatK selectively inhibits the collagenase and elastase activity of CatK (35). Exosite I partially overlaps with the protein dimer interaction site described in this report.…”

Collagen Type I as a Ligand for Receptor-Mediated Signaling

“…Since cathepsin K is the only papain-like cysteine protease capable of cleaving triple helical collagen, it is of significant interest as a pharmaceutical target [90,93]. Structural analysis revealed that for cathepsin K to demonstrate its collagenase activity, a dimer has to form an oligomeric complex with a glycosaminoglycan and dock onto a collagen molecule with its central grove [92,94,95]. The presence of glycosaminoglycans allows access to the triple helix, leading to the cutting of the fibril into smaller sub-fibrils and a simultaneous release of glycosaminoglycans.…”

Cathepsin K osteoporosis trials, pycnodysostosis and mouse deficiency models: Commonalities and differences

“…[50] Furthermore, specific exosites have been identified that are responsible for the selective binding to fibrillar collagens and elastin, for the binding of GAGs, and that act as protein-protein interaction sites in oligomeric complexes or potential allosteric sites. [38,49,51,52] The potent collagenase activity of CatK and its role in osteoclastic bone resorption has been the major reason for defining this protease as an anti-osteoporotic drug target.…”

“…[34,35] Whereas its major role in osteoclasts and fibroblasts has been linked to its potent collagenase and/or elastase activity, little is known about the functional role in other cell types and the nature of physiologically relevant substrates other than the two major extracellular matrix proteins, collagens and elastin. Other substrates identified (mostly by in vitro assays) include the tartrate-resistant acid phosphatase, [36] thyroglobulin, [37,38] osteopontin, osteonectin, fibrinogen, [39] endorphin-precursor, [40] TGF-1ß, [41] IGF, [42] and kinin. [43] More is known about the mechanism of matrix protein degradation by CatK.…”