CTSK

CTSK Background

Cathepsin K (CTSK), a cysteine endopeptidase with a preference for non-aromatic hydrophobic residues such as Leu, Ile, Met, and Val, in its S2 binding pocket ^[1][2]. The enzyme is produced as a single-polypeptide- chain precursor pro-cathepsin K and is subsequently secreted as an inactive form. Mature pro-cathepsin K is activated upon the removal of the propeptide from the N-terminus through proteolysis under the acidic environment of the lysosomes ^[3]. Active CTSK is released into bone resorption lacunae. Consistently, CTSK is expressed predominantly in osteoclasts and plays a key role in bone remodeling and resorption ^[4][10]. Mutations in CTSK are associated with pycnodysostosis, a hereditary bone disorder in which osteoclast function in bone resorption is defective ^[5]. Hence, CTSK is regarded as a drug target for osteoporosis ^[11]. A multitude of novel inhibitors displaying high potency and specificity were developed and synthesized. The pH optimum of CTSK for synthetic substrates is between pH 5.5 and 6.5. CTSK can hydrolyze connective tissue proteins such as collagen and elastin. Chapman et al. identified that the elastolytic activity of CTSK exceeds that of pancreatic elastase at pH 5.5 ^[6]. Besides, CTSK can catalyze the hydrolysis of type I, II, and IV collagens ^[7]. Unlike other collagenolytic cathepsins, CTSK cleaves collagen in the telopeptides and at multiple sites within the native triple helix, generating fragments of various sizes ^[8]. The regulation of CTSK expression is controlled by RANKL (receptor activator of NF-κB ligand) ^[9]. CTSK is also reported to participate in lysosomal rupture-induced apoptosis.

[1] McGrath, M.E., et al. Crystal structure of human cathepsin K complexed with a potent inhibitor [J]. Nature Struct. Biol. 1997, 4: 106-109.
[2] Zhao, B.G., et al. Crystal structure of human osteoclast cathepsin K complex with E64 [J]. Nature Struct. Biol. 1997, 4: 109-111.
[3] Michael S. McQueney, Bernard Y. Amegadzie, et al. Autocatalytic Activation of Human Cathepsin K [J]. J Biol Chem 1997 272: 13955.
[4] Drake F.H., Dodds R.A., et al. Cathepsin K, but not cathepsins B, L, or S, is abundantly expressed in human osteoclasts [J]. J. Biol. Chem. 1996;271:12511-12516.
[5] Gelb, B.D., et al., 1996, Pycnodysostosis is a lysosomal disease caused by cathepsin K deficiency. Science 273: 1236-1238.
[6] Chapman HA, Riese RJ, et al. Emerging roles for cysteine proteases in human biology [J]. Annu Rev Physiol. 1997;59:63-88.
[7] Kafienah, W., et al. Human cathepsin K cleaves native type I and II collagens at the N-terminal end of the triple helix. Biochem. [J] J. 1998, 331: 727-732.
[8] P Garnero, O Borel, et al. The Collagenolytic Activity of Cathepsin K Is Unique Among Mammalian Proteinases [J]. J Biol Chem, 1998, 273 (48), 32347-52.
[9] Troen, B.R. The regulation of cathepsin K gen.e expression [J]. Ann. N. Y. Acad. Sci. 2006, 1068, 165-172.
[10] M. Asagiri, H. Takayanagi. The molecular understanding of osteoclast differentiation [J]. Bone, 40 (2007), pp. 251-264.
[11] Smith, W.W., and Abdelmeguid, S.S. Cathepsin K as a target for the treatment of osteoporosis [J]. Expert Opin. Ther. Patents 1999, 9: 683-694.