Cathepsin K (CtsK) is a cysteine protease with potent collagenase activity. CtsK is highly expressed by bone-resorbing osteoclasts and plays an essential role in resorption of bone matrix. Although CtsK is known to bind heparan sulfate (HS), the structural details of the interaction, and how HS regulates the biological functions of CtsK, remains largely unknown. In this report, we discovered that HS is a multifaceted regulator of the structure and function of CtsK. Structurally, HS forms a highly stable complex with CtsK and induces its dimerization. Co-crystal structures of CtsK with bound HS oligosaccharides reveal the location of the HS binding site and suggest how HS may support dimerization. Functionally, HS plays a dual role in regulating the enzymatic activity of CtsK. While it preserves the peptidase activity of CtsK by stabilizing its active conformation, it inhibits the collagenase activity of CtsK in a sulfation level-dependent manner. These opposing effects can be explained by our finding that the HS binding site is remote from the active site, which allows HS to specifically inhibit the collagenase activity without affecting the peptidase activity. At last, we show that structurally defined HS oligosaccharides effectively block osteoclast resorption of bone without inhibiting osteoclast differentiation, which suggests that HS-based oligosaccharide might be explored as a new class of selective CtsK inhibitor for many diseases involving exaggerated bone resorption.

中文翻译：

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硫酸乙酰肝素选择性抑制组织蛋白酶 K 的胶原酶活性

组织蛋白酶 K (CtsK) 是一种具有强大胶原酶活性的半胱氨酸蛋白酶。 CtsK 在骨吸收破骨细胞中高表达，在骨基质吸收中发挥重要作用。尽管已知 CtsK 可以结合硫酸乙酰肝素 (HS)，但相互作用的结构细节以及 HS 如何调节 CtsK 的生物学功能仍然很大程度上未知。在本报告中，我们发现 HS 是 CtsK 结构和功能的多方面调节剂。在结构上，HS 与 CtsK 形成高度稳定的复合物并诱导其二聚化。 CtsK 与结合的 HS 寡糖的共晶结构揭示了 HS 结合位点的位置，并表明 HS 如何支持二聚化。从功能上来说，HS 在调节 CtsK 酶活性方面发挥双重作用。虽然它通过稳定其活性构象来保留 CtsK 的肽酶活性，但它以硫酸化水平依赖性方式抑制 CtsK 的胶原酶活性。这些相反的作用可以通过我们的发现来解释，HS 结合位点远离活性位点，这使得 HS 能够特异性抑制胶原酶活性而不影响肽酶活性。最后，我们发现结构明确的 HS 寡糖可有效阻止破骨细胞骨吸收，而不抑制破骨细胞分化，这表明基于 HS 的寡糖可能被探索作为一类新型选择性 CtsK 抑制剂，用于治疗许多涉及骨吸收过度的疾病。