CRYSTAL STRUCTURE OF THE NOVEL CYSTEINE PROTEASE, CATHEPSIN K, IN COMPLEX WITH THE INHIBITOR E-64. Baoguang Zhao, Cheryl A. Janson, Ward W. Smith, Mike McQueney, Christopher Jones, Sherin S. Abdel-Meguid, Departments of Macromolecular Sciences and Protein Biochemistry, SmithKline Beecham Pharmaceuticals, 709 Swedeland Rd, King of Prussia, PA 19406 USA

Cathepsin K is a recently discovered human cysteine protease with significant sequence homology to cathepsin S and cathepsin L. Cathepsin K is abundantly and selectively expressed in osteoclasts, the cells responsible for bone degradation. This observation has led to the suggestion that this enzyme plays an important role in bone resorption. The design of potent, selective inhibitors of cathepsin K should represent a new approach to the prevention of excessive bone loss in diseases such as osteoporosis.

We have determined the three dimensional structure of human cathepsin K in complex with the cysteine protease inhibitor E-64 at 2.2 angstroms resolution. The complex crystallizes in space group P2₁2₁2₁ with unit cell dimensions a = 38.4, b = 50.7, c = 104.9Å. The structure was solved using molecular replacement with the coordinates of papain as the basis for a search model. The resulting electron density confirms that cathepsin K has the identical secondary structure and the same overall fold as papain. The position and conformation of the E-64 inhibitor are clearly evident. We will describe the structure of cathepsin K including the active site of the enzyme and the interactions with the inhibitor and compare this structure with other known cysteine proteases. Knowledge of the structure of cathepsin K will be useful in the structure-based design of inhibitors of the enzyme.