CRYSTAL-STRUCTURE OF HUMAN CATHEPSIN-S

We have determined the 2.5 Angstrom structure (R-cryst = 20.5%, R-free = 28.5%) of a complex between human cathepsin S and the potent, irrev ersible inhibitor 4-morpholinecarbonyl-Phe-hPhe-vinyl sulfone-phenyl. Noncrystallographic symmetry averaging and other density modification techniques were used to improve electron density maps which were nonop timal due to systematically incomplete data. Methods that reduce the n umber of parameters were implemented for refinement. The refined struc ture shows cathepsin S to be similar to related cysteine proteases suc h as papain and cathepsins K and L. As expected, the covalently-bound inhibitor is attached to the enzyme at Cys 25, and enzyme binding subs ites S3-S1' are occupied by the respective inhibitor substituents. A s omewhat larger S2 pocket than what is found in similar enzymes is cons istent with the broader specificity of cathepsin S at this site, while Lys 61 in the S3 site may offer opportunities for selective inhibitio n of this enzyme. The presence of Arg 137 in the S1' pocket, and proxi mal to Cys 25 may have implications not only for substrate specificity C-terminal to the scissile bond, but also for catalysis.