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The in vitro activation of the recombinant purified human cathepsin K (EC 3.4.22.38) was examined by mutagenesis. Cathepsin K was expressed as a secreted proenzyme using baculovirus-infected Sf21 insect cells. Spontaneous in vitro activation of procathepsin K occurred at pH 4 and was catalyzed by exogenous mature cathepsin K. Three intermediates were identified as resulting from cleavages after ]Procathepsin K (containing mutation C139S,S163A) failed to spontaneously process and was only partially processed in the presence of 1% exogenous wild-type mature cathepsin K forming intermediates, which were identical to those observed in the activation of wild-type. [Ser 139 ,Ala-163 ]Procathepsin K could be fully processed to mature enzyme by including one equivalent of wild-type procathepsin K in the activation mixture. These results indicated that in vitro activation of the procathepsin K was an autocatalytic process.Bone remodeling is a constant process that involves bone resorption and rebuilding (for review, see Ref. 1). The resorption phase of this process is carried out by osteoclasts, which adhere to the surface of bone leading to the creation of an extracellular compartment termed the resorption pit. The resorption pit is maintained at an acidic pH, causing the dissolution of the mineral components of the underlying bone and exposure of the proteinaceous matrix to the action of proteolytic enzymes (2-6). The rebuilding phase of the remodeling process involves the recruitment of osteoblasts to the sites of prior bone resorption, where the layering of a new proteinaceous matrix occurs and becomes mineralized.Cathepsin K, a member of the papain cysteine protease family, has recently been implicated in the resorption of the bone matrix (7-12). The cDNA encoding this protease was cloned from human, rabbit, and mouse osteoclast libraries and expressed in baculovirus-infected insect cells by several independent groups as an inactive secreted proenzyme (7-13). Bossard (14) and Brömme (13) have demonstrated activation of the recombinant proenzyme in vitro by proteolytic degradation of the N-terminal 99-amino acid propeptide; however, different mechanisms leading to its activation were implicated.Activation of procathepsin K in vivo is likely to occur in the low pH environment of the resorption pit, via two possible mechanisms. The propeptide may be cleaved by another protease, such as cathepsin D as suggested by Brömme et al. or by an autocatalytic process, which is more consistent with the data presented by Bossard et al. (14).To elucidate the mechanism of activation of cathepsin K, we constructed a mutant in which the presumed active site Cys at position 139 was changed to Ser. The kinetics of activation of mutant and wild-type cathepsin K were studied in vitro.In this report we provide the following evidence for an autocatalytic activation mechanism. First, in vitro self-activation of wild-type procathepsin K occurs spontaneously at 4°C, pH 4 and is catalyzed by mature cathepsin K. Second, unlike wildtype enzyme, t...

The X-ray crystal structure of the proform of human matrix metalloproteinase MMP9 has been solved to 2.5 A resolution. The construct includes the prodomain, the catalytic domain and three FnII (fibronectin type II) domains. The prodomain is inserted into the active-site cleft, blocking access to the catalytic zinc. Comparison with the crystal structure of the most closely related MMP, MMP2, indicates that the conformations of residues in the active-site cleft and in the cysteine-switch peptide of the prodomain are highly conserved and that design of MMP9-specific inhibitors will be challenging. In common with MMP2, the MMP9 S1' inhibitor-binding pocket is large compared with that of other MMPs. One small point of difference in the S1' binding pockets of MMP9 and MMP2 may provide an opportunity to explore the design of specific inhibitors. The side chain of Arg424 in MMP9 is angled slightly away from the S1' pocket when compared with the corresponding residue in MMP2, Thr424. The secondary structure of the FnII domains is conserved between the two closely related MMPs, although the second FnII domain makes no contact with the catalytic domain in MMP9, while the same domain in MMP2 has a substantial area of interaction with the catalytic domain.

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Potent and selective active-site-spanning inhibitors have been designed for cathepsin K, a cysteine protease unique to osteoclasts. They act by mechanisms that involve tight binding intermediates, potentially on a hydrolytic pathway. X-ray crystallographic, MS, NMR spectroscopic, and kinetic studies of the mechanisms of inhibition indicate that different intermediates or transition states are being represented that are dependent on the conditions of measurement and the specific groups f lanking the carbonyl in the inhibitor. The species observed crystallographically are most consistent with tetrahedral intermediates that may be close approximations of those that occur during substrate hydrolysis. Initial kinetic studies suggest the possibility of irreversible and reversible active-site modification. Representative inhibitors have demonstrated antiresorptive activity both in vitro and in vivo and therefore are promising leads for therapeutic agents for the treatment of osteoporosis. Expansion of these inhibitor concepts can be envisioned for the many other cysteine proteases implicated for therapeutic intervention.