Type I collagen fibers account for 90% of the organic matrix of bone. The degradation of this collagen is a major event during bone resorption, but its mechanism is unknown. A series of data obtained in biological models strongly suggests that the recently discovered cysteine proteinase cathepsin K plays a key role in bone resorption. Little is known, however, about the actual action of cathepsin K on type I collagen. Here, we show that the activity of cathepsin K alone is sufficient to dissolve completely insoluble collagen of adult human cortical bone. We found that the collagenolytic activity of cathepsin K is directed both outside the helical region of the molecule, i.e. the typical activity of cysteine proteinases, and at various sites inside the helical region, hitherto believed to resist all mammalian proteinases but the collagenases of the matrix metalloproteinase family and the neutrophil elastase. This property of cathepsin K is unique among mammalian proteinases and is reminiscent of bacterial collagenases. It is likely to be responsible for the key role of cathepsin K in bone resorption.The only mammalian proteinases that have been shown to attack the native triple helical region of type I collagen are the collagenases of the MMP 1 family (1-3) and the neutrophil serine elastase (4) They cleave the type I collagen triple helix across all three chains (i.e. two ␣1 chains and one ␣2 chain) only at a specific point three-quarters of the way to the Nterminal end of the collagen molecule. Proteinases with broad specificity, such as cysteine proteinases, attack only the extrahelical regions that are located at either end of native collagen (telopeptides) and that represent only 4% of the molecule (5). Because the telopeptides are involved in intra-and intermolecular links, this attack may separate individual molecules. The latter proteinases may also attack destabilized triple helices, acting thereby as gelatinases. At 37°C, such a destabilization may transiently affect a small proportion of collagen molecules, because the melting temperature of soluble collagen is only a few degrees higher. It has also been emphasized that when collagen molecules are cross-linked and arranged in insoluble fibers they become more resistant to proteolysis (6). However, the co-operation of proteinases with distinct specificities toward the chemical bonds of collagen fibers has been shown to favor the efficiency of collagenolysis (7).Insoluble type I collagen fibers constitute 90% of the organic matrix of bone, and their degradation is necessary for bone resorption (8). The test tube experiments that have been performed so far showed that it is difficult to achieve complete degradation of adult lamellar bone with a single bone proteinase (9, 10). On the other hand, various biological approaches have shown that both MMPs and cysteine proteinases participate in the bone resorption processes (8,(11)(12)(13). Representatives of these two types of proteinases were identified in osteoclasts, the cells responsible for bone resorptio...

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.

Cathepsin C, or dipeptidyl peptidase I, is a lysosomal cysteine protease of the papain family that catalyzes the sequential removal of dipeptides from the free N-termini of proteins and peptides. Using the dipeptide substrate Ser-Tyr-AMC, cathepsin C was characterized in both steady-state and pre-steady-state kinetic modes. The pH(D) rate profiles for both log k cat/ K m and log k cat conformed to bell-shaped curves for which an inverse solvent kinetic isotope effect (sKIE) of 0.71 +/- 0.14 for (D)( k cat/ K a) and a normal sKIE of 2.76 +/- 0.03 for (D) k cat were obtained. Pre-steady-state kinetics exhibited a single-exponential burst of AMC formation in which the maximal acylation rate ( k ac = 397 +/- 5 s (-1)) was found to be nearly 30-fold greater than the rate-limiting deacylation rate ( k dac = 13.95 +/- 0.013 s (-1)) and turnover number ( k cat = 13.92 +/- 0.001 s (-1)). Analysis of pre-steady-state burst kinetics in D 2O allowed abstraction of a normal sKIE for the acylation half-reaction that was not observed in steady-state kinetics. Since normal sKIEs were obtained for all measurable acylation steps in the presteady state [ (D) k ac = 1.31 +/- 0.04, and the transient kinetic isotope effect at time zero (tKIE (0)) = 2.3 +/- 0.2], the kinetic step(s) contributing to the inverse sKIE of (D)( k cat/ K a) must occur more rapidly than the experimental time frame of the transient kinetics. Results are consistent with a chemical mechanism in which acylation occurs via a two-step process: the thiolate form of Cys-234, which is enriched in D 2O and gives rise to the inverse value of (D)( k cat/ K a), attacks the substrate to form a tetrahedral intermediate that proceeds to form an acyl-enzyme intermediate during a proton transfer step expressing a normal sKIE. The subsequent deacylation half-reaction is rate-limiting, with proton transfers exhibiting normal sKIEs. Through derivation of 12 equations describing all kinetic parameters and sKIEs for the proposed cathepsin C mechanism, integration of both steady-state and pre-steady-state kinetics with sKIEs allowed the provision of at least one self-consistent set of values for all 13 rate constants in this cysteine protease's chemical mechanism. Simulation of the resulting kinetic profile showed that at steady state approximately 80% of the enzyme exists in an active-site cysteine-acylated form in the mechanistic pathway. The chemical and kinetic details deduced from this work provide a potential roadmap to help steer drug discovery efforts for this and other disease-relevant cysteine proteases.

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