CTSH

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Abstract

Cathepsin H (EC 3.4.22.16), like other lysosomal cysteine proteases, is involved in intracellular protein degradation. There is increasing evidence that levels of cathepsin H or cathepsin H-like enzymes are increased in disease states, such as breast carcinoma and melanoma. The activity of cathepsin H is reduced in mucolipidosis II along with the activity of other cathepsins. Cysteine proteases are not just lysosomal enzymes, they are also involved in many processes outside the lysosome. For example, a cathepsin H-like enzyme, found in cells of human melanoma cell lines, degrades fibrinogen and fibronectin, and thus may participate in the destruction of extracellular matrix components along with other proteases, leading to cancer proliferation, migration and metastasis.

Mature cathepsin H consists of an N-terminal heavy chain, a C-terminal light chain, and an octapeptide called a mini-chain. The cleavage of the enzyme between the heavy and light chains is partial, and the ratio of the cleaved to uncleaved form varies between species and organs, but generally the double-stranded form is present in greater amounts than the triple-stranded form. Sequence homology shows that cathepsin H belongs to the papain-like cysteine protease superfamily, which also includes mammalian lysosomal cathepsins B, L, S, K and N and the plant enzymes papain, actinidin, aleurain and oryzains.

Cathepsin H is inhibited by stefins, cystatins, and kininogens. Cathepsin H interacts more strongly than cathepsin B, but not as potently as the endopeptidases papain and cathepsin L. An exception is stefin B, which is a more potent inhibitor of cathepsin H than endopeptidase. In contrast to cathepsin B, L, S and K, cathepsin H is only slowly inhibited by irreversible epoxysuccinyl-based inhibitors derived from E-64. Cathepsins H, B and C are the only known papain-like lysosomal cysteine exopeptidases. Cathepsin H is an aminopeptidase that cleaves a single N-terminal residue from a polypeptide chain, cathepsin C is an aminodipeptidase, and cathepsin B is a carboxydipeptidase.

Overall Structure

The structure confirmed the pattern of disulfide bridges in bovine cathepsin H, including t that formed between Cys205 and the minichain Cys80P. The three disulfide bridges, Cys22-Cys63, Cys56-Cys95, and Cys154-Cys200, are topologically equivalent to those in actinidin. Cathepsin H is an ellipsoidal molecule with dimensions of 32 × 26.5 × 24 Å. Its folding is typical of papain superfamily member. This structure includes the body of the enzyme consisting of right (R-) and left (L-) domains, and an octapeptide bound to the R-domain. The predominant secondary structure in the L-domain is alpha helix, while the R-domain is the beta barrel. The contact interface of these two domains is stabilized by hydrogen bonds and hydrophobic contacts. Superposition of the Cα atoms of cathepsin H on the Cα atoms of papain, actinidin and cathepsin B showed that the 180 amino-acid residues of cathepsin H and actinidin are topologically equivalent, 173 with papain, and 156 with human cathepsin B. Cathepsin H is most similar in structure to actinidin, and they differ mainly by insertions and deletions in the loop regions. Mature porcine cathepsin H has N-glycosylation sites on Asn112 on the enzyme body and Asn79P on the mini-chain. The Asn79P side chain is glycosylated, but the observed electron density map is not clear enough to allow us to identify the sugar rings.

Figure 1. The overall structure of cathepsin H (Guncar, G.; et al. 1998)

The active site cleft of the papain-like cysteine protease crosses the top of the molecule. The cleft is wide at the end and narrow in the middle, where the catalytic residues Cys25, His159 and Gln19 are located. In the cathepsin H structure, the front of the cleft is filled with carbohydrate residues on the left and mini-chain in the middle, while the back part is unoccupied and thus free for substrate binding.

Figure 2. Stereo view of the mini-chain binding in the active-site cleft (Guncar, G.; et al. 1998)