Cathepsin functions

A series of inhibitors containing an electrophilic keto-l,3,4-oxadiazole moiety as the warhead has been reported in which the substituent at the 5-position was varied resulting in the identification of furan as the optimal prime side substituent. Exploration of P3 substituents led to the identification of 10 with a K, of 1 nM against Cat K with > 700-fold selectivity over off-target cathepsins (Cat B Ki = 730 nM Cat L Rj = 960 nM Cat S Rj = 700 nM) [54], The potency of this compound was shifted in a functional bone resorption assay (Cat K IC50= 132 nM). 

This is a 29-kDa protein that has NH 2-terminal sequence homology with elastase and cathepsin G. However, it contains glycine and not serine at the predicted catalytic site, and so lacks protease and peptidase activity. Purified azurocidin kills a range of organisms (e.g. E. coli, S.faecalis, and C. albicans) in vitro. It functions optimally at pH 5.5 and in conditions of low ionic strength. 

One of the general principles of the Nomenclature Committee is that enzymes should be classified and named according to the reaction they catalyze. However, the overlapping specificities of and great similarities in the action of different peptidases render naming solely on the basis of function impossible [10]. For example, some enzymes can act as both endo- and exopeptidases. Thus, cathepsin H (EC 3.4.22.16) is not only an endopeptidase but also acts as an aminopeptidase (EC 3.4.11), and cathepsin B (EC 3.4.22.1) acts as an endopeptidase as well as a peptidyl-dipeptidase (EC 3.4.15). The actual classification of peptidases is, therefore, a compromise based not only on the reaction catalyzed but also on the chemical nature of the catalytic site, on physiological function, and on historical priority. 

Moreover, as expected disruption of genes coding for enzymes critical to the function of osteoclast such as tartrate-resistant acid phosphatase (Hayman et al., 1996) and cathepsin K (Gowen et al., 1999) also produced osteopetrosis. This complements earlier discussed spontaneous osteopetrotic phenotypes produced by interception of pathways generating either protons or chloride necessary for mineral dissolution. 

The functional proteins in the cell have to be protected in order to prevent premature degradation. Some of the intracellularly active proteolytic enzymes are therefore enclosed in lysosomes (see p. 234). The proteinases that act there are also known as cathepsins. Another carefully regulated system for protein degradation is located in the cytoplasm. This consists of large protein complexes (mass 2 10 Da), the proteasomes. Proteasomes contain a barrel-shaped core consisting of 28 subunits that has a sedimentation coef cient (see p. 200) of 20 S. Proteolytic activity (shown here by the scissors) is localized in the interior of the 20-S core and is therefore protected. The openings in the barrel are sealed by 19-S particles with a complex structure that control access to the core. 

The protease [APP secretase) responsible for the normal cleavage of APP through the PA4 segment is unknown, but a number of proteases, including calpain [D. H. Small et al. 1992), a serine protease, and cathepsin B [Cataldo et al. 1991), have been proposed. It remains to be determined whether therapy should be directed at enhancing the function of the secretase to accelerate the breakdown of the PA4 segment or at inhibiting the enzyme to reduce APP turnover. 

Most of the lysosomal proteases called cathepsins are small 20- to 40-kDa glycoproteins found in all animal tissues.313 Most are cysteine proteases which function best and are most stable in the low pH reducing environment of lysosomes. They resemble papain in size, amino acid sequence, and active site structures. Papain is nonspecific but most cathepsins have definite substrate preferences. Cathepsin B is the most abundant. There are smaller amounts of related cathepsins H (an aminopeptidase)314 and L315 and still less of cathepsins C, K, and others. Cathepsin B is both an endopep-tidase and an exopeptidase.316 It acts on peptides with arginine at either Pj or P2 but also accepts bulky hydro-phobic residues in Pj and prefers tyrosine at P3.317 Cathepsin S is less stable at higher pH than other cathepsins and has a more limited tissue distribution, being especially active in the immune system.318 319... 

Zymogens of cysteine proteases usually have a long terminal extension which is removed, sometimes by autoactivation. Propapain has a 107-residue extension.343 The 322-residue cathepsin B carries an unusually short 62-residue extension in its proenzyme form.315 343 344 In every case the N-terminal extension folds into a domain, one of whose functions is to block the active site cleft. 

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