Enzyme specificity cathepsin

This lysosomal enzyme [EC 3.4.22.1], also known as cathepsin Bl, is a member of the peptidase family Cl. The catalyzed reaction is the hydrolysis of peptide binds with a broad specificity. The enzyme prefers the ArgArg—Xaa bond in small peptide substrates (thus distinguishing this enzyme from cathepsin L). The enzyme also exhibits a peptidyl-dipeptidase activity, releasing C-terminal dipeptides from larger polypeptides. 

C5a is inactivated by the myeloperoxidase-H202 system, which oxidises a methionine residue (Met 70) on the molecule group A streptococcal endo-proteinases also abolish chemotactic activity of C5a and related compounds. Neutrophil lysosomal enzymes (e.g. elastase and cathepsin G) also destroy C5a chemotactic activity, but as these proteases are inhibited by the serum antiproteinases, a -antiproteinase and a2-macroglobulin, the physiological role of neutrophilic proteases in the inactivation of C5a is questionable. Two chemotactic factor inactivators have been found in human serum an a-globulin that specifically and irreversibly inactivates C5-derived chemotactic factors, and a / -globulin that inactivates bacterial chemotactic factors. These activities are heat labile (destroyed by treatment at 56 °C for 30 min) and are distinct from those attributable to anaphylatoxin inactivator. An apparently specific inhibitor of C5-derived chemotactic activity has also been described in human synovial fluid and peritoneal fluid. This factor (molecular mass of 40 kDa) is heat stable and acts directly on C5a. 

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. 

Tumor cells express many hydrolytic enzymes, particularly peptidases, some of which are partially specific for certain tumor types, e.g., plasmin, plasminogen activator protease, and cathepsins. A number of prodrug strategies have been developed for the tumor-selective delivery of cytotoxic drugs [45-47], as illustrated below with a few representative examples. 

This enzyme [EC 3.4.16.5] (also known as serine-type carboxypeptidase I, cathepsin A, carboxypeptidase Y, and lysosomal protective protein) is a member of the peptidase family SIO and catalyzes the hydrolysis of the peptide bond, with broad specificity, located at the C-terminus of a polypeptide. The pH optimum ranges from 4.5 to 6.0. The enzyme is irreversibly inhibited by diisopropyl fluorophosphate and is sensitive to thiolblocking reagents. 

This enzyme [EC 3.4.23.34], also known as slow-moving proteinase and erythrocyte membrane aspartic proteinase, is similar to cathepsin D, albeit with a slightly broader specificity. 

This peptidase family Cl enzyme [EC 3.4.22.15] is an lysosomal endopeptidase with specificity akin to papain. Cathepsin L displays a higher activity toward protein substrates than does cathepsin B. 

Cathepsin D (EC3.4.23.5). It has been known for more than 20 years that milk also contains an acid proteinase, (optimum pH ss 4.0) which is now known to be cathepsin D, a lysozomal enzyme. It is relatively heat labile (inactivated by 70°C x 10 min). Its activity in milk has not been studied extensively and its significance is unknown. At least some of the indigenous acid proteinase is incorporated into cheese curd its specificity on asl- and / -caseins is quite similar to that of chymosin but it has very poor milk-clotting activity (McSweeney, Fox and Olson, 1995). It may contribute to proteolysis in cheese but its activity is probably normally overshadowed by chymosin, which is present at a much higher level. 

Some of the above features can be seen in the spectra of cathepsin D (Fig. B3.5.10), where the intact single-chain bovine enzyme is compared with the same material cleaved at an exposed loop but without dissociation. The latter has 50% of the specific activity of the intact molecule. Phenylalanine residues can be seen to be present in specific environments in both forms. The fine structure of the Lb transition of tryptophan is superimposed on the broad peak of the La transition, which is apparently more intense in the intact enzyme. Alternatively, there could be a greater contribution from disulfide bonds, but the absence of ellipticity above 320 nm favors the former assignment and the CD is therefore consistent with a limited increase in dynamics of the molecule as a result of the chain... 

Like antithrombin, heparin cofactor II inhibits proteases by forming a I I stoichiometric complex with the enzyme. The protease attacks the reactive site of heparin cofactor II located on the C-terminus, resulting in the formation of a covalent bond. Heparin cofactor II has higher protease specificity than antithrombin. Of the coagulation enzymes, heparin cofactor II is known only to inhibit thrombin (92). Additionally heparin cofactor II has been shown to inhibit chymotrypsin (93) and leukocyte cathepsin G (94), This protease specificity appears to be due to the active site bond present in heparin cofactor II. Whereas antithrombin contains an Arg-Ser bond as its active site, heparin cofactor II is unique in containing a Leu-Ser bond. This suggests than another portion of the heparin cofactor II molecular may be required for protease binding,... 

The lysosomal enzymes most relevant to our discussion are the peptidases and the nucleases. The peptidases, also referred to as the cathepsins, comprise at least eight exopeptidases and nine endopeptidases, which between them have a broad range of specificities that enable them to reduce any proteins or peptides to their constituent amino acids. 

In the search for possible targets the inhibition activity of a range of RAPTA compounds was studied for two specific enzymes - namely thioredoxin reductases (TrxR) and cathepsin B (cat B), important targets in cancer chemotherapy [28]. TrxR was chosen as it is highly relevant with respect to gold-based drugs (see below) and cat B is implicated in various stages of metastasis which correlate to the observed in vitro effects of RAPTA-T [29], It turned out that while the selected compounds (Fig. 3) are not inhibitors of TrxR with the exception of carboRAPTA-C, they are active inhibitors of cat B [28],... 

Intracellular Turnover. There are many proteases involved in the turnover of proteins at the intracellular level, all of which have rather restricted specificity. These enzymes, while also present in the cytoplasm, are highly concentrated in the lysosomes. They generally are referred to as cathepsins and include both endo- and exosplitting hydrolases. 

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