Cathepsin amino acids

Cleavage occur s at the scissile bond. Residues in the substrate towards the N-terminus are numbered PI, P2, P3, etc, whereas residues towards the C-terminus are numbered PI, P2, P3 etc. Cleavage occurs between PI and P1. For a peptidase with limited specificity, only the residue in PI or PI is important for specificity. A peptidase with an extended substrate binding site will have a preference for residues in other positions. For example cathepsin L prefers substrates with phenylalanine in P2 and arginine in PI. However, this is a preference only, and cathepsin L cleaves substrates after other amino acids. Caspase-3 has a preference for Asp in both P4 and PI, but it is unusual for substrate specificity to extend much further from the scissile bond. The peptidase with the most extended substrate specificity may be mitochondrial intermediate peptidase that removes an octopeptide targeting signal from the N-terminus of cytoplasmically synthesized proteins that are destined for import into the mitochondrial lumen. 

Cathepsin G, a cationic, glycosylated protein of relative molecular mass -27 kDa, exists in four isoforms (25-29 kDa) that are identical in amino acid sequence but differ in levels of glycosylation. It is a component of azurophilic granules and present in human neutrophils at 1.5-3 jug/106 cells, but at lower levels in monocytes. cDNA has been cloned and sequenced (and the amino acid sequence predicted), and the gene has been localised to chromosome 14ql 1.2. The gene comprises five exons and four introns, a structure similar to that of the elastase gene. 

Cathepsin K, a member of the papain superfamily of cysteine proteinases, is selectively and highly expressed in osteoclasts (Drake et al., 1996 Bromme and Okamoto, 1995). It is secreted as a 314 amino acid proenzyme containing a 99 amino acid leader sequence (Bossard et al., 1996). Cathepsin K plays an important role in bone resorption and is a potential therapeutic target for the treatment of diseases involving excessive bone loss such as osteoporosis (Veberef al., 1997). 

The prerequisite is the knowledge of the interactions taking place in the enzyme-substrate complex. This was known for the model enzymes. However, for cathepsin B, the most important enzyme in the lysosomal compartment from the drug release point of view, only limited information was available [26], It was known that the amino acid residues in positions P2 and P3 should be... 

Fig. 14. Initial interval of cleavage of HPMA copolymer based polymeric substrates by lysosomal cysteine proteinase cathepsin B (isolated from bovine spleen). Only the cleavage of the bond between the distal amino acid residue and p-nitroaniline was monitored. Conditions of cleavage [Cathepsin B] = 1.9 x 10 7 M [NAp] = 1.2 x 1(T3 M [EDTA] = 1 x 10 3 M [Cys] = 2.5 x 10 2 M 0.1 M phosphate buffer pH = 6.0 40 °C. Data from [249]...

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... 

Final degradation of substrates to oligopeptides and free amino acids may involve gastro-dermal exopeptidases such as a cathepsin C (Caffrey et al., 2004), which removes dipeptides from the N-terminus of proteins, and a leucine aminopeptidase (LAP McCarthy et al., 2004), which is capable of releasing free amino acids from peptides and dipeptides. However, it is notable that cathepsin B also exhibits carboxydipeptidase activity and, therefore, may well play a dual role (Tort et al., 1999 Caffrey et al., 2004). 

Liver flukes also possess cathepsin C and LAP exopeptidases that are orthologous to the schistosome enzymes. These exopeptidases most likely complete the digestive process to yield free dipeptides and amino acids, respectively, from peptides generated by endoprote-olytic cysteine protease activity on host proteins. Both cathepsin C and LAP have been immunolocalized to gastrodermal cells (Carmona et al., 1994 Acosta et al., 1998 J.P. Dalton, unpublished data). 

The trematode cathepsin B cysteine proteases possess a 20-amino-acid insert termed the occluding loop, a feature that is used to classify them. The intact loop is found in all... 

Proteases have played a pivotal role in the development of parasitism. By the time flat-worms had emerged, members of all the major families of proteases had evolved such that parasites exhibit a complete profile of exo- and endoproteases. The best characterized of these proteases, including cathepsins B, L, C, D and LAP, are vital in the complex process of nutrient uptake from the host by sequentially degrading blood tissue proteins such as haemoglobin to free amino acids. However, other proteases are involved in separate essen-... 

The amino acid sequences of aspergillopepsin I, penicillopepsin, rhizopuspepsin, endothiapepsin, candidapepsin, mucorpepsin, porcine pepsin, and human cathepsin D are aligned. These enzymes are classified into two groups (class I and II). Class I enzymes have... 

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. 

Meloun BM, Baudys M, Pohl J, Pavlik M, Kostka V (1988) Amino acid sequence of bovine spleen cathepsin B. J Biol Chem 263 9087-9093... 

---