Cathepsins

The term cathepsin is derived from the Greek word meaning “to digest” and is used to describe a broad range of intracellular proteinases that serve important biological functions. Two major pathways have been identified that control the degradation of cellular proteins. One operates within the cytoplasm and is called the ubiquitin-mediated pathway. The other functions within specific cellular compartments, especially the lysosomes, but also in endosomes and the endoplasmic reticulum. Lysosomes are the major site of intracellular protein degradation (1). They are amply endowed for this function because they contain as many as 20 different proteolytic enzymes and as many other hydrolytic enzymes (eg, phosphatases, lipases, nucleases, and sulfatases) (1). The lysosomal proteinases are called cathepsins. They are single-chain proteins that range in molecular mass from 20 kDa to about 40 kDa. Typically they have a broad substrate specificity, are optimally active at somewhat acidic pH, and belong to either the thiol proteinase or carboxyl proteinase classes. For no particular reason, they have been designated by letters as cathepsins B, L, H, M, N, S, and T, all of which are thiol proteinases or, more commonly, cysteine proteinases; and cathepsins D and E, which are carboxyl (or aspartyl) proteinases (2). All of these are endopeptidases, enzymes that cleave peptide bonds in the internal part of a protein. In addition, there are several lysosomal exopeptidases, including cathepsin C, which removes dipeptides from the amino-terminus of polypeptide chains and is therefore a dipeptidyl aminopeptidase, and at least two carboxycathepsins that are carboxypeptidases. Cathepsin G is a chymotrypsin-like serine proteinase found in neutrophils, and cathepsin R is another serine proteinase found in the endoplasmic reticulum.

 The lysosomal cathepsins have been considered to be the most active proteinases in the body, because they degrade four times as much protein as the pancreatic and gastric proteinases combined (3). Much of this is essentially turnover of cytosolic proteins to balance protein synthesis and maintain homeostasis. However, cathepsins are also important components of the immune system: In order for a foreign protein to generate an antibody response, it must be taken up by a specialized antigen-presenting cell and degraded within endosomes. The resultant peptide fragments are then “presented” on the cell surface, where they trigger antibody production (4).

References

1. F. Authier, B. I. Posner, and J. J. M. Burgeron (1994) In Cellular Proteolytic Systems (A. J. Ciechanover and A. L. Schwartz, eds.), Wiley-Liss, New York, pp. 89–113. 

2. J. S. Bond and P. E. Butler (1987) Annu. Rev. Biochem. 56, 333–364. 

3. A. J. Barrett and H. Kirschke (1981) Methods Enzymol. 80, 535–561. 

4. C. V. Harding (1994) In Cellular Proteolytic Systems (A. J. Ciechanover and A. L. Schwartz, eds.), Wiley-Liss, New York, pp. 163–180.