Proteases selectively cleave proteins by cutting the peptide bonds that link specific amino acids. Cathepsins, abbreviated CTS are proteases found in all animals as well as other organisms. Cathepsins are part of a larger enzyme family called proteases, which are well known therapeutic targets. Although their activities are optimal at lysosomal acidic and reducing conditions, cathepsins can significantly contribute to the degradation of the extracellular matrix.
Cathepsins are a group of lysosomal proteases that have a key role in cellular protein turnover. The term cathepsin includes serine proteases, aspartic proteases, and cysteine proteases. Most cathepsins are endopeptidases, with the exception of cathepsin C and Z. All cysteine cathepsins are expressed as polypeptides consisting of a signal sequence, a propeptide, and a catalytic domain. The length of the domains is depicted in the graph. The signal peptide is cleaved off at the site of the translocation into the endoplasmic reticulum. The propeptide is cleaved in the increasingly acidic environment of the endosomal/lysosomal system resulting in a fully active catalytic domain of the cathepsins.
The main function of cathepsins is protein recycling within the lysosome but they are also known to be involved in a range of other physiological, as well as pathological processes, including maturation of the MHC class II complex, bone remodeling, keratinocyte differentiation, tumor progression and metastasis, rheumatoid arthritis, osteoarthritis and atherosclerosis.