Substrate selectivity, peptide site-selective protein cleavage

Carboxypeptidase A (CPA) is a pancreatic metaUoenz5Tne which catalyses the cleavage of a peptide link in a polypeptide chain. The site of cleavage is specific in two ways it occurs at the C-terminal amino acid (equation 28.22), and it exhibits a high selectivity for substrates in which the C-terminal amino acid contains a large aliphatic or Ph substituent. The latter arises from the presence, near to the active site, of a hydro-phobic pocket in the protein which is compatible with the accommodation of, for example, a Ph group (see below). 

Abstract Site-selective peptide/protein degradation through chemical cleavage methods is an important modification of biologically relevant macromolecules which complements enzymatic hydrolysis. In this review, recent progress in chemical, site-selective peptide btuid cleavage is overviewed, with an emphasis oti postulated mechanisms and their implications on reactivity, selectivity, and substrate scope. 

When the first inhibitors for this protease were developed, very litde was known about the enzyme other than its aspartate-catalysed mechanism of action there was no crystal struchire to propose inhibitors based on modelling, and differences with the host proteases were unclear. All that was known was that the HIV protease was a much smaller enzyme than the equivalent host aspartate proteases (arotmd 100 amino acids as opposed to 200), and was able to cleave substrates N-terminal to proline residues, and since mammalian proteases were unable to carry out such cleavages, the possibility of selectivity for the virus over the host arose. Additionally, even before the protease had been isolated, a study of peptides obtained from infected cells suggested that the Met-Met and Tyr-Pro sites were the likely positions of cleavage in the fusion protein. 

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