Chymosin Active site

Acid proteases are inactivated by active-site specific reagents, diazoacetylnorleucine ethyl ester and other diazo compounds, and epoxy (p-nitrophenoxy)propane. Covalently labelled aspartic acid peptides have been isolated from pepsin, chymosin (= rennin), and penicillopepsin. The peptides labelled with the diazo compounds have similar sequences and differ from the epoxy (p-nitrophenoxy)pro-pane labelled peptides. These results indicate two aspartic acids at the active site and suggest homology between the enzymes. The latter is confirmed by a comparison of the sequence data. Studies of the action of porcine pepsin and penicillopepsin on some dipeptides with free N-terminal groups show transpeptidation involving a covalent acyl intermediate. It is proposed that there are differences in the mechanism of action of pepsin which are determined by the nature of the substrate. 

Aspartyl proteases are an important class of digestive enzymes including pepsin, chymosin and renin, as well as a number of microbial and plant enzymes with tertiary structures showing a striking similarity to each other (Fruton, 1987). They are optimally active at acidic pH and have two aspartate side chains at the active site (Hsu... 

Visser, S., Slangen, C. J., and van Rooijen, P. J. (1987). Peptide substrates for chymosin (rennin). Interaction sites in K-casein-related sequences located outside the (103-108) hexapeptide region that fits into the enzyme s active-site cleft. Biochem. J. 244,553-558. 

Aspartic endopeptidase Aspartic acid (2 residues) in the active site Pepsin, cathepsin D, rennin (chymosin)... 

Penicillopepsin is an acid protease produced by the mold Penicillium janthinellum at pH s less than 4.1 (1). Enzyme production occurs after the mycelial growth has ceased and sporulation has begun (2). The specificity and catalytic mechanism of penicillopepsin are very similar to those of porcine pepsin (3). The two active site aspartic acid residues, Asp-32 and Asp-215, occur in peptide sequences of at least eight amino acid residues which are almost identical in penicillopepsin, pepsin and chymosin (1,4-10). 

The group name aspartic endopeptidases indicates that the carboxyl groups of two aspartic acid residues are the catalytic groups in the active site. The best studied of this group of enzymes is pepsin (EC 3.4.23.1), but chymosin (EC 3.4.23.4) and a large number of microbial proteases also belong to this group. Pepsin has a preference for hydrolysis at the aromatic amino acid residues. 

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