Carboxypeptidase, pancreatic molecular weight

The carboxypeptidases are released from their inactive precursors in the pancreatic juice of animals. The most studied example is bovine carboxypeptidase A, which contains one mole of zinc per protein molecular weight of 34 500. These enzymes cleave the C-terminal amino acid residue from peptides and proteins, when the side-chain of the C-terminal residue is aromatic or branched aliphatic of l configuration. At least the first five residues in the substrate affect the activity of the enzyme. The enzyme also shows esterase activity. Esters and peptides inhibit each other competitively, indicating that the peptidase and esterase sites overlap, even if they are not the same. 

The most studied member of zinc proteases is the digestive enzyme bovine pancreatic carboxypeptidase A (CPA) which is a metalloenzyme containing one atom of zinc bound to its single polypeptide side chain of 307 amino acids with a molecular weight of 34 kD. It is an exopeptidase, which catalyses the hydrolysis of C-terminal amino... 

Fig. 3.13. Proportionality of the loss of accessible surface area to the molecular weight of proteins (from Janin, 1976). Different proteins insulin, rubredoxin, pancreatic trypsin inhibitor, HIPIP, calcium binding protein, ribonuclease S, lysozyme, staphylococcal nuclease, papain, chymotrypsin, concanavalin A, subtilisin, thermolysin, carboxypeptidase A.

The formation of tertiary structure buries a certain amount of secondary structure surfaces which varies from 32% to 60%. The proportion of the surface which is buried varies for each protein and increases with the molecular weight of the protein. In fact, it is the proportion of the nonpolar surface that becomes buried during folding, which increases with the molecular weight. From 60% for pancreatic trypsin inhibitor, it varies up to 79% in carboxypeptidase among the six proteins considered by Chothia... 

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