Carboxypeptidase B specifically catalyses the hydrolysis of proteins at the pept
ID: 66295 • Letter: C
Question
Carboxypeptidase B specifically catalyses the hydrolysis of proteins at the peptide bond next to the C terminus when/if the penultimate AA residue (i.e RY in figure below) is 'aromatic' The substrate and active site Is represented below. Position (I.e YOU draw) the substrate as it would align In the active site to maximize both the NCls and the catalytic efficiency. Then, at the end of each arrow, describe the role of each AA residue and zinc cation In the catalysis substrate, i.e C terminus of a protein Having 'docked' the substrate correctly in the active site, draw the first step of the mechanism of the hydrolysis reaction (you will need a H2O) on the diagram. It has been shown by specifically mutating the D to a N in carboxypeptidase B. enzyme activity decreases by 10^3. Explain this data. The specificity of trypsin is given on WP p 111. The specificity for thrombin is the A A residue triad, PRG where the hydrolysis deavge is between the R&G.; Thus there is some similarity. Describe the reason for the difference on specificities between these two eniymes in light of their biological roles.Explanation / Answer
Aspartic acid (D) residue provides the negative charge in the binding site of carboxypeptidase, which plays an important role in hydrolysis reaction. When this amino acid is replaced with uncharged amino acid, asparagine (N), active site does not pose the necessary negative charge. Hence, the enzyme activity decreases.