Question

9. How is His able to act as both a general base and general acid catalyst at the same pH? b) How is Ser able to act as a nucleophile? c) How is the transition-state stabilized by chymotrypsin?

10. Explain why chymotrypsin would become inactive at a: a) low temperature. b) high temperature. c) pH = 4.0.

Answer 1

9.a) Basically, the imidazole side chain of histidine can function as either a general acid catalyst or a general base catalyst. This is because the imidazole group has a pKa in the physiological pH range. In the physiological pH range, the nitrogen in the ring can be easily protonated/deprotonated. Thatis why many acid-base catalysis reactions involve histidine because it has a pH close to 7, allowing it to act as both an acid and a base.

b) The serine has an -OH group and that is the reason it is able to act as a nucleophile, attacking the carbonyl carbon of the scissile peptide bond of the substrate.

c) Enzymes lower the free energy of the transition state (∆G‡) by stabilizing the transition state.

  This happens because of the result of the binding of the transition state to the enzyme. In particular, the precise positioning of the “oxyanion hole” which allows the stabilization of the charged transition state.

10. Enzymes, like other proteins, also undergo denaturation. If the crystalline proteinase chymotrypsin is subjected to an unfavourable pH, some part of protein becomes denatured. This percentage of denatured protein is usually found to be equal to the per cent loss in enzymic activity. Similarly at low temperature or high temperature, chymotrypsin would become inactive.