An experiment is performed in which the kinetics of an enzyme-catalyzed reaction at different pHs is monitored.It is found that the K_m does not change but that the k_cat increases as the pH goes above 7.Which of the following is true? A) A chemical group within the enzyme that has a pKa of around 7 is likely involved in the catalytic mechanism. B) A chemical group with a pKa of around 7 must be deprotonated in order for substrate to bind. C) A chemical group with a pKa of around 7 must be positively charged in order for the substrate to bind. D) Protons are acting as positive heterotropic allosteric effectors of this enzyme.

Question

Quizplus · Accepted Answer

The fact that only the kcat increases with pH while the KM remains constant suggests that the rate-limiting step in the reaction involves a chemical group whose pKa is near 7, which is likely involved in the catalytic mechanism. This chemical group could be an amino acid residue in the active site of the enzyme, for example, whose protonation state is important for catalyzing the reaction. The other answer choices are not supported by the information given in the problem.

An Experiment Is Performed in Which the Kinetics of an Enzyme-Catalyzed