Question

An experiment is performed in which the kinetics of an enzyme-catalyzed reaction at different pHs is monitored. It is found that the Km does not change but that the k cat increases as the pH goes above 7 . Which of the following is true?

1. A chemical group within the enzyme that has a pKa of around 7 is likely involved in the catalytic mechanism.
2. A chemical group with a pKa of around 7 must be positively charged in order for the substrate to bind.
3. A chemical group with a pKa of around 7 must be deprotonated in order for substrate to bind.
4. Protons are acting as positive heterotropic allosteric effectors of this enzyme.

Answer 1

In an enzyme-catalyzed reaction experiment where kcat increases with pH above 7 while Km remains constant, it's likely that an enzyme group with a pKa around 7 plays a role in the catalytic mechanism.

Step-by-step explanation:

From the experiment that monitors enzyme-catalyzed reaction kinetics at different pH levels, and noting that the Km remains constant while the kcat (turnover number) increases as pH goes above 7, we can infer certain aspects about enzyme activity and pH dependence. A constant Km suggests that the substrate binding affinity doesn't change with pH, while an increasing kcat implies enhanced catalytic efficiency at more alkaline conditions. The most likely explanation for this observation is that a specific chemical group within the enzyme, which has a pKa of around 7, becomes ionized (deprotonated) when the pH exceeds 7, thus becoming more effective in its catalytic function. Therefore, the first statement is true: A chemical group within the enzyme that has a pKa of around 7 is likely involved in the catalytic mechanism.