In an enzyme-catalyzed reaction studying competitive inhibition, with enzyme and inhibitor concentrations held constant, which should be varied as the independent variable?

• A. The initial substrate concentration
• B. The reaction temperature
• C. The concentration of the buffer
• D. The pH of the solution

Answer: (A)

In competitive inhibition, the blocker and the substrate compete for the same active site. Because the inhibitor binds only at the active site, changing how much substrate is present changes how often a substrate molecule rather than an inhibitor is bound. To study this relationship, you vary the substrate concentration while keeping the enzyme amount and inhibitor amount fixed. This lets you see how the reaction rate increases as more substrate is available, approaching the uninhibited rate at high substrate levels. It also shows the kinetic effect: the apparent Km goes up in the presence of inhibitor, while Vmax stays the same. The other experimental factors—temperature, buffer concentration, and pH—are kept constant to isolate the effect of changing substrate concentration.