Reaction rate for an enzyme depends on

• A. Temperature
• B. pH
• C. Enzyme concentration
• D. The concentration of the substrate

Answer: (D)

Enzyme-catalyzed reaction rate is directly tied to substrate availability. The more substrate molecules present, the more often they can collide with and bind to the enzyme’s active site, forming enzyme–substrate complexes that speed product formation. At low substrate levels, increasing substrate boosts the rate roughly in proportion to [substrate]. As substrate becomes abundant, the enzyme becomes saturated and nearly all active sites are occupied, so adding more substrate no longer increases the rate—the maximum rate is reached and now only the amount of enzyme (and its turnover number) limits further increases. Temperature and pH modulate how well the enzyme works, and enzyme concentration sets the ceiling for the rate, but the rate seen in typical measurements is most directly controlled by substrate concentration.