In first-order enzyme kinetics, which statement is correct about limiting components?

• A. Enzyme is in excess and substrate is limiting
• B. Substrate is in excess and enzyme is limiting
• C. Both are in equal amounts and equally limiting
• D. There is no limiting component in first-order kinetics

Answer: (A)

In this early, low-substrate range of enzyme kinetics, the rate is proportional to the substrate concentration because the enzyme is plentiful relative to the substrate. With enzyme in excess and substrate limiting, every molecule of substrate can be processed as it quickly encounters an active site, so doubling the substrate doubles the rate. Mathematically, when [S] << Km, v ≈ (Vmax/Km) [S], which shows the rate depends linearly on [S]. That’s why the substrate, not the enzyme, is the limiting component in this situation. If the enzyme were limiting, the rate would reflect the amount of enzyme (and could approach a maximum when substrate is abundant), and if there were no limiting component, the rate would be constant and independent of substrate, which doesn’t describe the first-order regime.