Substrate Depletion Causes Reaction Plateau

Steps:

• Graph shows product concentration rising then leveling off, indicating slowing reaction rate.
• Enzyme reactions follow Michaelis-Menten kinetics, where rate depends on substrate availability.
• As hydrogen peroxide (substrate) converts to product, its concentration decreases.
• Reaction stops when substrate is fully used up, causing product accumulation to cease.

Why D is correct:

• Substrate molecules were used up: In enzyme kinetics, the reaction rate drops to zero when substrate concentration reaches zero, as no reactant remains for catalysis (Michaelis-Menten equation: v = Vmax [S] / (Km + [S])).

Why the others are wrong:

• A: Active site saturation explains maximum initial rate (Vmax) but not the eventual stop.
• B: Enzyme denaturation would cause abrupt rate drop, not gradual leveling.
• C: Hydrogen peroxide inhibition might reduce rate but typically doesn't produce a clean plateau.

Final answer: D