Transient peak identifies enzyme-substrate complex

Steps:

• Substrate concentration decreases steadily as it's consumed.
• Product concentration increases over time as it's formed.
• Free enzyme concentration remains low and constant after initial binding.
• Enzyme-substrate complex rises quickly to a peak then declines as it breaks down to product.

Why D is correct:

• The enzyme-substrate complex (ES) forms rapidly but is transient, peaking early before declining per Michaelis-Menten kinetics (ES → E + P).

Why the others are wrong:

• A: Steady decline matches substrate depletion.
• B: Steady increase matches product formation.
• C: Constant low level matches free enzyme.

Final answer: D