Enzyme kinetics: rate decreases as substrate depletes

Steps:

• Fixed substrate and enzyme concentrations mean initial rate is at maximum (Vmax) due to ample substrate.
• Reaction consumes substrate over time, reducing its concentration.
• Per Michaelis-Menten kinetics, rate is proportional to [S] when [S] is limiting, so rate falls as [S] drops.
• At completion, [S] ≈ 0, so rate reaches zero.

Why C is correct:

• C depicts rate starting high and curving down to zero, matching Michaelis-Menten rate = (Vmax [S]) / (Km + [S]) as [S] → 0.

Why the others are wrong:

• A shows constant rate, ignoring substrate depletion.
• B shows increasing rate, contradicting initial Vmax.
• D shows abrupt stop, not gradual decline with [S].

Final answer: C