MCAT® ExamChemical and Physical Foundations of Biological SystemsMedium
A biochemist is studying a novel enzyme that catalyzes a specific reaction. They determine that the enzyme exhibits Michaelis-Menten kinetics. If the enzyme concentration is kept constant, but the substrate concentration is significantly increased beyond the K_M value, what will be the effect on the reaction rate?
- AThe reaction rate will increase linearly with substrate concentration.
- BThe reaction rate will approach V_max and become largely independent of further substrate increase.
- CThe reaction rate will decrease due to substrate inhibition.
- DThe reaction rate will remain unchanged, as it is already at its maximum.
Show answer & explanationAnswer & explanation
Correct answer: B. The reaction rate will approach V_max and become largely independent of further substrate increase.
In Michaelis-Menten kinetics, when substrate concentration is significantly higher than K_M, the enzyme becomes saturated with substrate. At this point, the enzyme is working at its maximum capacity (V_max), and further increases in substrate concentration will not significantly increase the reaction rate because all active sites are continuously occupied.
Why the other options are wrong
- A. Linear increase only occurs at very low substrate concentrations, well below K_M.
- C. Substrate inhibition is a specific phenomenon not generally assumed in basic Michaelis-Menten kinetics, and typically only occurs at extremely high, non-physiological substrate concentrations.
- D. The reaction rate will approach V_max, but it might not be exactly at V_max unless the substrate concentration is infinitely high. It will, however, become largely independent of further substrate increases.
Michaelis-Menten Kinetics
A model describing enzyme kinetics, relating reaction rate to substrate concentration. It assumes a reversible enzyme-substrate binding step followed by an irreversible catalytic step.
- V_max is the maximum reaction rate when the enzyme is saturated with substrate.
- K_M is the Michaelis constant, representing the substrate concentration at which the reaction rate is half of V_max.
- At low [S], rate is proportional to [S]; at high [S], rate approaches V_max.
- The Michaelis-Menten equation is V = (V_max * [S]) / (K_M + [S]).
Memory trick: When the enzyme is full, the rate maxes out!