rate-limiting

Meaning: The rate-limiting step is the slowest step in a sequence, like the narrowest point in a pipe. In a metabolic pathway it is usually an early, essentially irreversible step controlled by allosteric effectors or covalent modification. Examples are phosphofructokinase-1 in glycolysis, acetyl-CoA carboxylase in fatty acid synthesis, and glycogen phosphorylase in glycogenolysis. In a single reaction mechanism (general chemistry), it is the step with the highest-energy transition state, and the rate law is based on it.

Equation: rate = k[A], where rate is in mol/(L·s), k is the rate constant of the slow step (s⁻¹ for a first-order step), and [A] is the concentration of the reactant in that step (mol/L).

On the MCAT: A passage gives a pathway with an inhibitor, mutation, or activator and asks what happens to flux. Intermediates before the slow step build up and products after it fall. Other questions ask which enzyme is the regulatory point, or which step is rate-limiting on a reaction coordinate diagram (the highest peak).

Common confusion: Students assume the first step is always rate-limiting or that speeding up any enzyme speeds up the pathway. Only changing the slow step changes the overall rate substantially. Also, rate-limiting describes kinetics (the highest activation energy barrier), not the most favorable ΔG.