kcatKm

kcatKm is an informal way some texts use to refer to the catalytic efficiency of an enzyme, a parameter more correctly written as kcat/Km. This ratio combines the turnover number of the enzyme with the substrate affinity and is especially meaningful at low substrate concentrations.

kcat, the turnover number, is the maximum number of substrate molecules that a single enzyme molecule can convert to product per unit time when the substrate is saturating. It is measured in s^-1 and reflects the catalytic speed of the enzyme under optimal substrate conditions. Km, the Michaelis constant, is the substrate concentration at which the reaction rate is half of Vmax. It provides a rough sense of enzyme-substrate affinity under the assumptions of the Michaelis–Menten model; a lower Km indicates higher affinity, though Km is not a direct binding constant.

The catalytic efficiency, kcat/Km, is a single measure that describes how efficiently an enzyme converts substrate to product at low substrate concentrations. It has units of M^-1 s^-1 and can be derived from the limiting form of the Michaelis–Menten equation: v/[S] ≈ (kcat/Km)[E]t as [S] approaches zero. Higher values indicate greater efficiency. In many enzymes, kcat/Km approaches diffusion-limited values (around 10^8–10^9 M^-1 s^-1), reflecting near-optimal use of substrate.

Determination of kcat and Km typically requires initial-rate measurements across a range of substrate concentrations and fitting to the Michaelis–Menten equation. Cautions include deviations from simple kinetics, multi-substrate mechanisms, or allosteric effects that can complicate interpretation. While kcat/Km is widely used to compare enzymes and substrates, it summarizes a specific kinetic regime and should be interpreted within the model’s assumptions.