Does a low Km always mean a better enzyme?

Not by itself; Km reflects apparent affinity for substrate, but overall catalytic efficiency is best captured by kcat/Km, which combines turnover rate with substrate binding.

What is the turnover number?

The turnover number kcat is the maximum number of substrate molecules one enzyme active site converts to product per unit time when fully saturated with substrate.

Enzyme Kinetics

Enzyme kinetics quantifies how fast enzymes work and how reaction rate depends on substrate concentration, providing parameters that characterize and compare catalysts.

Definition

Enzyme kinetics is the study of the rates of enzyme-catalyzed reactions and how those rates respond to substrate concentration, enzyme concentration, inhibitors, and conditions, summarized by parameters derived from the Michaelis–Menten equation.

Scope

This topic covers the Michaelis–Menten model and its underlying steady-state assumption, the meaning of the kinetic constants Km, Vmax, kcat, and the specificity constant kcat/Km, graphical and regression-based analysis of rate data, and the kinetic signatures of competitive, uncompetitive, and noncompetitive inhibition.

Core questions

Why does reaction velocity saturate as substrate concentration increases?
What do Km and kcat physically represent?
How do different inhibitor types alter the apparent kinetic parameters?
How is kcat/Km used to judge catalytic efficiency?

Key theories

Michaelis–Menten model: Modeling catalysis as reversible substrate binding followed by product release yields a hyperbolic velocity–substrate relationship, defining Vmax (maximal rate) and Km (the substrate concentration at half-maximal velocity).
Steady-state (Briggs–Haldane) treatment: Briggs and Haldane generalized the original equilibrium assumption to a steady-state one, in which the enzyme–substrate complex concentration is approximately constant, giving the Michaelis–Menten form broader validity.

Mechanisms

Under the steady-state assumption the rate of formation of the enzyme–substrate complex equals its rate of breakdown, so its concentration stays nearly constant during the initial phase. Solving for initial velocity gives v = Vmax[S]/(Km + [S]). Km combines the binding and catalytic rate constants; kcat (Vmax divided by total enzyme) is the turnover number, and kcat/Km gauges efficiency near the diffusion limit.

Clinical relevance

Kinetic parameters are central tools in chemical and pharmaceutical research—for screening inhibitors, comparing engineered enzyme variants, and characterizing reaction conditions. The discussion remains analytical and non-prescriptive.

History

Michaelis and Menten's 1913 analysis of invertase set the quantitative framework; Briggs and Haldane's 1925 steady-state derivation removed the need for the equilibrium assumption, and later linearizations (Lineweaver–Burk, Eadie–Hofstee) and nonlinear regression refined parameter estimation.

Key figures

Leonor Michaelis
Maud Menten
George Edward Briggs
J. B. S. Haldane

Seminal works

michaelis1913
briggs1925
nelson2021

Frequently asked questions

Does a low Km always mean a better enzyme?: Not by itself; Km reflects apparent affinity for substrate, but overall catalytic efficiency is best captured by kcat/Km, which combines turnover rate with substrate binding.
What is the turnover number?: The turnover number kcat is the maximum number of substrate molecules one enzyme active site converts to product per unit time when fully saturated with substrate.