what is km in enzyme kinetics

In enzyme kinetics, Km (also known as the Michaelis constant) is the substrate concentration at which the reaction rate is 50% of the Vmax. It is a measure of the affinity an enzyme has for its substrate, with a lower value of Km indicating that the enzyme is more efficient at carrying out its function at a lower substrate concentration. For practical purposes, Km is the concentration of substrate which permits the enzyme to achieve half Vmax. An enzyme with a high Km has a low affinity for its substrate, and requires a greater concentration of substrate to achieve Vmax. Km is a generalized definition of the Michaelis constant, and is calculated using the Michaelis-Menten equation.