The Michaelis-Menten constant, represented as Km or Ks, is the substrate concentration where the reaction velocity is one-half of the maximum reaction velocity. It is a part of the Michaelis-Menten equation for modeling enzyme kinetics.
The equation relates reaction velocity to the concentration of the substrate. Maximum velocity is reached when the substrate is at a saturated concentration. This equation only works for reactions where the substrate initially creates a reversible bond with an enzyme and makes a enzyme-substrate complex. By using up the substrate, this complex creates a product. The enzyme is released and can bind with another substrate to reproduce the entire reaction. In 1925, George Briggs and J.B.S. Haldane gave a simple derivation of the equation that is still used today.