For example, superoxide dismutase is the enzyme responsible for the destruction of the superoxide ion in living organisms. Enzymes act either in solution or attached to membranes. The material whose reaction is catalyzed by an enzyme is called a substrate . In some reactions catalyzed by enzymes, the reaction appears to take place on the surface of the catalyst enzyme molecules themselves, which may or may not be in homogeneous solution. Most reactions in biological systems do not occur at any perceptible rate in the absence of an enzyme to catalyze them.
Enzymes are very selective catalysts.
A given enzyme will catalyze only specific reactions of very specific substrate molecules. The ES complex then dissociates to yield the products of the catalyzed reaction. Experimentally, it has been found that the rates of many enzyme-catalyzed reactions are directly proportional to substrate concentration when substrate concentration is low and become independent of substrate concentration as substrate concentration increases. This behavior is called Michaelis-Menten kinetics after Leonor Michaelis and Maud Menten, who in 1913 proposed the kinetic mechanism which accounts for this behavior. A more useful form of the Michaelis-Menten equation is obtained from the following argument.