Molecules block the action of enzymes in two distinct ways; competetive and noncompetetive inhibition.

In competitive inhibition the molecule acts on the same active site as the normal enzyme substrate. The substrate molecules cannot enter the active site while the inhibitor is there, and inhibitors cannot entere the site when the substrate is there. Characteristically increasing the concentration of substrate reduces the effect of the inhibitor, and vice-versa.

In noncompetitive inhibition the inhibitor works by occupying some other site on the enzyme. Because of this the substrate and inhibitor do not compete for access to the same site. Characteristically the activity of the enzyme is commpletely blocked by the inhibitor and increasing the concentration of substrate does not restore enzyme activity.

The kinetics of these activities are described by the Michaelis-Menten equations.