At higher substrate concentrations, the rate levels off and approaches a maximal rate, called the maximum velocity, V max . The V max is the measure of the rate of the reaction when all of the enzyme has substrate bound. Increasing the concentration of enzyme, at a fixed substrate concentration, leads to an increase in the rate of reaction. There are two classes of enzyme inhibition, reversible and irreversible . There are two sorts of reversible inhibition, competitive and non-competitive .
Competitive inhibition occurs when structural analogs of the substrate bind at the active site .
Since the forces holding this inhibitor are weak, it will eventually leave the site and can be replaced by substrate. The degree of inhibition depends of the relative concentrations of the substrate and the competitive inhibitor. Sulfa drugs are an example of this. Bacteria synthesize folic acid from PABA in an enzyme catalyzed reaction. They need the folic acid to to make cell walls.