Bio Chemistry

BIOCHEMISTRY 3(2+1)

Lesson 14 : Enzymes- Factors Affecting Enzyme Activity

Effects of Inhibitors on Enzyme Activity

Enzyme inhibitors are substances which alter the catalytic action of the enzyme and consequently slow down, or in some cases, stop catalysis. There are three common types of enzyme inhibition - competitive, non-competitive and substrate inhibition.

Competitive inhibition occurs when the substrate and a substance resembling the substrate are both added to the enzyme. A theory called the "lock-key theory" of enzyme catalysts can be used to explain why inhibition occurs.

Substrate inhibition will sometimes occur when excessive amounts of substrate are present. It is shown by the reaction velocity decreasing after the maximum velocity has been reached.

Additional amounts of substrate added to the reaction mixture after this point actually decrease the reaction rate. This is thought to be due to the fact that there are so many substrate molecules competing for the active sites on the enzyme surfaces that they block the sites (Figure 12) and prevent any other substrate molecules from occupying them.

This causes the reaction rate to drop since all of the enzyme present is not being used.