THE LIVING WORLD

Unit two. The Living Cell

5. Energy and Life

5.5. How Cells Regulate Enzymes

Because an enzyme must have a precise shape to work correctly, it is possible for the cell to control when an enzyme is active by altering its shape. Many enzymes have shapes that can be altered by the binding of “signal” molecules to their surfaces. Such enzymes are called allosteric (Latin, other shape). Enzymes can be inhibited or activated by the binding of signal molecules. For example, the upper tan panels in the Key Biological Process illustration above show an enzyme that is inhibited. The binding of a signal molecule, called a repressor (panel 2), alters the shape of the enzyme’s active site such that it cannot bind the substrate. In other cases, the enzyme may not be able to bind the reactants unless the signal molecule is bound to the enzyme. The lower set of panels shows a signal molecule serving as an activator. The red substrate cannot bind to the enzyme’s active site unless the activator (the yellow molecule) is in place, altering the shape of the active site. The site where the signal molecule binds to the enzyme surface is called the allosteric site.

Enzymes are often regulated by a mechanism called feedback inhibition, where the product of the reaction acts as the repressor. Feedback inhibition can occur in two ways: competitive inhibitors and noncompetitive inhibitors. The blue molecule in figure 5.8a functions as a competitive inhibitor, blocking the active site so that the substrate cannot bind. The yellow molecule in figure 5.8b functions as a noncompetitive inhibitor. It binds to an allosteric site, changing the shape of the enzyme such that it is unable to bind to the substrate.

Figure 5.8 How enzymes can be inhibited.

(a) In competitive inhibition, the inhibitor interferes with the active site of the enzyme. (b) In noncompetitive inhibition, the inhibitor binds to the enzyme at a place away from the active site, effecting a conformational change in the enzyme so that it can no longer bind to its substrate. In feedback inhibition, the inhibitor molecule is the product of the reaction.

Many drugs and antibiotics work by inhibiting enzymes. Statin drugs like Lipitor lower cholesterol by inhibiting a key enzyme cells used to make cholesterol. The antibiotic penicillin inhibits an enzyme bacteria used in making cell walls. Because humans lack this enzyme, we are not harmed by the drug.

Key Learning Outcome 5.5. An enzyme's activity can be affected by signal molecules that bind to it, changing its shape.