recap

8.5 recap

The rates of most enzyme-catalyzed reactions are affected by interacting molecules (such as inhibitors and activators) and by environmental factors (such as temperature and pH). Reversible phosphorylation is another important mechanism for regulating enzyme activity.

learning outcomes

You should be able to:

  • Explain how metabolic pathways can shift in different directions.

  • Distinguish between different types of inhibitors.

  • Apply the concept of allosteric regulation to explain experimental results.

Question 1

What is feedback inhibition? How might the reactions shown in Figure 8.18A fit into a systems diagram like the one shown in Figure 8.14?

Feedback inhibition occurs in a biochemical pathway when that pathway’s end product can act as an inhibitor of the enzyme that catalyzes the first step in the pathway. In a systems diagram, each node represents an enzymatic transformation. Feedback inhibition can cross multiple pathways, thereby allowing the changing concentration of one molecule to affect several pathways that lead to its synthesis.

Question 2

Consider an enzyme that is subject to allosteric regulation. If a competitive inhibitor (not an allosteric inhibitor) is added to a solution containing such an enzyme, the ratio of enzyme molecules in the active form to those in the inactive form increases. Explain this observation.

See Figure 8.17. A competitive inhibitor binds to the active site of the enzyme and shifts the equilibrium to enzyme molecules in the active form.

Question 3

In humans, hydrogen peroxide (H2O2) is a dangerous toxin produced as a by-product of several metabolic pathways. The accumulation of H2O2 is prevented by its conversion to harmless H2O, a reaction catalyzed by the appropriately named enzyme catalase. Air pollutants can inhibit this enzyme and leave individuals susceptible to tissue damage by H2O2. How would you investigate whether catalase has an allosteric or a nonallosteric mechanism and whether the pollutants are acting as competitive or noncompetitive inhibitors?

To determine whether catalase has an allosteric or a nonallosteric mechanism, perform an experiment with varying amounts of substrate and plot the rate of catalase-versus-substrate concentration. An S-shaped curve will indicate an allosteric mechanism. A hyperbolic curve will indicate a nonallosteric enzyme. To determine if a pollutant is a competitive or a noncompetitive inhibitor, add the pollutant to the catalase to lower the rate of reaction, then add increasing amounts of substrate. A competitive inhibitor will be removed from the active site and the rate of reaction will increase. A noncompetitive inhibitor will not allow the rate to increase as more substrate is added.