Chapter Summary, Study Questions

CHAPTER SUMMARY

Enzymes are protein catalysts that increase the velocity of a chemical reaction by lowering the energy of the transition state (Figure 5.23). Enzymes are not consumed during the reaction they catalyze. Enzyme molecules contain a special pocket or cleft called the active site. The active site contains amino acid side chains that participate in substrate binding and catalysis. The active site binds the substrate, forming an enzyme–substrate (ES) complex. Binding is thought to cause a conformational change in the enzyme (induced fit) that allows catalysis. ES is converted to enzyme-product (EP), which subsequently dissociates to enzyme and product. An enzyme allows a reaction to proceed rapidly under conditions prevailing in the cell by providing an alternate reaction pathway with a lower free energy of activation. The enzyme does not change the free energies of the reactants or products and, therefore, does not change the equilibrium of the reaction. Most enzymes show Michaelis-Menten kinetics, and a plot of the initial reaction velocity (v_o) against substrate concentration ([S]) has a hyperbolic shape similar to the oxygen-dissociation curve of myoglobin. Any substance that can diminish the velocity of such enzyme-catalyzed reactions is called an inhibitor. The two most commonly encountered types of reversible inhibition are competitive (which increases the apparent K_m) and noncompetitive (which decreases the apparent V_max). In contrast, the multisubunit allosteric enzymes frequently show a sigmoidal curve similar in shape to the oxygen-dissociation curve of hemoglobin. They typically catalyze the rate-limiting (slowest step) of a pathway. Allosteric enzymes are regulated by molecules called effectors that bind noncovalently at a site other than the active site. Effectors can be either positive (accelerate the enzyme-catalyzed reaction) or negative (slow down the reaction). An allosteric effector can alter the affinity of the enzyme for its substrate, modify the maximal catalytic activity of the enzyme, or both. Enzymes can also be regulated by covalent modification and by changes in the rate of synthesis or degradation. Enzymes have diagnostic and therapeutic value in medicine.

Figure 5.23 Key concept map for the enzymes. S = substrate; [S] = substrate concentration; P = product; E = enzyme; vo = initial velocity; V_max = maximal velocity; K_m = Michaelis constant; K_0.5 = substrate concentration which gives half maximal velocity.

Study Questions

Choose the ONE best answer.

5.1 In cases of ethylene glycol poisoning and its characteristic metabolic acidosis, treatment involves correction of the acidosis, removal of any remaining ethylene glycol, and administration of an inhibitor of alcohol dehydrogenase (ADH), the enzyme that oxidizes ethylene glycol to the organic acids that cause the acidosis. Ethanol (grain alcohol) frequently is the inhibitor given to treat ethylene glycol poisoning. Results of experiments using ADH with and without ethanol are shown to the right. Based on these data, what type of inhibition is caused by the ethanol?

A. Competitive

B. Feedback

C. Irreversible

D. Noncompetitive

Correct answer = A competitive inhibitor increases the apparent Km for a given substrate. This means that, in the presence of a competitive inhibitor, more substrate is needed to achieve 1⁄2 V_max. The effect of a competitive inhibitor is reversed by increasing substrate concentration ([S]). At a sufficiently high [S], the reaction velocity reaches the V_max observed in the absence of inhibitor.

5.2 ADH requires oxidized nicotinamide adenine dinucleotide (NAD⁺) for catalytic activity. In the reaction catalyzed by ADH, an alcohol is oxidized to an aldehyde as NAD⁺ is reduced to NADH and dissociates from the enzyme. The NAD⁺ is functioning as a (an):

A. apoenzyme.

B. coenzyme-cosubstrate.

C. coenzyme-prosthetic group.

D. cofactor.

E. heterotropic effector.

For Questions 5.3 and 5.4, use the graph below which shows the changes in free energy when a reactant is converted to a product in the presence and absence of an enzyme. Select the letter that best represents:

Correct answer = B. Coenzymes-cosubstrates are small organic molecules that associate transiently with an enzyme and leave the enzyme in a changed form. Coenzyme-prosthetic groups are small organic molecules that associate permanently with an enzyme and are returned to their original form on the enzyme. Cofactors are metal ions. Heterotropic effectors are not substrates.

5.3 The free energy of activation of the catalyzed forward reaction.

Correct answers = B; D. Enzymes (biocatalysts) provide an alternate reaction pathway with a lower free energy of activation. However, they do not change the free energy of the reactant or product. A is the free energy of the uncatalyzed reaction. C is the free energy of the catalyzed reverse reaction.

5.4 The free energy of the reaction.