MCAT Biochemistry Review
Chapter 2: Enzymes
1. Consider a biochemical reaction A → B, which is catalyzed by A–B dehydrogenase. Which of the following statements is true?
1. The reaction will proceed until the enzyme concentration decreases.
2. The reaction will be most favorable at 0°C.
3. A component of the enzyme is transferred from A to B.
4. The free energy change (ΔG) of the catalyzed reaction is the same as for the uncatalyzed reaction.
2. Which of the following statements about enzyme kinetics is FALSE?
1. An increase in the substrate concentration (at constant enzyme concentration) leads to proportional increases in the rate of the reaction.
2. Most enzymes operating in the human body work best at a temperature of 37°C.
3. An enzyme–substrate complex can either form a product or dissociate back into the enzyme and substrate.
4. Maximal activity of many human enzymes occurs around pH 7.4.
3. Some enzymes require the presence of a nonprotein molecule to behave catalytically. An enzyme devoid of this molecule is called a(n):
4. Which of the following factors determine an enzyme's specificity?
1. The three-dimensional shape of the active site
2. The Michaelis constant
3. The type of cofactor required for the enzyme to be active
4. The prosthetic group on the enzyme
5. Enzymes increase the rate of a reaction by:
1. decreasing the activation energy.
2. increasing the overall free energy change of the reaction.
3. increasing the activation energy.
4. decreasing the overall free energy change of the reaction.
6. In the equation below, substrate C is an allosteric inhibitor to enzyme 1. Which of the following is another mechanism necessarily caused by substrate C?
1. Competitive inhibition
2. Irreversible inhibition
3. Feedback enhancement
4. Negative feedback
7. The activity of an enzyme is measured at several different substrate concentrations, and the data are shown in the table below.
8. Km for this enzyme is approximately:
1. Questions 8 and 9 refer to the following statement:
2. Consider a reaction catalyzed by enzyme A with a Km value of 5 × 10–6 M and vmax of .
3. At a concentration of 5 × 10–6 M substrate, the rate of the reaction will be:
4. At a concentration of 5 × 10–4 M substrate, the rate of the reaction will be:
5. The graph below shows kinetic data obtained for flu virus enzyme activity as a function of substrate concentration in the presence and absence of two antiviral drugs.
Based on the graph, which of the following statements is correct?
1. Both drugs are noncompetitive inhibitors of the viral enzyme.
2. Tamiflu increases the Km value for the substrate compared to Relenza.
3. Relenza increases the vmax value for the substrate compared to Tamiflu.
4. Both drugs are competitive inhibitors of the viral enzyme.
6. The conversion of ATP to cyclic AMP and inorganic phosphate is most likely catalyzed by which class of enzyme?
7. Which of the following is NOT a method by which enzymes decrease the activation energy for biological reactions?
1. Modifying the local charge environment
2. Forming transient covalent bonds
3. Acting as electron donors or receptors
4. Breaking bonds in the enzyme to provide energy
8. A certain cooperative enzyme has four subunits, two of which are bound to substrate. Which of the following statements can be made?
1. The affinity of the enzyme for the substrate has just increased.
2. The affinity of the enzyme for the substrate has just decreased.
3. The affinity of the enzyme for the substrate is at the average for this enzyme class.
4. The affinity of the enzyme for the substrate is greater than with one substrate bound.
9. Which of the following is LEAST likely to be required for a series of metabolic reactions?
1. Triglyceride acting as a coenzyme
2. Oxidoreductase enzymes
3. Magnesium acting as a cofactor
4. Transferase enzymes
10.How does the ideal temperature for a reaction change with and without an enzyme catalyst?
1. The ideal temperature is generally higher with a catalyst than without.
2. The ideal temperature is generally lower with a catalyst than without.
3. The ideal temperature is characteristic of the reaction, not the enzyme.
4. No conclusion can be made without knowing the enzyme type.
Answers and Explanations
Enzymes catalyze reactions by lowering their activation energy, and are not changed or consumed during the course of the reaction. While the activation energy is lowered, the free energy of the reaction, ΔG, remains unchanged in the presence of an enzyme. A reaction will continue to occur in the presence or absence of an enzyme; it simply runs slower without the enzyme, eliminating choice (A). Most physiological reactions are optimized at body temperature, 37°C, eliminating choice (B). Finally, dehydrogenases catalyze oxidation–reduction reactions, not transfer reactions, eliminating choice (C).
Most enzymes in the human body operate at maximal activity around a temperature of 37°C and a pH of 7.4, which is the pH of most body fluids. In addition, as characterized by the Michaelis–Menten equation, enzymes form an enzyme–substrate complex, which can either dissociate back into the enzyme and substrate or proceed to form a product. So far, we can eliminate choices (B), (C), and (D), so let's check choice (A). An increase in the substrate concentration, while maintaining a constant enzyme concentration, leads to a proportional increase in the rate of the reaction only initially. However, once most of the active sites are occupied, the reaction rate levels off, regardless of further increases in substrate concentration. At high concentrations of substrate, the reaction rate approaches its maximal velocity and is no longer changed by further increases in substrate concentration.
An enzyme devoid of its necessary cofactor is called an apoenzyme and is catalytically inactive.
An enzyme's specificity is determined by the three dimensional shape of its active site. Regardless of which explanation for enzyme specificity we are discussing (lock and key or induced fit), the active site determines which substrate the enzyme will react with.
Enzymes increase the rate of a reaction by decreasing the activation energy. They do not affect the overall free energy, ΔG, of the reaction.
By limiting the activity of enzyme 1, the rest of the pathway is slowed, which is the definition of negative feedback. Choice (A) is incorrect because there is no competition for the active site with allosteric interactions. While many products do indeed competitively inhibit an enzyme in the pathway that creates them, this is an example of an allosterically inhibited enzyme. There is not enough information for choice (B) to be correct because we aren't told whether the inhibition is reversible. In general, allosteric interactions are temporary. Choice (C) is incorrect because it is the opposite of what occurs when enzyme 1 activity is reduced.
While the equations given in the text are useful, recognizing relationships is even more important. You can see that as substrate concentration increases significantly, there is only a small change in the rate. This occurs as we approach vmax. Because the vmax is near equals . The substrate concentration giving this rate is 0.5 mM and corresponds to Km; therefore, choice (A) is correct.
As with the last question, relationships are important. At a concentration of 5 × 10–6 M, enzyme A is working at one-half of its vmax because the concentration is equal to the Km of the enzyme. Therefore, one-half of is , which corresponds to choice (A).
At a concentration of 5 × 10–4 M, there is 100 times more substrate than present at half maximal velocity. At high values (significantly larger than the value of Km) the enzyme is at or near its vmax, which is .
Based on the graph, when the substrate is present, Tamiflu results in the same vmax and a higher Km compared to when no inhibitor is added. These are hallmarks of competitive inhibitors. Noncompetitive inhibitors result in decreased vmax and the same Km as the uninhibited reaction, which is shown by the Relenza line in the graph. Because the question is only comparing the values between the two inhibitors, and not the enzyme without inhibitor, the mechanism of inhibition is less important to determine than the values of Km and vmax. This is a great example of why previewing the answer choices works well in the sciences.
Lyases are responsible for the breakdown of a single molecule into two molecules without the addition of water or the transfer of electrons. Lyases often form cyclic compounds or double bonds in the products to accommodate this. Water was not a reactant, and no cofactor was mentioned; thus lyase, choice (C), remains the best answer choice.
Enzymes are not altered by the process of catalysis. A molecule that breaks intramolecular bonds to provide activation energy would not be able to be reused.
Cooperative enzymes demonstrate a change in affinity for the substrate depending on how many substrate molecules are bound and whether the last change was accomplished because a substrate molecule was bound or left the active site of the enzyme. Because we cannot determine whether the most recent reaction was binding or dissociation, choices (A) and (B) are eliminated. We can make absolute comparisons though. The unbound enzyme has the lowest affinity for substrate, and the enzyme with all but one subunit bound has the highest. The increase in affinity is not linear, and therefore choice (C) is not necessarily true. An enzyme with two subunits occupied must have a higher affinity for the substrate than the same enzyme with only one subunit occupied; thus, choice (D) is correct.
Triglycerides are unlikely to act as coenzymes for a few reasons, including their large size, neutral charge, and ubiquity in cells. Cofactors and coenzymes tend to be small in size, such as metal ions like choice (C) or small organic molecules. They can usually carry a charge by ionization, protonation, or deprotonation. Finally, they are usually in low, tightly regulated concentrations within cells. Metabolic pathways would be expected to include both oxidation–reduction reactions and movement of functional groups, thus eliminating choices (B) and (D).
The rate of reaction increases with temperature because of the increased kinetic energy of the reactants, but reaches a peak temperature because the enzyme denatures with the disruption of hydrogen bonds at excessively high temperatures. In the absence of enzyme, this peak temperature is generally much hotter. Heating a reaction provides molecules with an increased chance of achieving the activation energy, but the enzyme catalyst would typically reduce activation energy. Keep in mind that thermodynamics and kinetics are not interchangeable, so we are not considering the impact of heat on the equilibrium position.