MCAT Biochemistry Practice Test 2: Enzymes

1. Consider a biochemical reaction A → B, which is catalyzed by A–B dehydrogenase. Which of the following statements is true?

• A. The reaction will proceed until the enzyme concentration decreases.
• B. The reaction will be most favorable at 0°C.
• C. A component of the enzyme is transferred from A to B.
• D. 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?

• A. An increase in the substrate concentration (at constant enzyme concentration) leads to proportional increases in the rate of the reaction.
• B. Most enzymes operating in the human body work best at a temperature of 37°C.
• C. An enzyme–substrate complex can either form a product or dissociate back into the enzyme and substrate.
• D. 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):

• A. holoenzyme.
• B. apoenzyme.
• C. coenzyme.
• D. zymoenzyme.

4. Which of the following factors determine an enzyme's specificity?

• A. The three-dimensional shape of the active site
• B. The Michaelis constant
• C. The type of cofactor required for the enzyme to be active
• D. The prosthetic group on the enzyme

5. Enzymes increase the rate of a reaction by:

• A. decreasing the activation energy.
• B. increasing the overall free energy change of the reaction.
• C. increasing the activation energy.
• D. 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?

• A. Competitive inhibition
• B. Irreversible inhibition
• C. Feedback enhancement
• D. Negative feedback

7. The activity of an enzyme is measured at several different substrate concentrations, and the data are shown in the table below.

[S] (mM)
0.01	1
0.05	9.1
0.1	17
0.5	50
1	67
5	91
10	95
50	99
100	100

K_m for this enzyme is approximately:

• A. 0.5.
• B. 1
• C. 10
• D. 50

8. Consider a reaction catalyzed by enzyme A with a K_m value of 5 × 10^–6 M and v_max of

At a concentration of 5 × 10^–6 M substrate, the rate of the reaction will be:

• A. .
• B. .
• C. .
• D. .

9. Consider a reaction catalyzed by enzyme A with a K_m value of 5 × 10^–6 M and v_max of

At a concentration of 5 × 10^–4 M substrate, the rate of the reaction will be:

• A. .
• B. .
• C. .
• D. .

10. 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?

• A. Both drugs are noncompetitive inhibitors of the viral enzyme.
• B. Tamiflu increases the K_m value for the substrate compared to Relenza.
• C. Relenza increases the v_max value for the substrate compared to Tamiflu.
• D. Both drugs are competitive inhibitors of the viral enzyme.

11. The conversion of ATP to cyclic AMP and inorganic phosphate is most likely catalyzed by which class of enzyme?

• A. Ligase
• B. Hydrolase
• C. Lyase
• D. Transferase

12. Which of the following is NOT a method by which enzymes decrease the activation energy for biological reactions?

• A. Modifying the local charge environment
• B. Forming transient covalent bonds
• C. Acting as electron donors or receptors
• D. Breaking bonds in the enzyme to provide energy

13. A certain cooperative enzyme has four subunits, two of which are bound to substrate. Which of the following statements can be made?

• A. The affinity of the enzyme for the substrate has just increased.
• B. The affinity of the enzyme for the substrate has just decreased.
• C. The affinity of the enzyme for the substrate is at the average for this enzyme class.
• D. The affinity of the enzyme for the substrate is greater than with one substrate bound.

14. Which of the following is LEAST likely to be required for a series of metabolic reactions?

• A. Triglyceride acting as a coenzyme
• B. Oxidoreductase enzymes
• C. Magnesium acting as a cofactor
• D. Transferase enzymes

15. How does the ideal temperature for a reaction change with and without an enzyme catalyst?

• A. The ideal temperature is generally higher with a catalyst than without.
• B. The ideal temperature is generally lower with a catalyst than without.
• C. The ideal temperature is characteristic of the reaction, not the enzyme.
• D. No conclusion can be made without knowing the enzyme type.