MCAT Practice questions
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1. Consider a biochemical reaction A to B, which is catalyzed by A-B dehydroge-
nase. Which of the following statements is true?
A. The reaction will proceed until the enzyme concentration decreases.
B. The reaction will be more favorable at 0 degrees C.
C. A component of the enzyme transferred from A to B.
D. The free energy change (delta G) of the catalyzed reaction is the same as for
the uncatalyzed reaction.: D.
-In an enzyme-catalyzed reaction, the rate of a reaction is increased by a decrease in the activation energy. Furthermore,
enzymes are not changed or consumed during the course of the reaction. Also, the overall free energy change of the
reaction, delta G, remains unchanged in the presence of an enzyme.
2. Which of the following statements about enzyme kinetics is false?
A. An increase in the substrate concentration (at constant enzyme concentra-
tion) 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 degree 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 7.2.: A.
-Most enzymes in the human body operate a maximal activity around a temperature of 37 degrees C and a pH of
7.2, which is the pH of most body fluids. In addition, as characterized by the Michaelis-Menten model, enzymes form
an enzyme-substrate complex, which can either dissociate back into the enzyme and substrate or proceed to form
a product. We can eliminate B, C, and D. 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. Therefore statement A is not entirely true.
3. At which pH would pancreatic enzymes work at maximum activity?
A. 5.3
B. 6.7
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C. 7.2
D. 8.5: D.
-Pancreatic enzymes work optimally in the alkaline condition of the small intestine. It is not necessary to know the exact
pH at which these enzymes work because the only very basic pH is seen in (D), which is 8.5.
4. Some enzymes require the presence of a non-protein molecule to behave
catalytically. An enzyme devoid of this molecule is called a(n):
A. holoenzyme
B. apoenzyme
C. coenzyme
D. zymoenzyme: B.
-An enzyme devoid of its necessary cofactor is called an apo-enzyme and is catalytically inactive.
5. Which of the following factors determine an enzyme's specificity?
A. The 3D 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.: A.
-An enzyme's specificity is determined by the 3D shape of its active site. Regardless of which theory of enzyme specificity
we are discussing (lock and key or induced fit), the active site determines which substrate the enzyme will react with.
6. Enzymes increase the rate of a reaction by:
A. decreasing the activation energy.
B. increasing the overall free energy of the reaction.
C. both A and B.
D. none of the above.: A.
-Enzymes increase the rate of a reaction by decreasing the activation energy. They do not affect the overall free energy,
delta G, of the reaction.
7. Bonding between atoms of an enzyme such as trypsin is best described as:
A. peptide
B. saccharide
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C. ionic
D. van der Waals: A.
-Enzymes are proteins. Proteins are composed of amino acids linked together by peptide bonds. Choice (B) - Saccha-
ride, is a type of bond found in polysaccharides. Choice (C) - Ionic, is a chemical bond formed through electrostatic
interaction between positive and negative ions. Choice (D) may be formed in secondary or tertiary structures of an
enzyme but is not as good an option as (A).
8. In the equation below, substrate C is an allosteric inhibitor to enzyme 1. Which
of the following is another mechanism caused by substrate C?
A --> enzyme 1 --> B --> enzyme 2 --> C.
A. Competitive inhibition
B. Irreversible inhibition
C. Feedback enhancement
D. Negative feedback: D.
-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. There is not enough
information for (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 is it the opposite of what occurs when enzyme 1 activity
is reduced.
9. When lactose hydrolyzes its substrate, lactose, which of the following occurs?
A. Lactase retains it structure after the reaction.
B. Lactose retains its structure after the reaction.
C. Lactase increases the activation energy of the reaction.
D. Lactose decreases the activation energy of the reaction.: A.
-Choice (A) is correct answer because, by definition, an enzyme remains unchanged by the reaction that is catalyzes.
Choice (B) is incorrect because a substrate is changed by an enzymatic reaction. Choice (C) is not true, an enzyme would
decrease the activation energy. Choice (D) is also incorrect since a substrate does not affect the activation energy.
10. Discuss why competitive inhibition can be overcome with increasing sub-
strate concentration but non-competitive inhibition cannot be overcome.: -Com-
petitive inhibitors are structurally similar to the substrate of an enzyme. This means that in the course of catalyzing
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reactions the enzyme can bind the inhibitor instead of the substrate. Imagine there are an equal number of inhibitor and
substrate molecules. The chance that the enzyme will bind the inhibitor is equal to the chance that it will interact with the
substrate. Thus, the rate of catalysis will be decreased, effectively inhibiting the enzyme. However, if the concentration
of substrate if 1,000 or 100,000 times greater than the concentration of inhibitor, then the chance the enzyme will
randomly interact with the inhibitor instead of the substrate falls dramatically (practically to zero).
-Noncompetitive inhibitors do not associate with the enzyme at the active site. Instead, they bind at an allosteric site.
When a noncompetitive inhibitor is bound, the enzyme cannot catalyze bind the substrate. Increasing the concentration
of substrate will not decrease the likelihood of a noncompetitive inhibitor associating at an allosteric site.
11. Why is negative feedback inhibition more common than positive feedback
loops? Are there biological examples when positive feedback is necessary?: -Most
biological conditions, such as blood pH, body temperature, and blood pressure, must be kept within a narrow range.
Negative feedback allows for the maintenance of these conditions. If a homeostatic system moves too far to one extreme,
negative feedback will brings the levels back within biologically acceptable conditions. Positive feedback, however, has
the opposite effect. The more a process moves in a particular direction, the more positive feedback will keep it moving
in that same direction. Two examples in which positive feedback is important are blood clotting and childbirth.
12. Compare and contrast the the concept of allosteric effectors with non-com-
petitive inhibition.: -First, all noncompetitive inhibitors bind at allosteric sites. The binding of noncompetitive
inhibitors, however, does not affect the binding of substrate to the active site (and vice versa). What noncompetitive
inhibitors do affect is the ability for the enzyme to catalyze the reaction and release product. In contrast, allosteric
effectors allow the enzyme to exist in two different confirmations (T and R), in which one state binds substrate more
readily than the other. Again, this is in direct contract to noncompetitive inhibitors, which do not influence the binding
of substrate.
13. Consider the pathway:
A --1-> B --2-> C --3-> D --4-> E
--5-> X --6-> Y
An increase in [E] leads to the inhibition of enzyme 3. All of the following are
results of the process except:
A. an increase in [B].
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1. Consider a biochemical reaction A to B, which is catalyzed by A-B dehydroge-
nase. Which of the following statements is true?
A. The reaction will proceed until the enzyme concentration decreases.
B. The reaction will be more favorable at 0 degrees C.
C. A component of the enzyme transferred from A to B.
D. The free energy change (delta G) of the catalyzed reaction is the same as for
the uncatalyzed reaction.: D.
-In an enzyme-catalyzed reaction, the rate of a reaction is increased by a decrease in the activation energy. Furthermore,
enzymes are not changed or consumed during the course of the reaction. Also, the overall free energy change of the
reaction, delta G, remains unchanged in the presence of an enzyme.
2. Which of the following statements about enzyme kinetics is false?
A. An increase in the substrate concentration (at constant enzyme concentra-
tion) 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 degree 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 7.2.: A.
-Most enzymes in the human body operate a maximal activity around a temperature of 37 degrees C and a pH of
7.2, which is the pH of most body fluids. In addition, as characterized by the Michaelis-Menten model, enzymes form
an enzyme-substrate complex, which can either dissociate back into the enzyme and substrate or proceed to form
a product. We can eliminate B, C, and D. 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. Therefore statement A is not entirely true.
3. At which pH would pancreatic enzymes work at maximum activity?
A. 5.3
B. 6.7
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C. 7.2
D. 8.5: D.
-Pancreatic enzymes work optimally in the alkaline condition of the small intestine. It is not necessary to know the exact
pH at which these enzymes work because the only very basic pH is seen in (D), which is 8.5.
4. Some enzymes require the presence of a non-protein molecule to behave
catalytically. An enzyme devoid of this molecule is called a(n):
A. holoenzyme
B. apoenzyme
C. coenzyme
D. zymoenzyme: B.
-An enzyme devoid of its necessary cofactor is called an apo-enzyme and is catalytically inactive.
5. Which of the following factors determine an enzyme's specificity?
A. The 3D 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.: A.
-An enzyme's specificity is determined by the 3D shape of its active site. Regardless of which theory of enzyme specificity
we are discussing (lock and key or induced fit), the active site determines which substrate the enzyme will react with.
6. Enzymes increase the rate of a reaction by:
A. decreasing the activation energy.
B. increasing the overall free energy of the reaction.
C. both A and B.
D. none of the above.: A.
-Enzymes increase the rate of a reaction by decreasing the activation energy. They do not affect the overall free energy,
delta G, of the reaction.
7. Bonding between atoms of an enzyme such as trypsin is best described as:
A. peptide
B. saccharide
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C. ionic
D. van der Waals: A.
-Enzymes are proteins. Proteins are composed of amino acids linked together by peptide bonds. Choice (B) - Saccha-
ride, is a type of bond found in polysaccharides. Choice (C) - Ionic, is a chemical bond formed through electrostatic
interaction between positive and negative ions. Choice (D) may be formed in secondary or tertiary structures of an
enzyme but is not as good an option as (A).
8. In the equation below, substrate C is an allosteric inhibitor to enzyme 1. Which
of the following is another mechanism caused by substrate C?
A --> enzyme 1 --> B --> enzyme 2 --> C.
A. Competitive inhibition
B. Irreversible inhibition
C. Feedback enhancement
D. Negative feedback: D.
-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. There is not enough
information for (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 is it the opposite of what occurs when enzyme 1 activity
is reduced.
9. When lactose hydrolyzes its substrate, lactose, which of the following occurs?
A. Lactase retains it structure after the reaction.
B. Lactose retains its structure after the reaction.
C. Lactase increases the activation energy of the reaction.
D. Lactose decreases the activation energy of the reaction.: A.
-Choice (A) is correct answer because, by definition, an enzyme remains unchanged by the reaction that is catalyzes.
Choice (B) is incorrect because a substrate is changed by an enzymatic reaction. Choice (C) is not true, an enzyme would
decrease the activation energy. Choice (D) is also incorrect since a substrate does not affect the activation energy.
10. Discuss why competitive inhibition can be overcome with increasing sub-
strate concentration but non-competitive inhibition cannot be overcome.: -Com-
petitive inhibitors are structurally similar to the substrate of an enzyme. This means that in the course of catalyzing
, MCAT Practice questions
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reactions the enzyme can bind the inhibitor instead of the substrate. Imagine there are an equal number of inhibitor and
substrate molecules. The chance that the enzyme will bind the inhibitor is equal to the chance that it will interact with the
substrate. Thus, the rate of catalysis will be decreased, effectively inhibiting the enzyme. However, if the concentration
of substrate if 1,000 or 100,000 times greater than the concentration of inhibitor, then the chance the enzyme will
randomly interact with the inhibitor instead of the substrate falls dramatically (practically to zero).
-Noncompetitive inhibitors do not associate with the enzyme at the active site. Instead, they bind at an allosteric site.
When a noncompetitive inhibitor is bound, the enzyme cannot catalyze bind the substrate. Increasing the concentration
of substrate will not decrease the likelihood of a noncompetitive inhibitor associating at an allosteric site.
11. Why is negative feedback inhibition more common than positive feedback
loops? Are there biological examples when positive feedback is necessary?: -Most
biological conditions, such as blood pH, body temperature, and blood pressure, must be kept within a narrow range.
Negative feedback allows for the maintenance of these conditions. If a homeostatic system moves too far to one extreme,
negative feedback will brings the levels back within biologically acceptable conditions. Positive feedback, however, has
the opposite effect. The more a process moves in a particular direction, the more positive feedback will keep it moving
in that same direction. Two examples in which positive feedback is important are blood clotting and childbirth.
12. Compare and contrast the the concept of allosteric effectors with non-com-
petitive inhibition.: -First, all noncompetitive inhibitors bind at allosteric sites. The binding of noncompetitive
inhibitors, however, does not affect the binding of substrate to the active site (and vice versa). What noncompetitive
inhibitors do affect is the ability for the enzyme to catalyze the reaction and release product. In contrast, allosteric
effectors allow the enzyme to exist in two different confirmations (T and R), in which one state binds substrate more
readily than the other. Again, this is in direct contract to noncompetitive inhibitors, which do not influence the binding
of substrate.
13. Consider the pathway:
A --1-> B --2-> C --3-> D --4-> E
--5-> X --6-> Y
An increase in [E] leads to the inhibition of enzyme 3. All of the following are
results of the process except:
A. an increase in [B].
