MCAT Practice Questions and Answers Graded A+
1. Consider a biochemical reaction A D.
to B, which is catalyzed by A-B de- -In an enzyme-catalyzed reaction, the rate of a reaction
hydrogenase. Which of the follow- is increased by a decrease in the activation energy. Fur-
ing statements is true? thermore, enzymes are not changed or consumed during
the course of the reaction. Also, the overall free energy
A. The reaction will proceed un- change of the reaction, delta G, remains unchanged in
til the enzyme concentration de- the presence of an enzyme.
creases.
B. The reaction will be more favor-
able 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 reac-
tion.
2. Which of the following statements A.
about enzyme kinetics is false? -Most enzymes in the human body operate a maximal
activity around a temperature of 37 degrees C and a pH
A. An increase in the substrate of 7.2, which is the pH of most body fluids. In addition, as
concentration (at constant enzyme characterized by the Michaelis-Menten model, enzymes
concentration) leads to propor- form an enzyme-substrate complex, which can either
tional increases in the rate of the dissociate back into the enzyme and substrate or proceed
reaction. to form a product. We can eliminate B, C, and D. An in-
B. Most enzymes operating in the crease in the substrate concentration, while maintaining
human body work best at a tem- a constant enzyme concentration, leads to a proportional
perature of 37 degree C. increase in the rate of the reaction only initially. However,
C. An enzyme-substrate complex once most of the active sites are occupied, the reaction
can either form a product or dis- rate levels ott, regardless of further increases in substrate
sociate back into the enzyme and concentration. At high concentrations of substrate, the
substrate. reaction rate approaches its maximal velocity and is no
, MCAT Practice Questions and Answers Graded A+
D. Maximal activity of many hu- longer changed by further increases in substrate con-
man enzymes occurs around pH centration. Therefore statement A is not entirely true.
7.2.
3. At which pH would pancreatic en- D.
zymes work at maximum activity? -Pancreatic enzymes work optimally in the alkaline con-
dition of the small intestine. It is not necessary to know
A. 5.3 the exact pH at which these enzymes work because the
B. 6.7 only very basic pH is seen in (D), which is 8.5.
C. 7.2
D. 8.5
4. Some enzymes require the pres- B.
ence of a non-protein molecule -An enzyme devoid of its necessary cofactor is called an
to behave catalytically. An enzyme apo-enzyme and is catalytically inactive.
devoid of this molecule is called
a(n):
A. holoenzyme
B. apoenzyme
C. coenzyme
D. zymoenzyme
5. Which of the following factors de- A.
termine an enzyme's specificity? -An enzyme's specificity is determined by the 3D shape
of its active site. Regardless of which theory of enzyme
A. The 3D shape of the active site. specificity we are discussing (lock and key or induced fit),
B. The Michaelis constant. the active site determines which substrate the enzyme
C. The type of cofactor required for will react with.
the enzyme to be active.
D. The prosthetic group on the en-
zyme.
, MCAT Practice Questions and Answers Graded A+
6. Enzymes increase the rate of a re- A.
action by: -Enzymes increase the rate of a reaction by decreasing
the activation energy. They do not attect the overall free
A. decreasing the activation ener- energy, delta G, of the reaction.
gy.
B. increasing the overall free ener-
gy of the reaction.
C. both A and B.
D. none of the above.
7. Bonding between atoms of an en- A.
zyme such as trypsin is best de- -Enzymes are proteins. Proteins are composed of amino
scribed as: acids linked together by peptide bonds. Choice (B) -
Saccharide, is a type of bond found in polysaccharides.
A. peptide Choice (C) - Ionic, is a chemical bond formed through
B. saccharide electrostatic interaction between positive and negative
C. ionic ions. Choice (D) may be formed in secondary or tertiary
D. van der Waals structures of an enzyme but is not as good an option as
(A).
8. In the equation below, substrate C D.
is an allosteric inhibitor to enzyme -By limiting the activity of enzyme 1, the rest of the
1. Which of the following is anoth- pathway is slowed, which is the definition of negative
er mechanism caused by substrate feedback. Choice (A) is incorrect because there is no
C? competition for the active site with allosteric interactions.
There is not enough information for (B) to be correct be-
A --> enzyme 1 --> B --> enzyme 2 cause we aren't told whether the inhibition is reversible.
--> C. In general, allosteric interactions are temporary. Choice
(C) is incorrect because is it the opposite of what occurs
A. Competitive inhibition when enzyme 1 activity is reduced.
B. Irreversible inhibition
C. Feedback enhancement
D. Negative feedback
, MCAT Practice Questions and Answers Graded A+
9. When lactose hydrolyzes its sub- A.
strate, lactose, which of the follow- -Choice (A) is correct answer because, by definition,
ing occurs? an enzyme remains unchanged by the reaction that is
catalyzes. Choice (B) is incorrect because a substrate is
A. Lactase retains it structure after changed by an enzymatic reaction. Choice (C) is not true,
the reaction. an enzyme would decrease the activation energy. Choice
B. Lactose retains its structure af- (D) is also incorrect since a substrate does not attect the
ter the reaction. activation energy.
C. Lactase increases the activation
energy of the reaction.
D. Lactose decreases the activa-
tion energy of the reaction.
10. Discuss why competitive inhibi- -Competitive inhibitors are structurally similar to the sub-
tion can be overcome with increas- strate of an enzyme. This means that in the course of
ing substrate concentration but catalyzing reactions the enzyme can bind the inhibitor
non-competitive inhibition cannot instead of the substrate. Imagine there are an equal
be overcome. 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, ettectively 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 en-
zyme at the active site. Instead, they bind at an allosteric
site. When a noncompetitive inhibitor is bound, the en-
zyme cannot catalyze bind the substrate. Increasing the
1. Consider a biochemical reaction A D.
to B, which is catalyzed by A-B de- -In an enzyme-catalyzed reaction, the rate of a reaction
hydrogenase. Which of the follow- is increased by a decrease in the activation energy. Fur-
ing statements is true? thermore, enzymes are not changed or consumed during
the course of the reaction. Also, the overall free energy
A. The reaction will proceed un- change of the reaction, delta G, remains unchanged in
til the enzyme concentration de- the presence of an enzyme.
creases.
B. The reaction will be more favor-
able 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 reac-
tion.
2. Which of the following statements A.
about enzyme kinetics is false? -Most enzymes in the human body operate a maximal
activity around a temperature of 37 degrees C and a pH
A. An increase in the substrate of 7.2, which is the pH of most body fluids. In addition, as
concentration (at constant enzyme characterized by the Michaelis-Menten model, enzymes
concentration) leads to propor- form an enzyme-substrate complex, which can either
tional increases in the rate of the dissociate back into the enzyme and substrate or proceed
reaction. to form a product. We can eliminate B, C, and D. An in-
B. Most enzymes operating in the crease in the substrate concentration, while maintaining
human body work best at a tem- a constant enzyme concentration, leads to a proportional
perature of 37 degree C. increase in the rate of the reaction only initially. However,
C. An enzyme-substrate complex once most of the active sites are occupied, the reaction
can either form a product or dis- rate levels ott, regardless of further increases in substrate
sociate back into the enzyme and concentration. At high concentrations of substrate, the
substrate. reaction rate approaches its maximal velocity and is no
, MCAT Practice Questions and Answers Graded A+
D. Maximal activity of many hu- longer changed by further increases in substrate con-
man enzymes occurs around pH centration. Therefore statement A is not entirely true.
7.2.
3. At which pH would pancreatic en- D.
zymes work at maximum activity? -Pancreatic enzymes work optimally in the alkaline con-
dition of the small intestine. It is not necessary to know
A. 5.3 the exact pH at which these enzymes work because the
B. 6.7 only very basic pH is seen in (D), which is 8.5.
C. 7.2
D. 8.5
4. Some enzymes require the pres- B.
ence of a non-protein molecule -An enzyme devoid of its necessary cofactor is called an
to behave catalytically. An enzyme apo-enzyme and is catalytically inactive.
devoid of this molecule is called
a(n):
A. holoenzyme
B. apoenzyme
C. coenzyme
D. zymoenzyme
5. Which of the following factors de- A.
termine an enzyme's specificity? -An enzyme's specificity is determined by the 3D shape
of its active site. Regardless of which theory of enzyme
A. The 3D shape of the active site. specificity we are discussing (lock and key or induced fit),
B. The Michaelis constant. the active site determines which substrate the enzyme
C. The type of cofactor required for will react with.
the enzyme to be active.
D. The prosthetic group on the en-
zyme.
, MCAT Practice Questions and Answers Graded A+
6. Enzymes increase the rate of a re- A.
action by: -Enzymes increase the rate of a reaction by decreasing
the activation energy. They do not attect the overall free
A. decreasing the activation ener- energy, delta G, of the reaction.
gy.
B. increasing the overall free ener-
gy of the reaction.
C. both A and B.
D. none of the above.
7. Bonding between atoms of an en- A.
zyme such as trypsin is best de- -Enzymes are proteins. Proteins are composed of amino
scribed as: acids linked together by peptide bonds. Choice (B) -
Saccharide, is a type of bond found in polysaccharides.
A. peptide Choice (C) - Ionic, is a chemical bond formed through
B. saccharide electrostatic interaction between positive and negative
C. ionic ions. Choice (D) may be formed in secondary or tertiary
D. van der Waals structures of an enzyme but is not as good an option as
(A).
8. In the equation below, substrate C D.
is an allosteric inhibitor to enzyme -By limiting the activity of enzyme 1, the rest of the
1. Which of the following is anoth- pathway is slowed, which is the definition of negative
er mechanism caused by substrate feedback. Choice (A) is incorrect because there is no
C? competition for the active site with allosteric interactions.
There is not enough information for (B) to be correct be-
A --> enzyme 1 --> B --> enzyme 2 cause we aren't told whether the inhibition is reversible.
--> C. In general, allosteric interactions are temporary. Choice
(C) is incorrect because is it the opposite of what occurs
A. Competitive inhibition when enzyme 1 activity is reduced.
B. Irreversible inhibition
C. Feedback enhancement
D. Negative feedback
, MCAT Practice Questions and Answers Graded A+
9. When lactose hydrolyzes its sub- A.
strate, lactose, which of the follow- -Choice (A) is correct answer because, by definition,
ing occurs? an enzyme remains unchanged by the reaction that is
catalyzes. Choice (B) is incorrect because a substrate is
A. Lactase retains it structure after changed by an enzymatic reaction. Choice (C) is not true,
the reaction. an enzyme would decrease the activation energy. Choice
B. Lactose retains its structure af- (D) is also incorrect since a substrate does not attect the
ter the reaction. activation energy.
C. Lactase increases the activation
energy of the reaction.
D. Lactose decreases the activa-
tion energy of the reaction.
10. Discuss why competitive inhibi- -Competitive inhibitors are structurally similar to the sub-
tion can be overcome with increas- strate of an enzyme. This means that in the course of
ing substrate concentration but catalyzing reactions the enzyme can bind the inhibitor
non-competitive inhibition cannot instead of the substrate. Imagine there are an equal
be overcome. 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, ettectively 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 en-
zyme at the active site. Instead, they bind at an allosteric
site. When a noncompetitive inhibitor is bound, the en-
zyme cannot catalyze bind the substrate. Increasing the
