Deck 11: Enzymes: Biological Catalysts

Question 1

An enzyme that catalyzes the intramolecular movement of a functional group from one carbon atom to another would be called a(n)____________.

A)isomerase
B)transferase
C)oxidoreductase
D)kinase
E)ligase

Answer 1

A

Question 2

Which amino acid most commonly serves as a general acid and general base in an enzyme mechanism?

A)serine
B)arginine
C)aspartic acid
D)histidine
E)cysteine

Answer 2

D

Question 3

What type of inhibitor would give the results seen in the following plot?

A)competitive inhibitor
B)mixed inhibitor
C)noncompetitive inhibitor
D)uncompetitive inhibitor
E)none of the above

Answer 3

D

Question 4

If an enzyme gave a rate enhancement of 3.4 * 10⁷ over the noncatalyzed reaction, what is the $\Delta$$\Delta$ G1 ^{$\circ$ ‡} at 37 ^$\circ$ C?

A)5.3 kJ/mol
B)45 kJ/mol
C)9.5 kJ/mol
D)19 kJ/mol
E)none of the above

Question 5

Which of the following scenarios would result in a relatively low energy of activation?

A)an enzyme binds to the substrate with greater affinity than the transition state
B)an enzyme binds to both substrate and transition state with equal affinity
C)an enzyme binds to the transition state with greater affinity than the substrate
D)an enzyme binds only to the transition state
E)none of the above

Question 6

Which of the following cofactors is used during the activation and transfer of carbon dioxide?

A)nicotinamide adenine dinucleotide (NAD)
B)thiamine pyrophosphate
C)biotin
D)coenzyme A
E)tetrahydrofolate

Question 7

In the mechanism of chymotrypsin, which of the following amino acids found in the active site is correctly defined in terms of its role in the reaction?

A)serine: hydrogen bonds with carbonyl oxygen to withdraw electron density from the substrate
B)histidine: deprotonates aspartic acid to allow a nucleophilic attack to occur
C)aspartic acid: electrostatic stabilization of histidine to make a stronger base
D)cysteine: performs a nucleophilic attack on the carbonyl carbon of the substrate
E)none of the above

Question 8

In the following enzyme reaction scheme, what sort of multi-enzyme kinetics are shown?

A)ordered substrate binding with random product release
B)ordered substrate binding with ordered product release
C)random substrate binding with ordered product release
D)random substrate binding with random product release
E)ping-pong mechanism

Question 9

Since an enzyme is a catalyst, which of the following must be true?

A)an enzyme-catalyzed reaction is always exergonic
B)enzymes increase the rate of a reaction by providing a completely alternate mechanism to the uncatalyzed reaction
C)over the course of an enzyme-catalyzed reaction, the enzyme is not changed
D)in the absence of an enzyme, the reaction that is normally catalyzed by the enzyme will not occur
E)enzyme-catalyzed reactions never reach equilibrium

Question 10

Which of the following explains why enzymes are extremely effective catalysts?

A)an enzyme stabilizes the transition state
B)enzymes bind very tightly to substrates
C)enzymes release products very rapidly
D)an enzyme can convert a normally endergonic reaction into an exergonic reaction
E)an enzyme lowers the energy of activation only for the forward reaction

Question 11

What must be true if K_M is truly a measure of the affinity of enzyme and substrate?

A)k_cat must be larger than K_M
B)k_cat must be smaller than K_M
C)k_cat must be about equal to k₁
D)k_cat must be much smaller than k_-1
E)k_cat must be much larger than k_-1

Question 12

Since the product of the reaction catalyzed by hexokinase, glucose-6-phosphate (G6P), can act as both a competitive and uncompetitive inhibitor, what can be said about the interaction between G6P and hexokinase?

A)G6P binds only to active site of the enzyme
B)G6P binds only to a regulatory site of the enzyme
C)G6P binds to both the active site and a regulatory site of the enzyme
D)G6P binds to one of the substrates, ATP, thus preventing ATP from binding to the active site
E)none of the above

Question 13

In a Lineweaver-Burke plot, what does the slope represent?

A)K_M
B)V_max
C)V_max/K_M
D)K_M/V_max
E)none of the above

Question 14

Both ATP and CTP are allosteric regulators of the enzyme aspartate transcarbamoylase. Which of the following correctly identifies the activities of these two regulators?

A)ATP: activator; CTP: activator
B)ATP: activator; CTP: inhibitor
C)ATP: inhibitor; CTP: activator
D)ATP: inhibitor; CTP: inhibitor
E)none of the above

Question 15

In noncompetitive inhibition, which of the following best explains how the inhibitor binds to the enzyme?

A)the inhibitor binds to the active site only after the substrate binds
B)the inhibitor binds to a site other than the active site only before the substrate binds
C)the inhibitor binds to a site other than the active site only after the substrate binds
D)the inhibitor binds to a site other than the active site either before or after the substrate binds
E)none of the above

Question 16

The steady state assumption in enzyme kinetics:

A)insures that the product of an enzymatic reaction will always be formed
B)explains why enzymes are effective catalysts
C)states that the formation of ES is equal to its breakdown
D)is based upon the fact that the maximum velocity of an enzyme is very high
E)none of the above

Question 17

What are the expected changes in kinetics in the presence of a competitive inhibitor?

A)V_max decreases, K_M appears to decrease
B)V_max does not change, K_M appears to decrease
C)V_max decreases, K_M appears to increase
D)V_max does not change, K_M appears to increase
E)V_max decreases, K_M does not change

Question 18

Which of the following is the most common form of reversible covalent modification for control of enzyme activity?

A)acetylation/deacetylation
B)adenylation/deadenylation
C)phosphorylation/dephosphorylation
D)ADP-ribosylation/de-ADP-ribosylation
E)none of the above

Question 19

The activity of lysozyme is greater than 50% of maximum between pH 3.8 and 6.1 with peak activity occurring around pH 5. Below 3.8 and above 6.1, the activity drops rapidly. Which of the following provides the best explanation for this?

A)two histidine residues in the active site must be protonated for enzyme activity
B)the active site contains an aspartic acid and glutamic acid, both of which must be deprotonated
C)the active site contains an aspartic acid that must be deprotonated and a glutamic acid that must be protonated
D)the active site contains a histidine that must be protonated and a glutamic acid that must be deprotonated
E)none of the above

Question 20

An enzymatic reaction has a V_max of 100 $\mu$ M/min. At a substrate concentration of 5 $\mu$ M, the velocity is 25 $\mu$ M/min. What is the K_M for the reaction?

A)5 $\mu$ M
B)10 $\mu$ M
C)15 $\mu$ M
D)20 $\mu$ M
E)none of the above

Question 21

What term describes an inactive precursor of an enzyme such as the precursors to protease enzymes produced by the pancreas?

A)allosteric enzyme
B)zymogen
C)isozyme
D)ribozyme
E)hydrolase

Question 22

Below is a curve for the enzyme glycogen phosphorylase without any allosteric effectors present showing velocity as a function of the substrate, orthophosphate (P_i). Draw and label a curve that shows the result of addition of the allosteric activator, AMP. Draw and label a curve that shows the result of addition of the allosteric inhibitor, ATP. Glycogen phosphorylase is activated by phosphorylation. Which of these three curves most resembles what happens when glycogen phosphorylase is phosphorylated?

Question 23

Given the chymotrypsin catalytic triad and the peptide substrate, draw the first tetrahedral intermediate of the chymotrypsin mechanism.

Question 24

A plot of enzyme activity with and without an inhibitor present gave the following plot. What type of inhibitor is present? How does this inhibitor function? What changes are seen in V_max and K_M? Draw a line that approximates the result from addition of twice as much inhibitor to the reaction.