Question 1

A key role of the hydroxyl group at position 6 in the purine ring in the formation of a transition state by the enzyme adenosine deaminase is obtained by comparing a and a .
A) competitive inhibitor; noncompetitive inhibitor
B) transition state analog; normal substrate
C) noncompetitive inhibitor; transition state analog
D) competitive inhibitor; transition state analog
E) All of the above

Accepted Answer

The answer of A key role of the hydroxyl group...

Question 2

A reaction that occurs with every collision between reactant molecules is called a/an .
A) saturation point
B) diffusion-controlled reaction
C) rate-determining step
D) first order reaction

Accepted Answer

A diffusion-controlled reaction is one where every collision between reactant molecules leads to a reaction, as the reaction rate is limited by the rate of diffusion of the reactants.

Question 3

The graphs below all represent the same chemical reaction, but each employing a different catalyst. Which enzyme uses the most efficient mechanism of catalysis? 
A) see graph I
B) see graph II
C) see graph III
D) see graph IV

Accepted Answer

The answer of The graphs below all represent the same...

Question 4

The proximity effect of speeding an enzyme-catalyzed reaction is explained by .
A) a large loss of entropy when reactive groups are brought close to each other
B) a large gain in entropy due to binding of the substrate
C) a conversion of a reaction from endergonic to exergonic
D) increasing the flexibility of the substrate by increasing its degrees of freedom of rotation

Accepted Answer

The proximity effect refers to the increase in enzyme-catalyzed reaction rates that occurs when reactive groups in the enzyme and substrate are brought close together. This effect can be explained by a large loss of entropy when the reactive groups are brought close to each other, which leads to increased reactivity and faster reaction rates.

Question 5

In the nonpolar environment of most enzyme active sites, which statement applies to charge-charge interactions between the enzyme and the substrate?
A) They are rare due to the non-polar environment.
B) They are frequent, but not very strong in the nonpolar environment.
C) They are stronger in the nonpolar environment.
D) The polarity of the active site has no effect on the strength of the charge-charge interactions.

Accepted Answer

The answer of In the nonpolar environment of most enzyme...

Question 6

An enzymeʹs active site contains an arginine residue and a glutamate residue with pK_aʹs of 2.9 and 9.1, respectively. Both residues are actively involved in the catalytic mechanism and they are the only two ionizable residues in the active site. What would you expect for the optimum pH of the enzyme? A) 6.0 B) 7.0 C) 7.4 D) No pH can be determined since the information is irrelevant to the optimum pH.

Accepted Answer

The optimum pH is typically between the pKa values of the ionizable residues involved in catalysis. Here, it would be between 2.9 and 9.1, so around 6.0.

Question 7

The enzyme has an active site which
A) fits the substrate exactly.
B) fits the transition state.
C) may contain hydrogen bonds which are covalent-like.
D) A and C
E) B and C

Accepted Answer

The active site of an enzyme is specifically shaped to fit the transition state of the reaction, which is a high-energy intermediate state that occurs during the conversion of substrate to product. The active site may contain hydrogen bonds that are covalent-like, allowing for strong binding between the enzyme and substrate. Therefore, choices B and C are correct, and choice E is the best answer. Choice A is incorrect because the active site fits the transition state, not necessarily the substrate exactly.

Question 8

Acid-base catalysis is estimated to accelerate a typical enzymatic reaction by what factor? A) 1 to 2 fold increase B) 10 to 100 fold increase C) 10⁶fold increase D) 10²³fold increase

Accepted Answer

Acid-base catalysis can accelerate a typical enzymatic reaction by a factor of 10 to 100 fold.

Question 9

Some antibody molecules are enzymatic if they are formed against
A) enzymes.
B) enzymes bound to other proteins.
C) transition state analogs bound to other proteins.
D) transition state analogs.
E) All of the above

Accepted Answer

The answer of Some antibody molecules are enzymatic if they...

Question 10

The reaction catalyzed by a certain phosphatase enzyme is found to follow ping-pong kinetics and involves the transfer of a phosphate group from substrate A to substrate B. Which mode of catalysis is likely for this reaction?
A) sequential catalysis
B) acid-base catalysis
C) transfer catalysis
D) covalent catalysis

Accepted Answer

The answer of The reaction catalyzed by a certain phosphatase...

Question 11

A thermodynamic pit occurs when
A) ES is not very stable.
B) ES forms faster than it dissociates.
C) ES is highly stable.
D) S is not bound tightly to an enzyme.
E) S is positioned incorrectly to the enzyme.

Accepted Answer

A thermodynamic pit occurs when the enzyme-substrate complex (ES) is highly stable, meaning that the substrate is bound so tightly to the enzyme that it is not easily converted to product, leading to a decrease in reaction rate.

Question 12

An update of Fischerʹs lock-and-key theory of enzyme specificity view the as the lock and as the key.
A) enzyme; substrate
B) substrate; enzyme
C) enzyme; transition state
D) transition state; enzyme
E) substrate; transition state

Accepted Answer

The answer of An update of Fischerʹs lock-and-key theory of...

Question 13

Superoxide dismutase enzyme catalysis is faster than the rate of diffusion because it
A) is an acid-base catalyst.
B) is a two-step reaction.
C) has an electric field around the active site.
D) occurs in very high quantities in cells.

Accepted Answer

Superoxide dismutase enzyme catalysis is faster than the rate of diffusion because it has an electric field around the active site. This allows for the enzyme to attract and orient substrates more effectively, leading to faster catalysis. Acid-base catalysis (A) and two-step reactions (B) can also increase reaction rates, but they are not specific to superoxide dismutase. Occurring in high quantities (D) may increase the overall rate of the reaction, but it is not the reason why catalysis is faster than diffusion.

Question 14

The active site of a certain enzyme contains a serine residue. When the enzyme is incubated for a short time with its substrate, a form of the enzyme in which the active site serine is acetylated can be isolated and purified. In the native protein the serine is never found to be acetylated. This information supports .
A) a covalent catalysis mode
B) intermediate state stabilization effects
C) the acid-base catalysis mode
D) a polar group catalytic mechanism

Accepted Answer

The fact that the active site serine can be acetylated suggests that it is directly involved in the reaction mechanism and serves as a nucleophile. This supports a covalent catalysis mode, where the enzyme forms a covalent intermediate with the substrate during catalysis.

Question 15

What shape would a graph of reaction velocity versus pH have for an enzyme that uses both a proton donor and a proton acceptor during catalysis both acid and base catalysis)?
A) sigmoidal
B) hyperbolic
C) exponential
D) bell-shaped
E) linear

Accepted Answer

The graph would be bell-shaped because the enzyme activity depends on the presence of both a proton donor and a proton acceptor. At low pH, the enzyme may be too protonated, and at high pH, it may be too deprotonated, leading to optimal activity at an intermediate pH.

Question 16

Most K_mvalues of enzymes for their substrates are on the order of M. A) 10^-² B) 10^-³ C) 10^-⁴ D) 10^-⁵ E) 10^-⁶

Accepted Answer

The answer of Most K_mvalues of enzymes for their...

Question 17

When the concentration of a substrate inside a cell falls below the substrate concentration at half maximal velocity .
A) ES is formed more easily
B) ES dissociates to E + S
C) ES is closer to the ground state than to the transition state
D) E + S is in equilibrium with ES
E) the reaction will not proceed

Accepted Answer

When the substrate concentration falls below the concentration at half maximal velocity, there are fewer substrate molecules available to bind with the enzyme's active site. As a result, some of the ES complexes dissociate to regenerate the E and S molecules. This is known as substrate inhibition.

Question 18

Which graph might you expect for the pH profile of an enzymeʹs activity if the only ionizable residue in the active site is aspartate? 
A) I
B) II
C) III
D) IV

Accepted Answer

The answer of Which graph might you expect for the...

Question 19

Transition state analogs should A) stabilize transition states. B) have a dissociation constant of 10^-¹³M or less. C) bind very tightly to the enzyme. D) A and C E) B and C

Accepted Answer

The answer of Transition state analogs should
A) stabilize transition states.
B)...

Question 20

Aspartate and lysine are in the active site of an enzyme. They are both known to participate directly in catalysis. The pK_aʹs of the residues are found to be 3.2 and 9.6, respectively for aspartate and lysine. The optimum pH for the enzyme is 6.4. Which forms of these two residues will predominate when the enzyme is most active? A) aspartate is protonated; lysine is deprotonated B) both residues are protonated C) aspartate is deprotonated; lysine is protonated D) both residues are deprotonated

Accepted Answer

At pH 6.4, aspartate (pKa 3.2) will be deprotonated (negatively charged), and lysine (pKa 9.6) will be protonated (positively charged), which is optimal for enzyme activity.

Quiz 6: Mechanisms of Enzymes

A key role of the hydroxyl group at position 6 in the purine ring in the formation of a transition state by the enzyme adenosine deaminase is obtained by comparing a and a .

A reaction that occurs with every collision between reactant molecules is called a/an .

The graphs below all represent the same chemical reaction, but each employing a different catalyst. Which enzyme uses the most efficient mechanism of catalysis?

The proximity effect of speeding an enzyme-catalyzed reaction is explained by .

In the nonpolar environment of most enzyme active sites, which statement applies to charge-charge interactions between the enzyme and the substrate?

The enzyme has an active site which

Acid-base catalysis is estimated to accelerate a typical enzymatic reaction by what factor?

Some antibody molecules are enzymatic if they are formed against

The reaction catalyzed by a certain phosphatase enzyme is found to follow ping-pong kinetics and involves the transfer of a phosphate group from substrate A to substrate B. Which mode of catalysis is likely for this reaction?

A thermodynamic pit occurs when

An update of Fischerʹs lock-and-key theory of enzyme specificity view the as the lock and as the key.

Superoxide dismutase enzyme catalysis is faster than the rate of diffusion because it

What shape would a graph of reaction velocity versus pH have for an enzyme that uses both a proton donor and a proton acceptor during catalysis both acid and base catalysis) ?

Most K_mvalues of enzymes for their substrates are on the order of M.

When the concentration of a substrate inside a cell falls below the substrate concentration at half maximal velocity .

Which graph might you expect for the pH profile of an enzymeʹs activity if the only ionizable residue in the active site is aspartate?

Transition state analogs should