Question 1

A mixture of enzyme and inhibitor is run through a size-exclusion chromatography column. The activity of the enzyme is assessed before and after the chromatography. The enzyme has more activity after the chromatography step. Which of the following is true?
A) The enzyme was not eluted fully from the column.
B) The enzyme was denatured during chromatography.
C) The inhibitor is a reversible inhibitor.
D) The inhibitor is an irreversible inhibitor.

Accepted Answer

The increase in enzyme activity after size-exclusion chromatography suggests that the inhibitor was separated from the enzyme, which is consistent with the behavior of a reversible inhibitor. Reversible inhibitors can dissociate from the enzyme, allowing for the recovery of enzyme activity once the inhibitor is removed, such as through chromatography.

Question 2

Which of the following statements are true?
A) ATCase is regulated by feedback inhibition.
B) CTP is an allosteric activator of ATCase.
C) ATP is an allosteric inhibitor of ATCase.
D) GTP is an allosteric inhibitor of ATCase.

Accepted Answer

ATCase is regulated by feedback inhibition, where the end product of the pathway, CTP (cytidine triphosphate), acts as an allosteric inhibitor, not an activator. This means it binds to a site other than the active site on the enzyme, causing a conformational change that reduces the enzyme's activity. ATP, on the other hand, acts as an allosteric activator, countering the inhibitory effect of CTP and thus promoting the synthesis of pyrimidine nucleotides when purine nucleotides are abundant. This creates a balance between the production of purine and pyrimidine nucleotides.

Question 3

Which answer correctly classifies the compound with its relationship to aspartate transcarbamoylase?
A) ATP; allosteric inhibitor
B) ATP; allosteric activator
C) CTP; allosteric activator
D) CTP; substrate

Accepted Answer

ATP is an allosteric activator of aspartate transcarbamoylase, promoting the biosynthesis of pyrimidines when energy and building blocks are abundant.

Question 4

Which of the following is NOT a primary mechanism that affects catalytic efficiency?
A) binding of regulatory molecules
B) covalent modification
C) proteolytic processing
D) cofactor degradation

Accepted Answer

The binding of regulatory molecules, covalent modification, and proteolytic processing are all mechanisms that can affect catalytic efficiency. However, cofactor degradation is not a primary mechanism that affects catalytic efficiency. While the presence or absence of cofactors can affect enzyme activity, cofactor degradation is usually a secondary effect and not a direct mechanism of catalytic regulation.

Question 5

Below is a Lineweaver-Burk plot in which the axis labels have been removed. Interpret the plot to determine the v_max. A) 0.2 B) 5 C) 3 D) 0.33

Accepted Answer

The answer of Below is a Lineweaver-Burk plot in which...

Question 6

An experiment is performed in which the kinetics of an enzyme-catalyzed reaction at different pHs is monitored. It is found that the K_m does not change but that the k_cat increases as the pH goes above 7. Which of the following is true? A) A chemical group within the enzyme that has a pKa of around 7 is likely involved in the catalytic mechanism. B) A chemical group with a pKa of around 7 must be deprotonated in order for substrate to bind. C) A chemical group with a pKa of around 7 must be positively charged in order for the substrate to bind. D) Protons are acting as positive heterotropic allosteric effectors of this enzyme.

Accepted Answer

The fact that the kcat increases as pH goes above 7 suggests that a chemical group with a pKa around 7 is involved in the catalytic mechanism. This chemical group is likely a residue within the active site of the enzyme that is important for catalysis.

Question 7

The specificity constant is equal to A) [E_t][S] / v₀. B) K_m / v₀. C) k_cat / K_m. D) k_cat / [E_t].

Accepted Answer

The specificity constant is defined as kcat/Km, where kcat is the turnover rate (number of substrate molecules converted to product per enzyme molecule per unit time) and Km is the Michaelis constant (substrate concentration at which the initial reaction rate is half of the maximum). Therefore, the correct formula for the specificity constant is kcat/Km, which is equivalent to choice C: kcat/[S].

Question 8

A kinase adds a phosphate group to a target enzyme, altering the catalytic efficiency of the enzyme. This is an example of
A) covalent modification.
B) proteolytic processing.
C) binding of regulatory molecules.
D) feedback inhibition.

Accepted Answer

Addition of a phosphate group to an enzyme by a kinase is an example of covalent modification, as it involves a chemical alteration to the structure of the enzyme. Proteolytic processing involves the cleavage of a protein into smaller fragments. Binding of regulatory molecules involves the binding of a molecule to a protein to alter its activity. Feedback inhibition involves the product of a metabolic pathway inhibiting an earlier step in the pathway to regulate its activity.

Question 9

Acetylcholinesterase is an important enzyme in the nervous system. Acetylcholinesterase activity is blocked by the nerve agent sarin gas, which forms a covalent bond with a Ser in the active site of the enzyme. Sarin gas is a(n)
A) allosteric effector.
B) competitive inhibitor.
C) reversible inhibitor.
D) irreversible inhibitor.

Accepted Answer

Sarin gas forms a covalent bond with a Ser in the active site of acetylcholinesterase, which means it becomes an irreversible inhibitor. Therefore, the best choice is D - irreversible inhibitor.

Question 10

An inhibitor that binds only to the ES complex and not free enzyme is known as a(n) __________ inhibitor.
A) irreversible
B) competitive
C) uncompetitive
D) mixed

Accepted Answer

An uncompetitive inhibitor binds only to the ES complex and not free enzyme. This type of inhibitor decreases both the maximum rate of the reaction and the apparent Km (affinity of the enzyme for the substrate). It does not compete with the substrate for the active site on the enzyme, but instead binds to a different site on the enzyme-substrate complex, resulting in a conformational change that prevents the release of product.

Question 11

The Lineweaver-Burk plot shows data obtained for an enzyme in the absence and presence of a noncompetitive inhibitor. If the [I] is increased significantly in the experiment, the V_max^app would and the K ^appwould _. A) decrease; decrease B) stay the same; decrease C) decrease; stay the same D) stay the same; stay the same

Accepted Answer

The answer of The Lineweaver-Burk plot shows data obtained for...

Question 12

A plot of 1 / v₀ versus 1/[S] is called a plot. Data in this plot have a slope equal to _. A) Lineweaver-Burk; K_m/v_max B) Lineweaver-Burk; -1/K_m C) Michaelis-Menten; K_m/v_max D) Michaelis-Menten; v_max

Accepted Answer

The correct plot is called a Lineweaver-Burk plot, and its slope equals Km/Vmax.

Question 13

The Lineweaver-Burk plot shows data obtained for an enzyme in the absence and presence of a reversible inhibitor. Which type of inhibitor was used in the experiment? 
A) competitive
B) uncompetitive
C) mixed
D) noncompetitive

Accepted Answer

The answer of The Lineweaver-Burk plot shows data obtained for...

Question 14

A Lineweaver-Burk plot displays parallel lines for an enzyme in the absence and presence of increasing amounts of an inhibitor. The inhibitor in this experiment A) binds both the free enzyme and the ES complex. B) is competitive. C) alters the K_m but not the v_max. D) is uncompetitive.

Accepted Answer

Uncompetitive inhibitors bind only to the enzyme-substrate complex, leading to parallel lines on a Lineweaver-Burk plot, indicating that both $K_m$ and $V_{max}$ are affected proportionally.

Question 15

An enzyme undergoes a mutation that causes it to lose the ability to be regulated via phosphorylation. Which of the following mutations may lead to this loss of regulation? Assume that the overall structure is not altered by the mutation.
A) Ser $\rightarrow$Thr
B) Thr $\rightarrow$ Ser
C) Tyr $\rightarrow$ Phe
D) Ser $\rightarrow$ Tyr

Accepted Answer

The mutation from Tyrosine (Tyr) to Phenylalanine (Phe) removes a hydroxyl group, which is necessary for phosphorylation. Tyrosine can be phosphorylated, but Phenylalanine cannot, leading to the loss of regulation via phosphorylation.

Question 16

Which of the following pairs correctly matches the type of interaction observed between an inhibitor and an enzyme with the type of inhibition?
A) ionic; irreversible
B) covalent; irreversible
C) hydrogen bonding; reversible
D) hydrophobic; irreversible

Accepted Answer

Ionic and covalent interactions are generally irreversible inhibition, meaning that the inhibitor irreversibly binds to the enzyme, permanently deactivating it. Hydrophobic interactions may lead to irreversible inhibition in extreme cases but typically are reversible.
On the other hand, hydrogen bonding is a weaker, reversible interaction; therefore, the correct pair is hydrogen bonding and reversible inhibition.

Question 17

A plot of v_o versus [S] for aspartyl transcarbamoylase displays three sigmoidal lines. If the line in the middle represents the enzyme activity in the absence of any allosteric effectors, then the line to the __________ represents the enzyme in the __________ when bound to __________. A) right; R state; CTP B) right; T state; CTP C) left; R state; GTP D) left; T state; GTP

Accepted Answer

The sigmoidal curve in the middle represents the enzyme activity in the absence of any allosteric effectors, i.e. the enzyme in the R state. The curve to the right represents the enzyme in the R state when bound to CTP, which is an allosteric inhibitor. Therefore, the curve to the left must represent the enzyme in the T state when bound to an allosteric activator, and GTP is known to be an allosteric activator of aspartyl transcarbamoylase. The correct choice is B - right; T state; CTP.

Question 18

When compared with the T state of aspartate transcarbamoylase, the R state A) has dissociated into two C₃R₃ complexes. B) has greater separation of the catalytic subunits. C) is bound to CTP. D) has substrate bound in the ATP binding site.

Accepted Answer

The R state of aspartate transcarbamoylase (ATCase) has a greater separation of the catalytic subunits compared to the T state. This is due to a conformational change that occurs upon binding of substrate or activator molecules, which promotes the conversion of the enzyme from the T state to the R state. Dissociation into two complexes (A) or substrate bound in the ATP binding site (D) are not characteristic of the R state of ATCase. CTP binding (C) is associated with the T state of the enzyme.

Question 19

Which type of interaction is more likely to be found between an enzyme and an irreversible inhibitor?
A) covalent bond
B) hydrogen bond
C) ionic interaction
D) van der Waals interaction

Accepted Answer

Irreversible inhibitors form covalent bonds with the enzyme, making it impossible for the enzyme to function properly.

Question 20

In the formation of an ESI complex, __________ inhibition can result.
A) mixed
B) competitive
C) covalent
D) anticompetitive

Accepted Answer

Mixed inhibition can result in the formation of an ESI (enzyme-substrate-inhibitor) complex, as the inhibitor can bind to either the enzyme alone or the enzyme-substrate complex.

Quiz 7: Enzyme Mechanisms

A mixture of enzyme and inhibitor is run through a size-exclusion chromatography column. The activity of the enzyme is assessed before and after the chromatography. The enzyme has more activity after the chromatography step. Which of the following is true?

Which of the following statements are true?

Which answer correctly classifies the compound with its relationship to aspartate transcarbamoylase?

Which of the following is NOT a primary mechanism that affects catalytic efficiency?

Below is a Lineweaver-Burk plot in which the axis labels have been removed. Interpret the plot to determine the v_max.

An experiment is performed in which the kinetics of an enzyme-catalyzed reaction at different pHs is monitored. It is found that the K_m does not change but that the k_cat increases as the pH goes above 7. Which of the following is true?

The specificity constant is equal to

A kinase adds a phosphate group to a target enzyme, altering the catalytic efficiency of the enzyme. This is an example of

Acetylcholinesterase is an important enzyme in the nervous system. Acetylcholinesterase activity is blocked by the nerve agent sarin gas, which forms a covalent bond with a Ser in the active site of the enzyme. Sarin gas is a(n)

An inhibitor that binds only to the ES complex and not free enzyme is known as a(n) __________ inhibitor.

The Lineweaver-Burk plot shows data obtained for an enzyme in the absence and presence of a noncompetitive inhibitor. If the [I] is increased significantly in the experiment, the V_max^app would and the K ^appwould _.

A plot of 1 / v₀ versus 1/[S] is called a plot. Data in this plot have a slope equal to _.

The Lineweaver-Burk plot shows data obtained for an enzyme in the absence and presence of a reversible inhibitor. Which type of inhibitor was used in the experiment?

A Lineweaver-Burk plot displays parallel lines for an enzyme in the absence and presence of increasing amounts of an inhibitor. The inhibitor in this experiment

An enzyme undergoes a mutation that causes it to lose the ability to be regulated via phosphorylation. Which of the following mutations may lead to this loss of regulation? Assume that the overall structure is not altered by the mutation.

Which of the following pairs correctly matches the type of interaction observed between an inhibitor and an enzyme with the type of inhibition?

A plot of v_o versus [S] for aspartyl transcarbamoylase displays three sigmoidal lines. If the line in the middle represents the enzyme activity in the absence of any allosteric effectors, then the line to the represents the enzyme in the when bound to __.

When compared with the T state of aspartate transcarbamoylase, the R state

Which type of interaction is more likely to be found between an enzyme and an irreversible inhibitor?

In the formation of an ESI complex, __________ inhibition can result.