Question 1

Which of the following best describes the role of an enzyme in a biological reaction, as illustrated by the difference between the uncatalyzed and catalyzed reaction energy diagrams?

Accepted Answer

Enzymes lower the activation energy ($$E_A) $$required for the reaction without changing the overall free energy change ($$\Delta G^\circ).$$

Question 2

A patient is exposed to a nerve gas that acts as an irreversible inhibitor of acetylcholinesterase. What is the most likely molecular effect of this inhibitor?

Accepted Answer

It binds covalently to the enzyme, causing permanent loss of catalytic activity.

Question 3

Which of the following statements about the Michaelis–Menten equation is correct?

Accepted Answer

The Michaelis constant ($$K_m) is $$the substrate concentration at which the reaction velocity is half of $$V_{max}.$$

Question 4

Which of the following is a property shared by all biological catalysts, including enzymes and ribozymes?

Accepted Answer

They increase the rate of reaction by lowering the activation energy ($$E_A).$$

Question 5

A mutation in the active site of an enzyme changes a key histidine residue to alanine. Which catalytic mechanism is most likely to be affected?

Accepted Answer

Proton transfer

Question 6

Which enzyme class is responsible for catalyzing the transfer of a phosphate group from ATP to glucose, as shown in the reaction: $$	ext{Glucose} + 	ext{ATP} ightarrow 	ext{Glucose-6-phosphate} + 	ext{ADP}$$?

Accepted Answer

Transferase

Question 7

Which of the following best explains why enzymes are highly specific for their substrates?

Accepted Answer

The three-dimensional structure and chemical properties of the active site allow only certain substrates to bind with high affinity.

Question 8

A competitive inhibitor of an enzyme will have which of the following effects on enzyme kinetics?

Accepted Answer

It increases the apparent $$K_m$$ but does not affect $$V_{max}.$$

Question 9

Which of the following is an example of enzyme regulation by covalent modification?

Accepted Answer

Phosphorylation of serine residues by protein kinases

Question 10

Which of the following statements about ribozymes is TRUE?

Accepted Answer

They are RNA molecules with catalytic activity.

Question 11

A researcher observes that an enzyme's activity increases with substrate concentration but eventually plateaus even as more substrate is added. What is the best explanation for this observation?

Accepted Answer

The enzyme has reached saturation, and all active sites are occupied.

Question 12

Which of the following best describes the induced-fit model of enzyme-substrate interaction?

Accepted Answer

Substrate binding induces a conformational change in the enzyme, optimizing the fit for catalysis.

Question 13

Which of the following is a likely consequence of increasing the temperature above the optimal range for a human enzyme?

Accepted Answer

The enzyme will denature, losing its tertiary structure and activity.

Question 14

Which of the following mechanisms is used by cells to rapidly and reversibly regulate enzyme activity in response to changing cellular needs?

Accepted Answer

Allosteric regulation by non-covalent binding of effectors

Question 15

Which of the following statements about enzyme cofactors is correct?

Accepted Answer

Cofactors can be metal ions or small organic molecules called coenzymes.

Question 16

A patient has a genetic mutation that causes the enzyme carboxypeptidase A to lose its group specificity. Which of the following is the most likely result?

Accepted Answer

The enzyme will no longer efficiently hydrolyze the carboxyl terminal peptide bond in polypeptides.

Question 17

Which of the following best describes feedback inhibition in a metabolic pathway?

Accepted Answer

The final product of the pathway inhibits an enzyme that acts earlier in the pathway.

Question 18

Which of the following is true regarding the effect of pH on enzyme activity?

Accepted Answer

Enzyme activity is highest within a narrow pH range, and deviations can disrupt ionic and hydrogen bonds.

Question 19

Which of the following is an example of an enzyme that is activated by proteolytic cleavage?

Accepted Answer

Trypsin, which is synthesized as inactive trypsinogen and activated in the intestine

Enzymes: The Catalysts of Life – Structure, Function, and Regulation

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