Question 1

Which statement best describes the role of enzymes in biological systems?

Accepted Answer

Enzymes decrease the activation energy, thereby increasing reaction rates.

Question 2

A reaction has a standard free-energy change ($$\Delta G$$^{\circ'}$$) of +23.8$ kJ/mol. Under cellular conditions, the concentrations of products and reactants are such that $$\Delta G$$ is -7.9$ kJ/mol. What does this indicate about the reaction?

Accepted Answer

The reaction is endergonic under standard conditions but exergonic in the cell.

Question 3

Which of the following is TRUE regarding cofactors required by enzymes?

Accepted Answer

Cofactors can be either metal ions or vitamin-derived organic molecules.

Question 4

What is the equation that relates the free-energy change of a reaction to the standard free-energy change and the actual concentrations of reactants and products?

Accepted Answer

$$\Delta G = \Delta G$$^{\circ'}$$ + RT \ln \dfrac{[C][D]}{[A][B]}$$

Question 5

Ferrochelatase catalyzes the insertion of $$Fe^{2+}$$ into protoporphyrin IX. What structural change in protoporphyrin IX is necessary for this reaction to occur?

Accepted Answer

The porphyrin must bend, increasing exposure of pyrrole nitrogen lone pairs to solvent.

Question 6

Which of the following best explains why enzymes do not alter the equilibrium position of a chemical reaction?

Accepted Answer

Enzymes only increase the rate at which equilibrium is reached by lowering activation energy.

Question 7

A catalytic antibody (abzyme) was generated against a transition state analog of N-methylmesoporphyrin. What is the significance of this approach?

Accepted Answer

It shows that transition state binding is crucial for enzyme catalysis.

Question 8

Which of the following is a correct statement about the formation of the enzyme-substrate complex?

Accepted Answer

Water is largely excluded from the active-site cleft when the substrate is bound.

Question 9

In the glycolytic pathway, aldolase catalyzes the conversion of fructose 1,6-bisphosphate (FBP) to dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P). Given the following concentrations: $$[FBP] = 1.5 	imes 10$$^{-4}$$ M, $$[DHAP] = 4.3 	imes $$10^{-6} M$$, $$[G3P] = 9.6 	imes 10$$^{-5}$$ M, calculate the value of $$\ln \dfrac{[DHAP][G3P]}{[FBP]}$$ for use in the $$\Delta G$$ equation.

Accepted Answer

Question 10

Which of the following is a plausible reason why N-methylmesoporphyrin is a potent inhibitor of ferrochelatase?

Accepted Answer

It mimics the transition state and binds tightly to the enzyme, preventing substrate access.

Question 11

Which of the following best describes the catalytic power of enzymes?

Accepted Answer

Enzymes can enhance reaction rates by factors of $$10^6 or $$more.

Question 12

Why do biochemists often use $$\Delta G$$^{\circ'}$$ rather than $$\Delta $$G^{\circ}$$ when discussing biochemical reactions?

Accepted Answer

$$\Delta G$$^{\circ'}$$ is measured at pH 7, which is relevant for biological systems.

Question 13

Which of the following is a key step in enzyme catalysis?

Accepted Answer

Formation of the enzyme-substrate complex at the active site.

Question 14

What is the effect of an enzyme binding more tightly to the transition state than to the substrate?

Accepted Answer

It lowers the activation energy and increases the reaction rate.

Question 15

Which of the following statements about abzymes (catalytic antibodies) is correct?

Accepted Answer

Abzymes can catalyze reactions by binding transition state analogs as antigens.

Question 16

Which of the following is NOT a function of enzymes?

Accepted Answer

Changing the equilibrium constant of a reaction.

Question 17

In the context of enzyme catalysis, what is the 'transition state'?

Accepted Answer

The highest-energy species along the reaction pathway.

Question 18

Which of the following is a consequence of substrate binding to the enzyme active site?

Accepted Answer

Conformational changes in the enzyme that facilitate transition state formation.

Question 19

Which of the following best explains why the reaction catalyzed by aldolase is exergonic under intracellular conditions, even though it is endergonic under standard conditions?

Accepted Answer

The actual concentrations of reactants and products in the cell favor product formation.

Biochemistry: A Short Course 6

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