Question
Aminolevulinate dehydratase (ALAD) is an enzyme that relies on zinc as a cofactor. A zinc ion binds to the ALAD active site, where it forms favorable interactions with the side chains of three amino acids. Researchers have found that substituting a lead ion for a zinc ion in the ALAD active site causes inhibition of ALAD.
Which of the following statements best helps explain how the lead ion causes inhibition of ALAD?
A. It changes the shape and charge of the substrate so that it becomes more compatible with ALAD’s active site.
B. It changes the amino acid sequence of the ALAD protein so that the enzyme catalyzes a different reaction.
C. It changes the three-dimensional structure of the active site so that ALAD is no longer compatible with its substrate.
D. It changes the enzyme-substrate complex so that the transition state is more stable and the reaction proceeds at a faster rate.
▶️Answer/Explanation
Ans: C
The substitution of a lead ion for a zinc ion most likely changes the three-dimensional structure of the ALAD active site. The three-dimensional structure of the active site plays an important role in the compatibility between an enzyme and its substrate. Changing the structure of the active site will likely interfere with enzyme function by disrupting the enzyme-substrate interaction.
Question
Acetylcholinesterase (AChE) is a protein that catalyzes the conversion of acetylcholine to acetate and choline. When the concentration of AChE in an aqueous solution is held constant, the rate of the reaction catalyzed by AChE increases with increasing concentrations of substrate. At low concentrations of acetylcholine, a small increase in the substrate concentration results in a large increase in the reaction rate. At high concentrations of acetylcholine, however, a large increase in the substrate concentration results in only a small increase in the reaction rate.
Which of the following statements correctly explains the observed effect of the acetylcholine concentration on the rate of the enzyme-catalyzed reaction?
A. The active site of AChE is specific for acetylcholine, and only one substrate molecule can occupy the active site at a time.
B. AChE begins converting product into substrate as the acetylcholine concentration changes from low to high.
C. The AChE protein becomes denatured as the acetylcholine concentration changes from low to high.
D. The substrate specificity of AChE changes as the acetylcholine concentration changes from low to high.
▶️Answer/Explanation
Ans: A
Acetylcholine is the substrate for AChE, and since the shape and charge of the active site is specific for acetylcholine, only one substrate molecule can occupy the active site at a time. Therefore, at high concentrations of acetylcholine, AChE becomes saturated with substrate. When an enzyme is saturated with substrate, adding more substrate to the solution will not lead to an observable increase in the reaction rate.
Question
A researcher proposes a model to explain how enzyme-substrate interactions determine enzyme specificity. The model is based on the idea that substrate molecules form favorable interactions with the amino acid side chains in an enzyme’s active site.
Based on the model, which of the following statements best explains an enzyme’s specificity for a particular substrate molecule?
A. A hydrophilic molecule interacts with nonpolar side chains in the enzyme’s active site.
B. A hydrophobic molecule interacts with polar side chains in the enzyme’s active site.
C. A molecule with positive charges interacts with positively charged side chains in the enzyme’s active site.
D. A molecule with negative charges interacts with positively charged side chains in the enzyme’s active site.
▶️Answer/Explanation
Ans: D
Because opposite charges attract, a molecule with negative charges will form favorable interactions with positively charged side chains in an enzyme’s active site. Based on the model, the favorable interactions helps explain the enzyme’s specificity for the molecule as a substrate.
Question
Which of the following statements best helps explain the reaction specificity of an enzyme?
A. The free energy of the reactants is greater than the free energy of the products.
B. The equilibrium constant of the reaction is much greater than 1.
C. The shape and charge of the substrates are compatible with the active site of the enzyme.
D. The concentration of the enzyme inside living cells is greater than the concentration of substrate.
▶️Answer/Explanation
Ans: C
The reaction specificity of an enzyme is determined by the compatibility of the substrate with the enzyme’s active site. A substrate’s shape and charge affect how it fits into an enzyme’s active site and whether it forms favorable interactions with the enzyme.