Unit 3 Progress Check MCQ

Scoring Guide Unit 3 Progress Check: MCQ Researchers investigated the influence of environmental on the activity of peroxidase, an enzyme that catalyzes the conversion of hydrogen peroxide to water and oxygen gas. In an experiment, the researchers added a hydrogen peroxide solution containing guaiacol to several identical test tubes and adjusted the solution in each test tube to a different . The researchers included the guaiacol because it caused the solutions to change color as the reactions proceeded, which the researchers relied on for measuring reaction rates. Finally, the researchers added the same amount of peroxidase to each test tube and measured the rate of each reaction at . The results of the experiment are represented in Figure 1. Figure 1. The effect of 1. on peroxidase activity Based on Figure 1, which of the following statements best predicts the effect that a change from a moderately acidic environment ( near 6) to a basic environment will have on peroxidase activity? (A) Peroxidase activity will decrease. (B) Peroxidase activity will increase. (C) Peroxidase activity will stay the same. (D) Peroxidase activity will increase at first and then decrease. Answer A Correct. Based on Figure 1, peroxidase’s optimal environment ( near 6) to a basic environment ( activity. 2. is 5. A change from a moderately acidic above 7) will result in a decrease in peroxidase Which of the following actions will provide a negative control for the investigation? (A) Repeating the experiment at (B) Repeating the experiment using twice the amount of hydrogen peroxide (C) Repeating the experiment using twice the amount of peroxidase (D) Repeating the experiment using heat-denatured peroxidase Scoring Guide Unit 3 Progress Check: MCQ Answer D Correct. A negative control is a group in which no response is expected. It is the opposite of the positive control, in which a known response is expected. Heat-denatured peroxidase will be inactive. Repeating the experiment using heat-denatured peroxidase will allow the researcher to measure reaction rates in the presence of inactive peroxidase. 3. One of the researchers proposes using oxygen gas production to measure reaction rates. Which of the following statements best justifies the use of the proposed modification as a way of creating an appropriate control for the investigation? (A) The experiment can be repeated without hydrogen peroxide, which will help eliminate an uncontrolled variable. (B) The experiment can be repeated without peroxidase, which will introduce a second independent variable. (C) The experiment can be repeated without guaiacol, which will reveal the effect of guaiacol on the reaction rates. (D) The experiment can be repeated without water, which will reveal whether the reaction can occur inside a living cell. Answer C Correct. In the original experiment, the researchers used guaiacol as a reaction indicator. By using oxygen gas production to measure reaction rates, the researchers no longer need the guaiacol and can repeat the experiment without it, which will reveal the effect of guaiacol on the reaction rates. 4. Aminolevulinate dehydratase ( ) is an enzyme that relies on zinc as a cofactor. A zinc ion binds to the 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 active site causes inhibition of . Which of the following statements best helps explain how the lead ion causes inhibition of ? (A) It changes the shape and charge of the substrate so that it becomes more compatible with active site. ’s (B) It changes the amino acid sequence of the reaction. (C) It changes the three-dimensional structure of the active site so that 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. protein so that the enzyme catalyzes a different is no longer compatible with Scoring Guide Unit 3 Progress Check: MCQ Answer C Correct. The substitution of a lead ion for a zinc ion most likely changes the three-dimensional structure of the 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. 5. Acetylcholinesterase is a protein that catalyzes the conversion of acetylcholine to acetate and choline. When the concentration of in an aqueous solution is held constant, the rate of the reaction catalyzed by 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) (B) The active site of active site at a time. high. (C) The (D) is specific for acetylcholine, and only one substrate molecule can occupy the begins converting product into substrate as the acetylcholine concentration changes from low to protein becomes denatured as the acetylcholine concentration changes from low to high. The substrate specificity of high. changes as the acetylcholine concentration changes from low to Answer A Correct. Acetylcholine is the substrate for , 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, 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. 6. 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? Scoring Guide Unit 3 Progress Check: MCQ (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 D Correct. 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. 7. Catalase is an enzyme found in yeast cells that facilitates the chemical breakdown of hydrogen peroxide to water and oxygen gas. An experiment was conducted to determine the effect of on catalase function. Five buffer solutions of varying (2, 4, 6, 8, and 10) were prepared and added to separate test tubes. Hydrogen peroxide was added to each test tube. Yeast was added, and the reactions were timed. After 1 minute the amount of oxygen gas released was determined by measuring the foam layer produced in each test tube. Figure 1 illustrates the experimental setup. Figure 1. Illustration of experimental procedure A set of five additional test tubes were prepared and used as controls. Which of the following best describes the contents expected to be contained in one of the five control test tubes?