A. Incorrect! Enzymes are not altered or consumed by the reactions they catalyze.

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1 CLEP Biology - Problem Drill 04: Enzymes and Cellular Metabolism No. 1 of Which of the following statements about enzymes is correct? (A) Enzymes are consumed in a reaction. (B) Enzymes act by lowering the activation energy of a reaction. (C) Enzymes form an irreversible complex with the substrate. (D) Enzymes do not require cofactors. (E) Enzymes are created in a reaction. Enzymes are not altered or consumed by the reactions they catalyze. B. Correct! Enzymes lower the energy barrier, or the activation energy, of a reaction. The enzyme-substrate complex is a transient step in catalysis. Some enzymes do require cofactors in order to catalyze chemical reactions. Enzymes are not altered or consumed by the reactions they catalyze. Certain facts about enzymes are important to keep in mind. Enzymes do not alter ΔG of a reaction or its equilibrium. They are not consumed or altered by the reaction they catalyze (therefore they cannot make permanent bonds to their substrates), and they affect reactions by lowering the amount of energy required for the reactants to reach the transition state (i.e., the energy barrier, or activation energy). Some enzymes require inorganic or organic cofactors for their activity. The correct answer is (B).

2 No. 2 of Which of the following statements about active sites is not true? (A) Active sites are found in a cleft on the enzyme. (B) The binding site holds the substrate by covalent interactions. (C) The catalytic site is where the chemical reaction occurs. (D) The active site exhibits elasticity. (E) Every statement is correct. By definition, active sites are located in a cleft or depression on the enzyme s 3- dimensional structure. B. Correct! Substrates interact with the amino acids that line the active site by non-covalent interactions. The catalytic site is indeed the site of the chemical reaction. By the induced-fit hypothesis, the active site is a changeable structure that responds to substrate binding with a conformational change. There is only one statement above is not true. Note that the question is asking which answer choice is NOT correct. Based on your knowledge of enzyme active sites, you should be prepared to answer this question by process of elimination. Active sites are located in clefts, or depressions, on the surface of an enzyme. They are lined by amino acid residues whose side chains can interact with the substrate by weak, non-covalent interactions, and the induced-fit model of substrate binding states that the active site adjusts itself for the correct substrate (i.e., it is not a pre-formed, rigid structure). The active site is the place where the chemical reaction will occur. The correct answer is (B).

3 No. 3 of Which of the following is not a cofactor? (A) Monovalent metal ions. (B) A coenzyme. (C) A prosthetic group. (D) The substrate. (E) None of the above Monovalent or divalent metal ions can act as cofactors for enzymes. In particular, they are known as enzyme activators. Coenzymes are organic cofactors. Prosthetic groups are tightly-bound cofactors. D. Correct! The substrate is the reactant for a given reaction that is acted upon by the enzyme. The substrate itself is not required to produce a catalytically-active enzyme. There is one option above as the right choice. Many enzymes require a cofactor for their enzymatic activity. Cofactors are nonprotein molecules, either inorganic or organic in nature, that bind enzymes either by non-covalent or covalent bonds. The enzyme without its cofactor is catalytically inactive and is called an apoenzyme. Once the cofactor is bound, the enzyme is called a holoenzyme. The holoenzyme is catalytically active and ready to accept substrates. The correct answer is (D).

4 No. 4 of The specificity of an enzyme towards a type of chemical bond is referred to as. (A) Group specificity. (B) Absolute specificity. (C) Stereospecificity. (D) Linkage specificity. (E) Proton specificity Group specificity refers to an enzyme that can act on a group of molecules sharing a similar structure. Absolute specificity indicates that an enzyme is specific for only one reaction. Stereospecificity refers to the preference of an enzyme for one isomer over another. D. Correct! As its name implies, enzymes with linkage specificity are specific for one type of chemical bond, or link, regardless of the rest of the molecular structure. This has nothing to do with the proton. Enzymes with linkage specificity are specific for one type of chemical bond, or link, regardless of the rest of the molecular structure. Enzymes must be able to distinguish between a number of possible substrates to find the right one(s). The different types of specificity exhibited by enzymes are: group, absolute, stereo, and linkage. The correct answer is (D).

5 No. 5 of A competitive inhibitor. (A) Binds at the active site. (B) Does not bind at the active site. (C) Alters V max only. (D) Binds at the allosteric site. (E) Can t bind at the active site A. Correct! As their name implies, competitive inhibitors compete with the substrate for access to the enzyme s active site. Non-competitive inhibitors bind enzymes at sites other than the active site, and so do not compete with substrates. Competitive inhibitors do not alter V max. Allosteric inhibitors bind the allosteric site. Competitive inhibitors do bind at the active site, as they compete with the substrate for access to the enzyme s active site. Competitive inhibitors are often structural analogues of the substrate. This is why they are able to compete with the substrate for binding to the enzyme s active site. Competitive inhibitors have medical significance in that a number of drugs act by competitive inhibition on several enzymes. The correct answer is (A).

6 No. 6 of An allosteric inhibitor influences an enzyme s activity by. (A) Competing with the substrate for the catalytic site. (B) Changing the specificity of the enzyme for the substrate. (C) Changing the nature of the products formed. (D) Changing the enzyme s conformation by binding to a site different from the active site. (E) Altering the concentration of the enzyme. Allosteric regulators do not bind to the active site of an enzyme, so they will not compete with the substrate. Allosteric inhibitors do not affect the specificity of an enzyme, though they may affect its affinity for a given substrate. Allosteric regulators do not affect the nature of the products formed. D. Correct! Allosteric regulators bind to the enzyme at a site other than the active site, inducing changes in its conformation. Conformation, not concentration is the one being altered in enzyme s activity. Allosteric regulators may be activators or inhibitors and bind to an enzyme at a site other than the active site to induce a conformational change in the protein. The effect may be a stabilization of the active conformation of the enzyme, or a stabilization of the inactive form of the enzyme. In addition, allosteric modulation may alter the affinity of the enzyme for its substrate. Often, enzymes subject to allosteric regulation are multi-subunit protein complexes, and allosteric modulation is commonly seen in cases of feedback inhibition. The correct answer is (D).

7 No. 7 of The increase in velocity of an enzyme-catalyzed reaction with an increase in ph, until the optimum ph is reached, is due to. (A) Altered ionization of the enzyme. (B) Increased stability of the Enzyme-Substrate complex. (C) A decrease in the reverse reaction. (D) Increased interaction of the enzyme with its cofactors. (E) The ionization of the substrate. A. Correct! The ionization state (i.e., whether there are positive or negative charges) of the substrate and the enzyme are important factors in determining whether they will bind, and is directly influenced by ph. If the Enzyme-Substrate complex were stabilized, the time to reach the transition state would be longer, and the reaction velocity would decrease instead of increase. An increase or decrease in ph affects an enzyme s activity, but not the equilibrium of the reaction. Cofactors are part of the holoenzyme and their status is not directly affected by changes in ph. It is not due to the ionization of substrates, rather the enzyme itself. The ionization state of the substrate and the enzyme are important factors in determining whether they will bind, and is directly influenced by ph. A number of factors will determine whether a substrate will locate and bind an enzyme s active site. These can include the concentrations of both the enzyme and the substrate in solution, their proximity and orientations, and their surface charges. The surface charges stem from the ionization state of the functional groups within the substrate and enzyme. For example, consider the change that occurs in a carboxyl group or an amino group at low and high ph. At low ph, the carboxyl group remains uncharged, but the amino group acquires a positive charge. At high ph, the carboxyl groups will be deprotonated to yield a negative charge, while the amino group will be uncharged. In this way, ph directly affects whether the substrate will be attracted to the active site. The correct answer is (A).

8 No. 8 of Where does the carbon dioxide that humans and other organisms exhale come from? (A) Chemical Gradients. (B) Protein Anabolism. (C) Krebs Cycle. (D) Oxidative Phosphorylation. (E) Stomach Acids Chemical gradients simply describe the differences in concentration of a molecule between two areas. Indeed, many examples of chemical gradients exist in the body; however, their existence is not the source of the carbon dioxide we exhale. We need to look for an answer involving chemical reactions. Protein anabolism describes the production of proteins from amino acids. Peptide bonds involve condensation reactions that release H 2 O, not CO 2. C. Correct! The Krebs Cycle is a series of redox reactions that ultimately produces the loaded electron carriers NADH and FADH 2. A byproduct of the Krebs Cycle is CO 2. Oxidative phosphorylation is important because it is the energetic payoff of aerobic respiration, with the production of 32 ATP by ATP synthase. CO 2, however, is not involved. It is the byproduct of the Krebs Cycle, which has nothing to do with the acids in a human s digestive system. Take the answer choices one by one and use the process of elimination. Chemical gradients describe a difference in concentration between two areas but they are not themselves a chemical reaction that would produce CO 2. Peptide bonds are formed when the amino group of one amino acid is linked to the carboxyl group of a different amino acid, and a molecule of water is lost as a result. This clearly does not produce CO 2 as a byproduct. The last two choices are steps 2 and 3 of aerobic respiration. Each turn of the Krebs Cycle produces CO 2, so choice C is correct. The correct answer is (C).

9 No. 9 of If aerobic respiration produces 36 ATP and anaerobic respiration produces only 2 ATP, why do cells use anaerobic respiration at all? (A) Advanced organisms do not perform anaerobic respiration. (B) Anaerobic respiration produces ATP much more quickly in times of stress. (C) Anaerobic respiration does not produce as much waste as aerobic respiration. (D) Anaerobic respiration is important for genetic reproduction. (E) None of the above. Anaerobic respiration is indeed an option for complex organisms, like humans. It is not the preferred method, but it acts as a backup system for those situations where the available oxygen is already being used for aerobic respiration but the cell needs to still produce more energy. B. Correct! Though it only produces 2 ATP, anaerobic respiration is a very rapid process. In the course of fermentation, which is the second step in anaerobic respiration, the intermediates of glycolysis are produced. This allows anaerobic respiration to cycle over and over again with very little delay. In terms of energetic efficiency, aerobic respiration produces 36 ATP from 1 glucose while anaerobic respiration produces just 2 ATP from 1 glucose. In this sense, anaerobic respiration is more wasteful than aerobic respiration, not less. Anaerobic respiration is not a salient feature of genetic reproduction. This choice can be eliminated quickly. There is actually one correct answer above. There are times when aerobic respiration will not be enough. For example, an organism may require a large amount of ATP in a short period of time. In this case, though energetically inefficient, anaerobic respiration offers a quick way to produce additional energy in the form of ATP. The correct answer is (B).

10 No. 10 of What is the final electron acceptor in aerobic respiration? (A) NADH. (B) Mitochondria. (C) Carbon Dioxide. (D) Oxygen. (E) Water NADH is indeed an electron acceptor (i.e., a molecule that can transport electrons between locations) that links the Krebs Cycle with Oxidative Phosphorylation; however, it is not the final electron acceptor in aerobic respiration. The Krebs Cycle takes place in the mitochondrial matrix (i.e., its central space) and oxidative phosphorylation occurs in the inner mitochondrial membrane, but the mitochondria itself is not an electron acceptor. CO 2 is a waste product of the Krebs Cycle, and not the final electron acceptor. D. Correct! The final electron transport chain of aerobic respiration occurs during oxidative phosphorylation. As the electrons complete the transport chain, they are passed to oxygen. This step is the aerobic (i.e., oxygen-requiring) feature of aerobic respiration. The final electron transport chain of aerobic respiration occurs during oxidative phosphorylation. As the electrons complete the transport chain, they are passed to oxygen. This step is the aerobic (i.e., oxygen-requiring) feature of aerobic respiration. This question forces you to break down aerobic respiration into its three steps: glycolysis, the Krebs cycle, and oxidative phosphorylation. Taking each step in turn, glycolysis does not involve an electron transport chain, the Krebs cycle involves electron transport, and oxidative phosphorylation involves electron transport. Of the above choices, NADH and oxygen can act as electron acceptors; however, NADH is involved in the Krebs cycle while oxygen comes into play during oxidative phosphorylation. Therefore, choice D oxygen is the correct answer. The correct answer is (D).