Chapter 9 Review Quiz

Aerobic Metabolism I: The Citric Acid Cycle

. The citric acid cycle is occasionally called the Krebs Cycle in honor of its discoverer, Hans Krebs. It is also occasionally called the tricarboxylic acid cycle. Why?

. Glycols are bifunctional alcohols having hydroxyl groups on adjacent carbons and in structure, looking as though hydroxyl groups had been added to a double bond. Ethylene glycol (1,2-dihydroxyethane) is a common component of automotive antifreeze and results in some accidental and deliberate poisonings. Propylene glycol (1,2-dihydroxypropane) may also be used as antifreeze and is advertized as being "environmentally friendly." Actually, it might be appropriate to say that propylene glycol is friendlier to children and pets, as it is not poisonous if consumed in limited quantities. While we often overlook dicarboxylic acids, several take part in metabolism, occasionally with dangerous results. The danger of toxic ethylene glycol is that it is oxidized in the body to form oxalic acid, the smallest dicarboxylic acid, which chelates calcium ion in tissues, resulting in tetany and death. If propylene glycol were oxidized by the body, both the first and second carbon would be oxidized as completely as possible without breaking the carbon chain. Why would the product of the oxidation be unlikely to be poisonous?

. The Krebs cycle provides ample opportunity to practice using the IUPAC enzyme classification system. Which of the following is catalyzed by an oxidoreductase?

. Facultative anaerobes have metabolic processes which allow them to:

. What are the first organisms on earth thought to have lived upon?

. If we consider a mole of NADH to be equivalent in energy to 2.5 ATP and a mole of FADH2 to be equivalent in energy to 1.5 ATP, what is the total number of moles of ATP Which could be generated by oxidation of a mole of pyruvic acid via the citric acid cycle?

. The reduced coenzymes of the TCA cycle, such as FADH2 and NADH, must pass their electrons into the electron transport chain in order to produce ATP, but one step of the TCA cycle yields a compound with high transfer potential which can provide energy for phosphorylation of ADP at the substrate level. What is this high-energy compound?

. The theft of a five lb. package of a poisonous compound called "1080" recently caused a panic in the police department of a Council Bluffs, Iowa. 1080 Relies for its toxicity on the presence of fluoroacetic acid. Fluoroacetic acid is metabolized to fluorocitric acid, which blocks the citric acid cycle, resulting in an accumulation of citrate in cells. Ranchers in western states once used meat baited with 1080 to kill coyotes. Environmentalists are concerned that eagles may take the poisoned bait. Farmers growing corn are not as concerned with fluoroacetic acid. Why is fluoroacetic acid toxic to coyotes, but not corn?

. Enzymes of gluconeogenesis can use some intermediates from the citric acid cycle in the formation of glucose. Why, then, can't we put carbons from fatty acids into the cycle as acetyl- CoA and then take molecules out to make glucose?

. You may have learned the the term "amphoteric" used in general chemistry to describe a substance which could function as either an acid or a base. When we describe the TCA cycle as amphibolic, we mean that it:

. Which of the following substances would slow the citric acid cycle?

. The first step of the citric acid cycle is usually considered to be formation of citric acid. Prior to that step, however, pyruvate must be converted to acetyl Coenzyme A in a reaction catalyzed by pyruvate dehydrogenase. Among the coenzymes need for pyruvate dehydrogenase is thiamine pyrophosphate, also known as vitamin B1. Lack of this vitamin leads to beri-beri, a debilitating disease often suffered by prisoners of war. Which other enzyme system in the citric acid cycle also has a requirement for thiamine pyrophosphate?

. Ordinarily, entrance of the carbons of pyruvic acid into the TCA cycle requires loss of one of the carbons. There is, however, an enzyme which can insert all three of the carbons of pyruvate into the TCA cycle. Its name is:

. In addition to the regulators of enzyme activity within the citric acid cycle, two enzymes outside the cycle profoundly affect its activity. Which ones?

. Oxaloacetic acid can pass into the mitochondrial matrix if it is first converted into:

. Consider the standard reduction potentials of the following reactions:



The free energy change for a typical electrochemical reaction can be expressed by the equation:
ΔG = -nFE
Given the table above and your understanding of the equation for free energy change, which metal from the list would you predict would react most vigorously if dropped into water?

. The citric acid cycle is a major source of NADH, but on occasion, an excess of citric acid accumulates. Under those conditions, the carbons from citric acid are most likely to wind up in the body as:

. Clare Booth Luce, United States ambassador to the Vatican, was once poisoned by exposure to fumes from her wallpaper. The wallpaper in her villa was colored with a beautiful green pigment containing mercury. Apparently, her bedroom was located below the laundry, and vibration from the machines released dust containing mercury. Chief among the problems caused by mercury exposure is the fact that mercury attaches to sulfhydryl groups such as those of lipoic acid. Which step of metabolism would be blocked by mercury?

. NADH and NADPH are major coenzymes for oxidoreductases. While nearly equivalent in redox potential, the two coenzymes often function in different metabolic actions. In general:

. Animals oxidize carbohydrate and can use fragments of extra carbohydrate such as acetyl- CoA to form lipids. Animals can oxidize lipids and use the energy for metabolic processes, but are incapable of forming carbohydrate from lipids. Plants can form carbohydrate from lipids and from fragments such as acetyl-CoA by which of the following mechanisms:

. Early life forms lived in an atmosphere largely devoid of free oxygen molecules. Most present-day life forms use molecular oxygen as:

. It is estimated that, in 1916, 10,000 sharecroppers are estimated to have died in the southern United States due to pellagra, a disease characterized by dermatitis, dementia and weakness caused by lack of nicotinamide. Nicotinamide is an essential component of coenzymes:

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