20.1: ATP- the Universal Energy Currency Concept Review Exercise Why is ATP referred to as the energy currency of the cell? Answer ATP is the principal molecule involved in energy exchange reactions in biological systems. Exercises How do ATP and ADP differ in structure? Why does the hydrolysis of ATP to ADP involve the release of energy? Identify whether each compound would be classified as a high-energy phosphate compound. ATP glucose 6-phosphate creatine phosphate Identify whether each compound would be classified as a high-energy phosphate compound. ADP AMP glucose 1-phosphate Answers ATP has a triphosphate group attached, while ADP has only a diphosphate group attached. yes no yes 20.2: Stage I of Catabolism Concept Review Exercises Distinguish between each pair of compounds. pepsin and pepsinogen chymotrypsin and trypsin aminopeptidase and carboxypeptidase What are the primary end products of each form of digestion? carbohydrate digestion lipid digestion protein digestion In what section of the digestive tract does most of the carbohydrate, lipid, and protein digestion take place? Answers Pepsinogen is an inactive form of pepsin; pepsin is the active form of the enzyme. Both enzymes catalyze the hydrolysis of peptide bonds. Chymotrypsin catalyzes the hydrolysis of peptide bonds following aromatic amino acids, while trypsin catalyzes the hydrolysis of peptide bonds following lysine and arginine. Aminopeptidase catalyzes the hydrolysis of amino acids from the N-terminal end of a protein, while carboxypeptidase catalyzes the hydrolysis of amino acids from the C-terminal end of a protein. glucose, fructose, and galactose monoglycerides and fatty acids amino acids the small intestine Exercises What are the products of digestion (or stage I of catabolism)? What is the general type of reaction used in digestion? Give the site of action and the function of each enzyme. chymotrypsin lactase pepsin maltase Give the site of action and the function of each enzyme. α-amylase trypsin sucrase aminopeptidase What is the meaning of the following statement? “Bile salts act to emulsify lipids in the small intestine.” Why is emulsification important? Using chemical equations, describe the chemical changes that triglycerides undergo during digestion. What are the expected products from the enzymatic action of chymotrypsin on each amino acid segment? gly-ala-phe-thr-leu ala-ile-tyr-ser-arg val-trp-arg-leu-cys What are the expected products from the enzymatic action of trypsin on each amino acid segment? leu-thr-glu-lys-ala phe-arg-ala-leu-val ala-arg-glu-trp-lys Answers proteins: amino acids; carbohydrates: monosaccharides; fats: fatty acids and glycerol Chymotrypsin is found in the small intestine and catalyzes the hydrolysis of peptide bonds following aromatic amino acids. Lactase is found in the small intestine and catalyzes the hydrolysis of lactose. Pepsin is found in the stomach and catalyzes the hydrolysis of peptide bonds, primarily those that occur after aromatic amino acids. Maltase is found in the small intestine and catalyzes the hydrolysis of maltose. Bile salts aid in digestion by dispersing lipids throughout the aqueous solution in the small intestine. Emulsification is important because lipids are not soluble in water; it breaks lipids up into smaller particles that can be more readily hydrolyzed by lipases. gly-ala-phe and thr-leu ala-ile-tyr and ser-arg val-trp and arg-leu-cys 20.3: Overview of Stage II of Catabolism Concept Review Exercises What is a metabolic pathway? What vitamin is required to make coenzyme A? What is the net yield of Glycolysis as far as ATP? Name the enzymes that are key regulatory sites in Glycolysis. Why are the enzymes in the previous question targets for regulation? Why is the priming phase necessary? Draw the entire pathway for glycolysis including enzymes, reactants and products for each step. Where does beta-oxidation occur? What is the average net yield of ATP per carbon? Where exactly is water formed during the process of fatty acid degradation? (Hint: H 2 O is formed when when the one of the products of beta-oxidation is passed through another of the metabolic pathways) During the process of beta-oxidation, why is it that [FAD] is used to oxidize an alkane to an alkene while NAD + is used to oxidize an alchol to a carbonyl Draw out the entire process of the degradation of a triglyceride, include enzymes and products and reactants for each step. Answers A metabolic pathway is a series of biochemical reactions by which an organism converts a given reactant to a specific end product. pantothenic acid 20.4: Stage III of Catabolism Concept Review Exercises What is the main function of the citric acid cycle? Two carbon atoms are fed into the citric acid cycle as acetyl-CoA. In what form are two carbon atoms removed from the cycle? What are mitochondria and what is their function in the cell? Answers the complete oxidation of carbon atoms to carbon dioxide and the formation of a high-energy phosphate compound, energy rich reduced coenzymes (NADH and FADH 2 ), and metabolic intermediates for the synthesis of other compounds as carbon dioxide Mitochondria are small organelles with a double membrane that contain the enzymes and other molecules needed for the production of most of the ATP needed by the body. Exercises Replace each question mark with the correct compound. \(\mathrm{?\xrightarrow{aconitase}isocitrate}\) \(\mathrm{?\, +\, ? \xrightarrow{citrate\: synthase} citrate + coenzyme\: A}\) \(\mathrm{fumarate \xrightarrow{fumarase}\, ?}\) \(\mathrm{isocitrate + NAD^+ \xrightarrow{?} \alpha\textrm{-ketoglurate} + NADH + CO_2}\) Replace each question mark with the correct compound. \(\mathrm{malate + NAD^+ \xrightarrow{?} oxaloacetate + NADH}\) \(\mathrm{?\, +\, ? \xrightarrow{nucleoside\: diphosphokinase} GDP + ATP}\) \(\mathrm{\textrm{succinyl-CoA} \xrightarrow{\textrm{succinyl-CoA synthetase}} \,?\, +\, ?}\) \(\mathrm{succinate + FAD \xrightarrow{succinate\: dehydrogenase}\, ? + FADH_2}\) From the reactions in Exercises 1 and 2, select the equation(s) by number and letter in which each type of reaction occurs. isomerization hydration synthesis From the reactions in Exercises 1 and 2, select the equation(s) by number and letter in which each type of reaction occurs. oxidation decarboxylation phosphorylation What similar role do coenzyme Q and cytochrome c serve in the electron transport chain? What is the electron acceptor at the end of the electron transport chain? To what product is this compound reduced? What is the function of the cytochromes in the electron transport chain? What is meant by this statement? “Electron transport is tightly coupled to oxidative phosphorylation.” How are electron transport and oxidative phosphorylation coupled or linked? Answers citrate oxaloacetate + acetyl-CoA malate α-ketoglutarate hydrogenase complex reaction in 1a reaction in 1c reaction in 1b Both molecules serve as electron shuttles between the complexes of the electron transport chain. Cytochromes are proteins in the electron transport chain and serve as one-electron carriers. 20.5: Stage II of Carbohydrate Catabolism Concept Review Exercises In glycolysis, how many molecules of pyruvate are produced from one molecule of glucose? In vertebrates, what happens to pyruvate when plenty of oxygen is available? oxygen supplies are limited? In anaerobic glycolysis, how many molecules of ATP are produced from one molecule of glucose? Answers two Pyruvate is completely oxidized to carbon dioxide. Pyruvate is reduced to lactate, allowing for the reoxidation of NADH to NAD + . There is a net production of two molecules of ATP. Exercises Replace each question mark with the correct compound. \(\mathrm{fructose\: 1,6\textrm{-bisphosphate} \xrightarrow{aldolase}\, ?\, +\, ?}\) \(\mathrm{? + ADP \xrightarrow{pyruvate\: kinase} pyruvate + ATP}\) \(\mathrm{dihydroxyacetone\: phosphate \xrightarrow{?} glyceraldehyde\: 3\textrm{-phosphate}}\) \(\mathrm{glucose + ATP \xrightarrow{hexokinase} \, ? + ADP}\) Replace each question mark with the correct compound. \(\mathrm{fructose\: 6\textrm{-phosphate} + ATP \xrightarrow{?} fructose\: 1,6\textrm{-bisphosphate} + ADP}\) \(\mathrm{? \xrightarrow{phosphoglucose\: isomerase} fructose\: 6\textrm{-phosphate}}\) \(\mathrm{glyceraldehyde\: 3\textrm{-phosphate} + NAD^+ + P_i \xrightarrow{?} 1,3\textrm{-bisphosphoglycerate} + NADH}\) \(\mathrm{3\textrm{-phosphoglycerate} \xrightarrow{phosphoglyceromutase} \, ?}\) From the reactions in Exercises 1 and 2, select the equation(s) by number and letter in which each type of reaction occurs. hydrolysis of a high-energy phosphate compound synthesis of ATP From the reactions in Exercises 1 and 2, select the equation(s) by number and letter in which each type of reaction occurs. isomerization oxidation What coenzyme is needed as an oxidizing agent in glycolysis? Calculate the total number of molecules of ATP produced for each molecule of glucose converted to pyruvate in glycolysis. the number of molecules of ATP hydrolyzed in phase I of glycolysis. the net ATP production from glycolysis alone. How is the NADH produced in glycolysis reoxidized when oxygen supplies are limited in muscle cells? yeast? Calculate the number of moles of ATP produced by the aerobic oxidation of 1 mol of glucose in a liver cell. Of the total calculated in Exercise 9a, determine the number of moles of ATP produced in each process. glycolysis alone the citric acid cycle the electron transport chain and oxidative phosphorylation Answers glyceraldehyde 3-phosphate + dihydroxyacetone phosphate phosphoenolpyruvate triose phosphate isomerase glucose 6-phosphate reactions 1b, 1d, and 2a reaction 1b NAD + Pyruvate is reduced to lactate, and NADH is reoxidized to NAD + . Pyruvate is converted to ethanol and carbon dioxide, and NADH is reoxidized to NAD + . 20.6: Stage II of Lipid Catabolism Concept Review Exercises How are fatty acids activated prior to being transported into the mitochondria and oxidized? Draw the structure of hexanoic (caproic) acid and identify the α-carbon and the β-carbon. Answers They react with CoA to form fatty acyl-CoA molecules. Key Takeaways Fatty acids, obtained from the breakdown of triglycerides and other lipids, are oxidized through a series of reactions known as β-oxidation. In each round of β-oxidation, 1 molecule of acetyl-CoA, 1 molecule of NADH, and 1 molecule of FADH 2 are produced. The acetyl-CoA, NADH, and FADH 2 are used in the citric acid cycle, the electron transport chain, and oxidative phosphorylation to produce ATP. Exercises For each reaction found in β-oxidation, identify the enzyme that catalyzes the reaction and classify the reaction as oxidation-reduction, hydration, or cleavage. What are the products of β-oxidation? How many rounds of β-oxidation are necessary to metabolize lauric acid (a saturated fatty acid with 12 carbon atoms)? How many rounds of β-oxidation are necessary to metabolize arachidic acid (a saturated fatty acid with 20 carbon atoms)? When myristic acid (a saturated fatty acid with 14 carbon atoms) is completely oxidized by β-oxidation, how many molecules of each are formed? acetyl-CoA FADH 2 NADH When stearic acid (a saturated fatty acid with 18 carbon atoms) is completely oxidized by β-oxidation, how many molecules of each are formed? acetyl-CoA FADH 2 NADH What is the net yield of ATP from the complete oxidation, in a liver cell, of one molecule of myristic acid? What is the net yield of ATP from the complete oxidation, in a liver cell, of one molecule of stearic acid? Answers enoyl-CoA hydratase; hydration thiolase; cleavage acyl-CoA dehydrogenase; oxidation-reduction five rounds 7 molecules 6 molecules 6 molecules 112 molecules 20.7: Stage II of Protein Catabolism Concept Review Exercises Write the equation for the transamination reaction between alanine and oxaloacetate. Name the two products that are formed. What is the purpose of oxidative deamination? Answers pyruvate and aspartate Oxidative deamination provides a reaction in which the amino group [as the ammonium (NH 4 + ) ion] is removed from a molecule, not simply transferred from one molecule to another. Most of the NH 4 + ion is converted to urea and excreted from the body. Exercises Write the equation for the transamination reaction between valine and pyruvate. Write the equation for the transamination reaction between phenylalanine and oxaloacetate. What products are formed in the oxidative deamination of glutamate? Determine if each amino acid is glucogenic, ketogenic, or both. phenylalanine leucine serine Determine if each amino acid is glucogenic, ketogenic, or both. asparagine tyrosine valine Answers α-ketoglutarate, NADH, and NH 4 + glucogenic both glucogenic Additional Exercises Hydrolysis of which compound—arginine phosphate or glucose 6-phosphate—would provide enough energy for the phosphorylation of ATP? Why? If a cracker, which is rich in starch, is chewed for a long time, it begins to develop a sweet, sugary taste. Why? Indicate where each enzymes would cleave the short peptide ala-ser-met-val-phe-gly-cys-lys-asp-leu. aminopeptidase chymotrypsin Indicate where each enzymes would cleave the short peptide ala-ser-met-val-phe-gly-cys-lys-asp-leu. trypsin carboxypeptidase If the methyl carbon atom of acetyl-CoA is labeled, where does the label appear after the acetyl-CoA goes through one round of the citric acid cycle? If the carbonyl carbon atom of acetyl-CoA is labeled, where does the label appear after the acetyl-CoA goes through one round of the citric acid cycle? The average adult consumes about 65 g of fructose daily (either as the free sugar or from the breakdown of sucrose). In the liver, fructose is first phosphorylated to fructose 1-phosphate, which is then split into dihydroxyacetone phosphate and glyceraldehyde. Glyceraldehyde is then phosphorylated to glyceraldehyde 3-phosphate, with ATP as the phosphate group donor. Write the equations (using structural formulas) for these three steps. Indicate the type of enzyme that catalyzes each step. What critical role is played by both BPG and PEP in glycolysis? How is the NADH produced in glycolysis reoxidized when oxygen supplies are abundant? When a triglyceride is hydrolyzed to form three fatty acids and glycerol, the glycerol can be converted to glycerol 3-phosphate and then oxidized to form dihydroxyacetone phosphate, an intermediate of glycolysis. (In this reaction, NAD + is reduced to NADH.) If you assume that there is sufficient oxygen to completely oxidize the pyruvate formed from dihydroxyacetone phosphate, what is the maximum amount of ATP formed from the complete oxidation of 1 mol of glycerol? How is the FADH 2 from β-oxidation converted back to FAD? If 1 mol of alanine is converted to pyruvate in a muscle cell (through transamination) and the pyruvate is then metabolized via the citric acid cycle, the electron transport chain, and oxidative phosphorylation, how many moles of ATP are produced? If the essential amino acid leucine (2-amino-4-methylpentanoic acid) is lacking in the diet, an α-keto acid can substitute for it. Give the structure of the α-keto acid and the probable reaction used to form leucine from this α-keto acid. Answers The hydrolysis of arginine phosphate releases more energy than is needed for the synthesis of ATP, while hydrolysis of glucose 6-phosphate does not. The enzyme will cleave off amino acids one at a time beginning with alanine (the N-terminal end). following phenylalanine Half of the label will be on the second carbon atom of oxaloacetate, while the other half will be on the third carbon atom. When oxygen is abundant, NADH is reoxidized through the reactions of the electron transport chain. FADH 2 is reoxidized back to FAD via the electron transport chain.