Sample Problem 22.1 Metabolism Identify each of the following as catabolic or anabolic: a. digestion of lipids b. synthesis of lactase c. hydrolysis of sucrose d. synthesis of adenine monophosphate from adenine, ribose, and phosphate Solution a. The breakdown of large molecules involves catabolic reactions. b. The synthesis of large molecules involves anabolic reactions. c. The breakdown of sucrose to smaller molecules is a catabolic reaction. d. The combination of small molecules to give a large molecule is an anabolic reaction. Study Check 8.1 Identify the oxidation of glucose to CO2 and H2O as catabolic or anabolic. Answer catabolic

Sample Problem 22.2 ATP and Energy The reaction of glutamate (Glu) with ammonia (NH3) produces the amino acid glutamine (Gln). The reaction requires 3.4 kcal/mole. a. Is the formation of glutamine a catabolic or an anabolic reaction? b. Write the equation for the formation of glutamine (Gln) including the energy. c. Write the equation for the hydrolysis of ATP including the energy. d. Write the overall equation for the combined reactions including the net energy change. Solution a. anabolic b. Glu + NH3 + 3.4 kcal/mole → Gln c. ATP → ADP + Pi + 7.3 kcal/mole d. Glu + NH3 + ATP → Gln + ADP + Pi + 3.9 kcal/mole Study Check 22.2 Why are anabolic reactions that require energy coupled with the hydrolysis of a high-energy compound such as ATP? Answer The hydrolysis of a high-energy compound such as ATP provides energy to drive reactions that require energy.

Sample Problem 22.3 Coenzymes Describe the reactive part of each of the following coenzymes and the way each participates in metabolic pathways: a. FAD b. NAD+ Solution a. The oxidized form of FAD undergoes reduction when two nitrogen atoms in the flavin part react with 2H+ and 2 e– to give FADH2. The coenzyme FAD is utilized in oxidation reactions that produce carbon–carbon (C = C) double bonds. b. The oxidized form of NAD+ undergoes reduction when a carbon atom in the nicotinamide part reacts with 2H+ and 2 e–, to give NADH and H+. The coenzyme NAD+ is utilized in oxidation reactions that produce carbon–oxygen (C = O) double bonds. Study Check 22.3 Describe the reactive part of coenzyme A and how it participates in metabolic reactions. Answer In coenzyme A, the thiol group (—SH) of aminoethanethiol combines with an acetyl group to form acetyl coenzyme A. The HS—CoA participates in the transfer of acyl groups, usually acetyl groups.

Sample Problem 22.4 Digestion of Carbohydrates Indicate the carbohydrate that undergoes digestion in each of the following sites: a. mouth b. stomach c. small intestine Solution a. starches amylose and amylopectin (α(1 → 4)-glycosidic bonds only) b. essentially no digestion of carbohydrates c. dextrins, maltose, sucrose, and lactose Study Check 22.4 Describe the digestion of amylose, a polymer of glucose molecules joined by α(1 → 4)-glycosidic bonds. Answer The digestion of amylose begins in the mouth when salivary amylase hydrolyzes some of the α(1 S 4)-glycosidic bonds. In the small intestine, pancreatic amylase hydrolyzes more glycosidic bonds, and finally maltose is hydrolyzed by maltase to yield glucose.

Sample Problem 22.5 Reactions in Glycolysis Identify each of the following reactions as an isomerization, phosphorylation, or dehydration: a. A phosphate group is transferred to ADP to form ATP. b. 3-Phosphoglycerate is converted to 2-phosphoglycerate. c. A water molecule is removed from 2-phosphoglycerate. Solution a. phosphorylation b. isomerization c. dehydration Study Check 22.5 Indicate the reaction and enzyme that catalyzes each of the reactions in Sample Problem 22.5. Answer a. Phosphorylation in reaction 7 is catalyzed by phosphoglycerate kinase and in reaction 10 by pyruvate kinase. b. Isomerization in reaction 8 is catalyzed by phosphoglycerate mutase. c. Dehydration in reaction 9 is catalyzed by enolase.

Sample Problem 22.6 Regulation of Glycolysis How is glycolysis regulated by each of the following enzymes? a. hexokinase b. phosphofructokinase c. pyruvate kinase Solution a. High levels of glucose-6-phosphate inhibit hexokinase, which stops the addition of a phosphate group to glucose in reaction 1. b. Phosphofructokinase, which catalyzes the formation of fructose-1,6-bisphosphate, is inhibited by high levels of ATP, and activated by high levels of ADP and AMP. c. High levels of ATP or acetyl CoA inhibit pyruvate kinase, which stops the formation of pyruvate in reaction 10. Study Check 22.6 Name the molecule that inhibits hexokinase by feedback regulation in step 1 of glycolysis. Answer glucose-6-phosphate

Sample Problem 22.7 Pathways for Pyruvate Identify the pathway(s) that has each of the following characteristics: a. In this pathway, NAD+ is reduced to NADH + H+. b. The product of this pathway contains three carbon atoms. c. NADH is the oxidizing agent in this anaerobic pathway to give a two-carbon molecule. d. CO2 is a product of this aerobic pathway. Solution a. NAD+ is used to oxidize pyruvate to acetyl CoA with the loss of one carbon atom as CO2. b. Pyruvate is reduced to a three-carbon molecule of lactate by lactate dehydrogenase and NADH. c. Under anaerobic conditions, pyruvate is reduced to ethanol, a two-carbon molecule, and CO2 by NADH. d. Under aerobic conditions, pyruvate is oxidized to acetyl CoA and CO2 by NAD+. Study Check 22.7 After strenuous exercise, lactate is oxidized back to pyruvate by lactate dehydrogenase using NAD+. Write an equation to show this reaction. Answer

Sample Problem 22.8 Glycogen Metabolism Identify each of the following as part of the reaction pathways of glycogenolysis or glycogenesis: a. Glucose-1-phosphate is converted to glucose-6-phosphate. b. Glucose-1-phosphate forms UDP-glucose. c. The pathway that is activated by insulin. Solution a. glycogenolysis b. glycogenesis c. glycogenesis Study Check 22.8 Why do cells in the liver and kidneys provide glucose to raise blood glucose levels, but cells in skeletal muscle do not? Answer Only the cells in the liver and kidneys provide glucose-6-phosphatase, which is needed to catalyze the hydrolysis of glucose-6-phosphate to yield free glucose.

Sample Problem 22.9 Glycogen Metabolism The conversion of fructose-1,6-bisphosphate to fructose-6-phosphate is an irreversible reaction using a glycolytic enzyme. How does gluconeogenesis make this reaction happen? Solution This reaction is catalyzed by fructose-1,6-bisphosphatase, which cleaves a phosphate group using a hydrolysis reaction, a reaction that is energetically favorable. Study Check 22.9 Why is hexokinase in glycolysis replaced by glucose-6-phosphatase in gluconeogenesis? Answer The reaction catalyzed by hexokinase in glycolysis is irreversible.