Answers and Solutions to Text Problems

Transcription

1 24 Answers and Solutions to Text Problems 24.1 Other names for the citric acid cycle are the Krebs cycle and tricarboxlyic acid cycle The citric acid cycle begins by combining acetyl CoA and oxaloacetate One turn of the citric acid cycle converts 1 acetyl CoA to 2CO 2, 3NADH + 3H +, FADH 2, GTP (ATP), and HS CoA Oxaloacetate is regenerated at the end of the citric acid cycle? 24.5 The reactions in steps 3 and 4 involve oxidative decarboxylation, which reduces the length of the carbon chain by one carbon in each reaction There is a dehydration in reaction 2, 24.7 NAD + is reduced by the oxidation reactions 3, 4, and 8 of the citric acid cycle FAD is reduced in reaction 6 when succinate is converted to fumarate In reaction 5, GDP undergoes a direct substrate phosphorylation to yield GTP, which converts ADP to ATP and regenerates GDP for the citric acid cycle NADH and 1 FADH 2 are produced in one turn of the citric acid cycle a. The six-carbon compounds in the citric acid cycle are citrate and isocitrate. b. Decarboxylation reactions remove carbon atoms as CO 2, which reduces the number of carbon atoms in a chain (reactions 3 and 4). c. The one five-carbon compound is _-ketoglutarate. d. Several reactions are oxidation reactions; isocitrate _-ketoglutarate; _-ketoglutarate succinyl CoA; succinate fumarate; malate oxaloacetate e. Secondary alcohols are oxidized in reactions 3 and a. Two CO 2 per citric acid cycle b. Succinyl CoA, succinate, fumarate, malate, oxaloacetate c. One GTP d. Oxidations of isocitrate (reaction 3) and α-ketoglutarate (reaction 4) e. Hydration of aconitate (reaction 1) and fumarate (reaction 6) a. Citrate synthase combines oxaloacetate with acetyl CoA. b. Succinate dehydrogenase and aconitase converts a carbon-carbon single bond to a double bond. c. Fumarase adds water to the double bond in fumarate; aconitase adds water to the double bond in iconitate from citrate a. aconitase b. α-ketoglutarate dehydrogenase c. nucleotide phosphokinase a. NAD + accepts 2H from the oxidative decarboxylation of isocitrate. b. GDP is phosphorylated in the formation of succinate. c. FAD accepts 2H when the carbon-carbon single bond in succinate is oxidized to a carboncarbon double bond in fumarate a. NAD + b. NAD + c. NAD +

2 Chapter 24 Answers and Solutions Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase are allosteric enzymes, which increase or decrease the flow of materials through the citric acid cycle Low levels of NADH activate the citric acid cycle and high levels of NADH inhibit the cycle High levels of ADP means there are low levels of ATP. To provide more ATP for the cell, the reaction rate of the citric acid cycle increases When NADH and ATP levels are high, there is a decrease in the production of acetyl CoA from pyruvate, which slows the rate of the citric acid cycle. Low levels of ATP stimulate the conversion of pyruvate to acetyl CoA providing fuel for the citric acid cycle and increasing the rate The Fe 3+ is the oxidized form of the iron in cytochrome c FMNH 2 is the reduced form a. The loss of 2H + and 2e - is oxidation. b. The gain of 2H + + 2e - is reduction a. reduction b. oxidation NADH and FADH 2 produced in glycolysis, oxidation of pyruvate, and the citric acid cycle provide the electrons for electron transport As electrons move along the electron transport chain, the energy level drops FAD is reduced to FADH 2, which provides 2H + and 2e - for coenzyme Q, then cytochrome b, and then cytochrome c NAD +, FMN, cytochrome a 3, O The mobile carrier coenzyme Q (or Q) transfers electrons from complex I to III. It also transfers electrons from complex II to complex III Cytochrome c is the mobile carrier that carries an electron from complex III to complex IV> When NADH transfers electrons to FMN in complex I, NAD + is produced The electrons from FADH 2 are transferred to coenzyme Q to give QH 2 and oxidized FAD a. NADH + H + + FMN NAD + + FMNH 2 b. QH Fe 3+ cyt b Q + 2 Fe 2+ cyt b + 2H a. Q + FADH 2 QH 2 + FAD b. 2 cyt a (Fe 3+ ) + 2 cyt a 3 (Fe 2+ ) 2 cyt a (Fe 2+ ) + 2 cyt a 3 (Fe 3+ ) In oxidative phosphorylation, the energy from the oxidation reactions in the electron transport chain is used to drive ATP synthesis As energy is released from oxidations along the transport chain, it is used to move protons into the intermembrane space. The accumulation of high-energy protons creates a proton gradient.

3 Metabolism and Energy Production Protons must pass through F 0 channel of ATP synthase to return to the matrix. During the process, energy is released to drive the synthesis of ATP in F When high-energy protons move through ATP synthetase, energy is provided to combine a phosphate with ADP, a process called oxidative phosphorylation The oxidation of the reduced coenzymes NADH and FADH 2 by the electron transport chain generates energy to drive the synthesis of ATP The electrons from NADH enter the electron chain at a higher energy level than the electrons from FADH 2. Thus, the greater energy difference for electrons from NADH provides energy that drives the synthesis of three ATP ATP synthase consists of two protein complexes known as F 0 and F Electrons flowing through F 0 provide energy that turns the center unit in F 1. In F 1, ADP and P i combine to form ATP, which is released as the shape of the sites in F 1 change The loose (L) site in ATP synthase begins the synthesis of ATP by binding ADP and P i Energy is required to convert the tight site of ATP synthase to an open(o) site, which releases the ATP Glycolysis takes place in the cytoplasm, not in the mitochondria. Because NADH cannot cross the mitochondrial membrane, one ATP is hydrolyzed to transport the electrons from NADH to FAD. The resulting FADH 2 produces only 2 ATP for each NADH produced in glycolysis Under anaerobic conditions, the maximum yield is 36 ATP per glucose molecule a. 3 ATP are produced by the oxidation of NADH in electron transport. b. 2 ATP are produced in glycolysis when glucose degrades to 2 pyruvate. c. 6 ATP are produced when 2 pyruvate are oxidized to 2 acetyl CoA and 2 CO 2. d. 12 ATP are produced in one turn of the citric acid cycle as acetyl CoA is converted to 2 CO a. 2 ATP b. 36 ATP c. 2 ATP d. 2 ATP The oxidation reactions of the citric acid cycle produce a source of reduced coenzymes for the electron transport chain and ATP synthesis In the citric acid cycle, oxaloacetate is regenerated, which is available to pick up acetyl CoA and start the cycle again The electron transport chain regenerates the oxidized forms of the coenzymes NAD + and FAD for use again by the citric acid cycle The reduced coenzymes NADH/H + and FADH 2 are needed for the electron transport chain a. Citrate and isocitrate are six-carbon compounds in the citric acid cycle. b. α-ketoglutarate is a five-carbon compound. c. The compounds α-ketoglutarate, succinyl-coa, and oxaloacetate have keto groups a. succinyl CoA, succinate, fumarate, malate, and oxaloacetate b. Citrate, isocitrate, and malate have hydroxyl groups. c. There are carbon-carbon double bonds in aconitate and fumarate. There are carbon-oxygen double bonds in many of the compounds in the citric acid cycle.

4 Chapter 24 Answers and Solutions a. In Reaction 4, α-ketoglutarate, a five-carbon keto acid, is decarboxylated. b. In Reaction 1 and 7, double bonds in aconitate and fumarate are hydrated. c. NAD + is reduced in Reactions 3, 4, and 8. d. In Reactions 3 and 8, a secondary hydroxyl group in isocitrate and malate is oxidized a. FAD is reduced in reaction 6. b. In reaction 3, isocitrate is decarboxylated to give _-ketoglutarate. c. In reaction 6, a carbon-carbon double bond forms when fumarate undergoes dehydrogenation with FAD. d. In reaction 5, GDP undergoes direct phosphorylation to give GTP a. NAD + is the coenzyme for the oxidation of a secondary hydroxyl group in isocitrate to a keto group in α-ketoglutarate. b. NAD + and CoA are needed in the oxidative decarboxylation of α-ketoglutarate to succinyl CoA a. succinate dehydrogenase b. malate dehydrogenase a. High levels of NADH inhibit isocitrate dehydrogenase and α-ketoglutarate dehydrogenase to slow the rate of the citric acid cycle. b. High levels of ATP inhibit the citric acid cycle a. increase the rate of the citric acid cycle to provide more ATP b. increase the rate of the citric acid cycle to provide more ATP a. A heme group is found in all the cytochromes (4). b. FMN (1) contains a ribitol group a. (1) FMN b. (3) CoQ a. CoQ is a mobile carrier. b. Fe-S clusters are found in complexes I, III, and IV. c. cyt a 3 is part of complex IV a. cyt b is part of complex II b. cyt c is a mobile carrier c. FMN is part of complex I a. FADH 2 is oxidized in complex II: FADH 2 + Q FAD + QH 2 b. Cyt a (Fe 2+ ) is oxidized in complex IV: Cyt a (Fe 2+ ) + Cyt a 3 (Fe 3+ ) Cyt a (Fe 3+ ) + Cyt a 3 (Fe 2+ ) a. cyt c (Fe 3+ ) + cyt a (Fe 2+ ) b. NAD + + FMNH 2

5 Metabolism and Energy Production Complete the following by adding the substances that are missing: a. FMN 2Fe 2+ S FMNH 2 2Fe 3+ S b. FAD QH 2 cyt b (Fe 3+ ) FADH 2 Q cyt b (Fe 2+ ) a. NADH + H + ; FMN b. 2 cyt a (Fe 2+ ); 2 cyt a3 (Fe 2+ ); H 2 O The transfer of electrons by complexes I, III, and IV generate energy to pump protons out of the matrix into the inner membrane space The accumulation of protons decreases ph in the intermembrane space and creates a proton gradient In the chemiosmotic model, energy released by the flow of protons through the ATP synthase is utilized for the synthesis ATP The synthesis of ATP occurs in the ATP synthase part of the electron transport chain In the inner membrane space, there is a higher concentration of protons, which reduces the ph and forms an electrochemical gradient. As a result protons flow into the matrix where the proton concentration is lower and the ph is higher The protons obtained from the oxidations reactions must be pumped from the end in the matrix to be released at the other end, which is in the intermembrane space Two ATP molecules are produced from the energy generated by the electrons from FADH 2 moving through electron transport to oxygen ATP are produced from each NADH + H a. Amytal and rotenone inhibit the transfer of electrons in NADH dehydrogenase (complex I). b. Antimycin inhibits electron flow from cyt b to cyt c 1 in complex III. c. Cyanide and carbon monoxide inhibit the flow of electron through cytochrome c oxidase (complex IV) a. The coenzymes that precede the blocked site remain in their reduced forms. b. The coenzymes that follow the blocked site remain in their oxidized forms.

6 Chapter 24 Answers and Solutions The oxidation of glucose to pyruvate by glycolysis produces 6 ATP. 2 ATP are formed by direct phosphorylation along with 2 NADH. Because the 2 NADH are produced in the cytosol, the electrons are transferred to form 2 FADH 2, which produces an additional 4 ATP. The oxidation of glucose to CO 2 and H 2 O produces 36 ATP The NADH produced from glycolysis cannot pass through the mitochondrial membrane. Therefore, the hydrogen ions and electrons from the NADH in the cytoplasm are transferred to dihydroxyacetone phosphate, which is reduced to glycerol-3-phosphate to regenerate NAD +. After glycerol-3-phosphate moves into the mitochondria, the hydrogen ions and electrons reduced FAD to give FADH 2 and FADH 2 produces two ATP in the electron transport chain a. 4 ATP x 7.3 kcal/mole = 29 kcal (actual ATP produced from glycolysis because the protons from the NADH in the cytoplasm are shuttled to FAD in the mitochondria). b. 6 ATP x 7.3 kcal/mole = 44 kcal (2 pyruvate to 2 acetyl CoA) c. 24 ATP x 7.3 kcal/ mole = 175 kcal (2 acetyl CoA citric acid cycle) d. 36 x 7.3 kcal/mol = 263 kcal (complete oxidation of glucose to CO 2 and H 2 O) a. 29 kcal/ 687 kcal 100 = 4.2% b. 44 kcal/ = 6.4% c. 175 kcal/ = 35% In a calorimeter, the complete combustion of glucose gives 687 kcal. The efficiency of ATP synthesis is determined by comparing the total kcal in 36 ATP (283 kcal in problem 24.81) to the energy obtained from glucose in a calorimeter. 283 /687 x 100 = 38.0% efficient moles glucose 283 moles ATP/1 mole glucose 7.3 kcal/1 mole ATP = 8260 kcal When 4 moles of glucose are completely oxidized at 38% efficiency, 8260 kcal of energy would be conserved The ATP synthase extends through the inner mitochondrial membrane with the F 0 part in contact with the proton gradient in the intermembrane space, while the F 1 complex is in the matrix Protons from the matrix where oxidation reactions take place are pumped through complexes I, IIII, and IV into the intermembrane space where they accumulate and form a proton gradient As protons from the proton gradient move through the ATP synthase to return to the matrix, energy is released and used to drive ATP synthesis at F ADP and P i enter the loose (L) site on ATP synthase. When the shape of the site changes to tight (T), ATP is formed. The site then changes to open (O), and ATP is released A hibernating bear has stored fat as brown fat, which can be used during the winter for heat rather than ATP energy A diet medication that is an uncoupler may transport protons through the inner membrane or block the changed in the F 0 part of ATP synthase. Because ATP synthesis is bypassed, the energy of electron transport is released as heat, which will cause an increase in body temperature.