Chapter 23 Metabolism and Energy Production 23.1 The Citric Acid Cycle Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Citric Acid Cycle The citric acid cycle (stage 3) Operates under aerobic conditions only. Oxidizes the two-carbon acetyl group in acetyl CoA to 2CO2. Produces reduced coenzymes NADH and FADH2 and one ATP directly.

Citric Acid Cycle Overview In the citric acid cycle, Acetyl (2C) bonds to oxaloacetate (4C) to form citrate (6C). Oxidation and decarboxylation reactions convert citrate to oxaloacetate. Oxaloacetate bonds with another acetyl to repeat the cycle. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Reaction 1 Formation of Citrate Oxaloacetate Combines with the two-carbon acetyl group to form citrate. citrate synthase + +

Reaction 2 Isomerization to Isocitrate Isomerizes to isocitrate. Has a tertiary —OH group converted to a secondary —OH in isocitrate that can be oxidized.

Summary of Reactions 1 and 2 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Reaction 3 Oxidative Decarboxylation Isocitrate Undergoes decarboxylation (carbon removed as CO2). Oxidizes the —OH to a ketone releasing H+ and 2e−. Provides H to reduce coenzyme NAD+ to NADH. + + + -ketoglutarate

Reaction 4 Oxidative Decarboxylation -Ketoglutarate Undergoes decarboxylation to form succinyl CoA. Produces a 4-carbon compound that bonds to CoA. Provides H+ and 2e− to reduce NAD+ to NADH. -ketoglutarate succinyl CoA -ketoglutarate dehydrogenase +

Summary of Reactions 3 and 4 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Reaction 5 Hydrolysis Succinyl CoA Undergoes hydrolysis of the thioester bond. Provides energy to add phosphate to GDP and form GTP, a high­energy compound. succinyl CoA synthetase + + GDP + Pi + CoASH + GTP ATP succinate succinyl CoA

Reaction 6 Dehydrogenation Succinate Undergoes dehydrogenation. Loses two H and forms a double bond. Provides 2H to reduce FAD to FADH2. succinate dehydrogenase + FAD + FADH2

Summary of Reactions 5 and 6 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Reaction 7 Hydration of Fumarate Undergoes hydration. Adds water to the double bond. Is converted to malate. fumarase

Reaction 8 Dehydrogenation Malate Undergoes dehydrogenation. Forms oxaloacetate with a C=O double bond. Provides 2H that reduce NAD+ to NADH + H+. malate dehydrogenase + NAD+ +

Summary of Reactions 7 and 8 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Summary of the Citric Acid Cycle In the citric acid cycle, An acetyl group bonds with oxaloacetate to form citrate. Two decarboxylations remove two carbons as 2CO2. Four oxidations provide hydrogen for 3NADH and one FADH2. A direct phosphorylation forms GTP (ATP).

Overall Chemical Reaction for the Citric Acid Cycle acetylS-CoA + 3NAD+ + FAD + GDP + Pi + 2H2O 2CO2 + 3NADH + 3H+ + FADH2 + HS-CoA + GTP One turn of the citric acid cycle produces: 2 CO2 1 GTP (1ATP) 3 NADH 1 HS-COA 1 FADH2

Learning Check How many of each are produced in one turn of the citric acid cycle? A ___ CO2 B. ___ NADH C. ___ FADH2 D. ___ GTP

Solution How many of each are produced in one turn of the citric acid cycle? A 2 CO2 B. 3 NADH C. 1 FADH2 D. 1 GTP

Regulation of Citric Acid Cycle The reaction rate for the citric acid cycle Increases when low levels of ATP or NAD+ activate isocitrate dehydrogenase. Decreases when high levels of ATP or NADH inhibit citrate synthetase (first step in cycle). Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Chapter 23 Metabolism and Energy Production 23.2 Electron Carriers Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Electron Carriers Electron carriers Are oxidized and reduced as hydrogen and/or electrons are transferred from one carrier to the next. Are FMN, Fe-S clusters, Coenzyme Q, and cytochromes. electron carrier AH2(reduced) electron carrier B (oxidized) electron carrier A (oxidized) electron carrier BH2(reduced)

Electron Carriers Electron carriers Accept hydrogen and electrons from the reduced coenzymes. Are oxidized and reduced to provide energy for the synthesis of ATP. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

FMN (Flavin mononucleotide) FMN coenzyme Contains flavin, ribitol,and phosphate. Accepts 2H+ + 2e- to form reduced coenzyme FMNH2. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Iron-Sulfur (Fe-S) Clusters Are groups of proteins containing iron ions and sulfide. Accept electrons to reduce Fe3+ to Fe2+, and lose electrons to re-oxidize Fe2+ to Fe3+. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Coenzyme Q (Q or CoQ) Coenzyme Q (Q or CoQ) is A mobile electron carrier derived from quinone. Reduced when the keto groups accept 2H+ and 2e-. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Cytochromes Cytochromes (cyt) are Proteins containing heme groups with iron ions. Fe3+ + 1e- Fe2+ Abbreviated as cyt a, cyt a3, cyt b, cyt c, and cyt c1. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Learning Check Write the abbreviation for each: A. Reduced form of coenzyme Q. B. Oxidized form of flavin mononucleotide. C. Reduced form of iron in cytochrome c.

Solution Write the abbreviation for each: A. Reduced form of coenzyme Q. CoQH2 or QH2 B. Oxidized form of flavin mononucleotide. FMN C. Reduced form of cytochrome c. Cyt c (Fe2+)

Learning Check A. FMNH2 FMN B. Cyt b (Fe3+) Cyt b (Fe2+) C. Q QH2 Indicate whether the electron carrier in each is oxidized or reduced: A. FMNH2 FMN B. Cyt b (Fe3+) Cyt b (Fe2+) C. Q QH2 D. Cyt c (Fe2+) Cyt c (Fe3+)

Solution A. FMNH2 FMN oxidized B. Cyt b (Fe3+) Cyt b (Fe2+) reduced Indicate whether the electron carrier in each is oxidized or reduced: A. FMNH2 FMN oxidized B. Cyt b (Fe3+) Cyt b (Fe2+) reduced C. Q QH2 reduced D. Cyt c (Fe2+) Cyt c (Fe3+) oxidized

Chapter 23 Metabolism and Energy Production 23.3 Electron Transport Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Electron Transport Electron transport Uses electron carriers. Transfers hydrogen ions and electrons from NADH and FADH2 until they combine with oxygen. Forms H2O. Produces ATP energy.

Electron Transport System In the electron transport system, The electron carriers are attached to the inner membrane of the mitochondrion. There are four protein complexes: Complex I NADH dehydrogenase Complex II Succinate dehydrogenase Complex III CoQ-Cytochrome c reductase Complex IV Cytochrome c oxidase

Electron Transport Chain Cyt c1 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Complex I NADH Dehydrogenase At Complex I, Hydrogen and electrons are transferred from NADH to FMN. NADH + H+ + FMN NAD+ + FMNH2 FMNH2 transfers hydrogen to Fe-S clusters and then to coenzyme Q reducing Q and regenerating FMN. Q + FMNH2 QH2 + FMN The overall reaction is NADH + H+ + Q QH2 + NAD+ QH2, a mobile carrier, transfers hydrogen to Complex III.

Complex II Succinate Dehydrogenase At Complex II, with a lower energy level than Complex I, FADH2 transfers hydrogen and electrons to coenzyme Q. Q is reduced to QH2 and FAD is regenerated. FADH2 + Q QH2 + FAD QH2, a mobile carrier, transfers hydrogen to Complex III.

Complex III CoQ-Cytochrome c reductase At Complex III, Electrons are transferred from QH2 to two Cyt b. Each Cyt b (Fe3+) is reduced to Cyt b (Fe2+). Q is regenerated. 2Cyt b (Fe3+) + QH2 2Cyt b (Fe2+) + Q + 2H+ Electrons are transferred from Cyt b to Fe-S clusters, to Cyt c1, and to Cyt c, the second mobile carrier. 2Cyt c (Fe3+) + 2Cyt b (Fe2+) 2Cyt c (Fe2+) + 2Cyt b (Fe3+)

Complex IV Cytochrome c Oxidase At Complex IV, electrons are transferred from: Cyt c to Cyt a. 2Cyt c (Fe2+) + 2Cyt a (Fe3+) 2Cyt a (Fe2+) + 2Cyt c (Fe3+) Cyt a to Cyt a3. 2Cyt a (Fe2+) + 2Cyt a3 (Fe3+) 2Cyt a (Fe3+) + 2Cyt a3 (Fe2+) Cyt a3 to oxygen and H+ to form water. 4H+ + O2 + 4e- (from Cyt a3 ) 2H2O

Learning Check Match each with their function: 1) FMN 2) Q 3) Cyt c A. Accepts H and electrons from NADH + H+. B. A mobile carrier between Complex II and III. C. Carries electrons from Complex I and II to Complex III. D. Accepts H and electrons from FADH2.

Solution Match each with their function: 1) FMN 2) Q 3) Cyt c A. 1 Accepts H and electrons from NADH + H+. B. 3 A mobile carrier between Complex II and III. C. 2 Carries electrons from Complex I and II to Complex III. D. 2 Accepts H and electrons from FADH2.

Learning Check Classify each as a product of the 1) Citric acid cycle 2) Electron transport chain A. CO2 B. FADH2 C. NAD+ D. NADH E. H2O

Solution Classify each as a product of the 1) citric acid cycle 2) electron transport chain A. 1 CO2 B. 1 FADH2 C. 2 NAD+ D. 1 NADH E. 2 H2O

Chapter 23 Metabolism and Energy Production 23.4 Oxidative Phosphorylation and ATP Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Chemiosmotic Model In the chemiosmotic model Complexes I, III, and IV pump protons into the intermembrane space creating a proton gradient. Protons pass through ATP synthase to return to the matrix. The flow of protons through ATP synthase provides the energy for ATP synthesis (oxidative phosphorylation): ADP + Pi + Energy ATP

Chemiosmotic Model of Electron Transport Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

ATP Synthase In ATP synthase Protons flow back to the matrix through a channel in the F0 complex. Proton flow provides the energy that drives ATP synthesis by the F1 complex. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

ATP Synthase F1 Complex In the F1 complex of ATP synthase A center subunit () is surrounded by three protein subunits: loose (L), tight (T), and open (O). Energy from the proton flow through F0 turns the center subunit (). The shape (conformation) of the three subunits changes.

ATP Synthesis in F1 During ATP synthesis ADP and Pi enter the loose L site. The center subunit turns changing the L site to a tight T conformation. ATP is formed in the T site where it remains strongly bound. The center subunit turns changing the T site to an open O site, which releases the ATP.

ATP Synthase F1 Diagram Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Learning Check Match the following 1) F0 complex 2) F1 complex 3) L site 4) T site 5) O site A. Contains subunits for ATP synthesis. B. Contains the channel for proton flow. C. The subunit in F1 that binds ADP and Pi. D. The subunit in F1 that releases ATP. E. The subunit in F1 where ATP forms.

Solution Match the following 1) F0 complex 2) F1 complex 3) L site 4) T site 5) O site A. 2 Contains subunits for ATP synthesis. B. 1 Contains the channel for proton flow. C. 3 The subunit in F1 that binds ADP and Pi. D. 5 The subunit in F1 that releases ATP. E. 4 The subunit in F1 where ATP forms.

Electron Transport and ATP In electron transport, the energy level decrease for electrons From NADH (Complex I) provides sufficient energy for 3ATPs. NADH + 3ADP + 3Pi NAD+ + 3ATP From FADH2 (Complex II) provides sufficient energy for 2ATPs. FADH2 + 2ADP + 2Pi FAD + 2ATP

ATP from Electron Transport Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Regulation of Electron Transport The electron transport system is regulated by Low levels of ADP, Pi, oxygen, and NADH that decrease electron transport activity. High levels of ADP that activate electron transport.

Chapter 23 Metabolism and Energy Production 23.5 ATP Energy from Glucose Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

ATP Energy from Glucose The complete oxidation of glucose yields 6 CO2 6 H2O 36 ATP Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

ATP from Glycolysis In glycolysis Glucose forms 2 pyruvate, 2 ATP and 2NADH. NADH produced in the cytoplasm cannot enter the mitochondria. A shuttle compound (glycerol-3-phosphate) moves hydrogen and electrons into the mitochondria to FAD, which forms FADH2. Each FADH2 provides 2 ATP. Glucose 2 pyruvate + 6 ATP

ATP from Glycolysis Reaction Pathway ATP for One Glucose Activation of glucose -2 ATP Oxidation of 2 NADH (as FADH2) 4 ATP Direct ADP phosphorylation (two triose) 4 ATP 6 ATP Summary: C6H12O6 2 pyruvate + 2H2O + 6 ATP glucose

ATP from Two Pyruvate Under aerobic conditions 2 pyruvate are oxidized to 2 acetyl CoA and 2 NADH. 2 NADH enter electron transport to provide 6 ATP. Summary: 2 Pyruvate 2 Acetyl CoA + 6 ATP

ATP from Citric Acid Cycle One turn of the citric acid cycle provides: 3 NADH x 3 ATP = 9 ATP 1 FADH2 x 2 ATP = 2 ATP 1 GTP x 1 ATP = 1 ATP Total = 12 ATP Acetyl CoA 2 CO2 + 12 ATP For two acetyl CoA from one glucose, two turns of the citric acid cycle produce 24 ATP. 2 Acetyl CoA 4 CO2 + 24 ATP

ATP from Citric Acid Cycle Reaction Pathway ATP (One Glucose) ATP from Citric Acid Cycle Oxidation of 2 isocitrate (2NADH) 6 ATP Oxidation of 2 -ketoglutarate (2NADH) 6 ATP 2 Direct substrate phosphorylations (2GTP) 2 ATP Oxidation of 2 succinate (2FADH2) 4 ATP Oxidation of 2 malate (2NADH) 6 ATP Summary: 2Acetyl CoA 4CO2 + 2H2O + 24 ATP

ATP from Glucose From glycolysis 6 ATP From 2 pyruvate 6 ATP One glucose molecule undergoing complete oxidation provides: From glycolysis 6 ATP From 2 pyruvate 6 ATP From 2 acetyl CoA 24 ATP Overall ATP Production for one glucose C6H12O6 + 6O2 + 36ADP + 36Pi glucose 6CO2 + 6H2O + 36ATP

Learning Check Indicate the ATP yield for each under aerobic conditions. A. Complete oxidation of glucose B. FADH2 C. Acetyl CoA in citric acid cycle D. NADH E. Pyruvate decarboxylation

Solution Indicate the ATP yield for each under aerobic conditions. A. Complete oxidation of glucose 36 ATP B. FADH2 2 ATP C. Acetyl CoA in citric acid cycle 12 ATP D. NADH 3 ATP E. Pyruvate decarboxylation 3 ATP