CHAPTER 09

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Chapter 9:

From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Chapter 9 Section 9.1: Oxidation-Reduction Reactions Section 9.2: Citric Acid Cycle Biochemistry in Perspective Overview Aerobic Metabolism I: The Citric Acid Cycle

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Section 9.1: Oxidation-Reduction Reactions From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Organisms deal with oxygen in many different ways: Obligate anaerobes Aerotolerant anaerobes Facultative anaerobes Obligate aerobes Figure 9.1 Overview of Aerobic Metabolism

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Section 9.1: Oxidation-Reduction Reactions From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Facultative anaerobes and obligate aerobes use oxygen to generate energy Use the processes: citric acid cycle , electron transport chain , and oxidative phosphorylation Important intermediates: NADH and FADH 2 Figure 9.1 Overview of Aerobic Metabolism

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Section 9.1: Oxidation-Reduction Reactions From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Aerobic processes occur in the mitochondrion Figure 9.2 Aerobic Metabolism in the Mitochondrion

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Section 9.1: Oxidation-Reduction Reactions From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Oxidation-Reduction Reactions In living organisms, energy capturing and energy releasing processes involve redox reactions Many redox reactions have both an electron ( e - ) and proton (H + ) transferred Conversion of pyruvate to lactate (shown above) is under anaerobic conditions Figure 9.3 Reduction of Pyruvate by NADH

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Section 9.1: Oxidation-Reduction Reactions From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Oxidation-Reduction Reactions Half-reactions make redox reactions more easily understood Biochemical reference half-reaction is: 2H + + 2 e -  H 2 (reversible) Gives a reduction potential of -0.42 V Figure 9.4 An Electrochemical Cell

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Section 9.1: Oxidation-Reduction Reactions From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Oxidation-Reduction Reactions Continued The relationship between standard reduction potentials ( D Eº′)and standard free energy ( D Gº′) is: D Gº′ = - nF D Eº′ Electrons flow spontaneously from a species with a more negative Eº′ to a species with a more positive Eº′ Living organisms utilize redox coenzymes as high-energy electron carriers (e.g., NADH and FADH 2 )

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Section 9.1: Oxidation-Reduction Reactions From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Redox Coenzymes: Nicotinic Acid Two coenzyme forms: nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) Have oxidized (NAD + and NADP + ) and reduced forms (NADH and NADPH) NADP + involved in biosynthetic reactions and NAD + involved in catabolic reactions

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Section 9.1: Oxidation-Reduction Reactions From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Figure 9.5 Nicotinamide Adenine Dinucleotide (NAD)

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Section 9.1: Oxidation-Reduction Reactions From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Redox Coenzymes: Riboflavin Riboflavin (vitamin B 2 ) is a component of two coenzymes: flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) Function in a class of enzymes known as flavoproteins Function as dehydrogenases , oxidases , and hydroxylases

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Section 9.1: Oxidation-Reduction Reactions From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Figure 9.6 Flavin Coenzymes

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Section 9.1: Oxidation-Reduction Reactions From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Aerobic Metabolism Electron transport chain captures most of aerobic cell’s free energy Energy transferred from NADH to O 2 ½O 2 + NADH + H +  H 2 0 + NAD + (-220 kJ/mol) Figure 9.7 Electron Flow and Energy

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Figure 9.9 Coenzyme A Citric acid cycle is used to harvest energy from acetyl group of acetyl-CoA Acetyl is derived from catabolism of carbohydrates (e.g., pyruvate), lipids, and some amino acids Coenzyme A is an acyl carrier molecule Section 9.2: Citric Acid Cycle

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Section 9.2: Citric Acid Cycle From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press In the citric acid cycle, the acetyl group’s carbon atoms are ultimately oxidized to form CO 2 Figure 9.8 The Citric Acid Cycle

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Section 9.2: Citric Acid Cycle Figure 9.8 The Citric Acid Cycle

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Section 9.2: Citric Acid Cycle Figure 9.8 The Citric Acid Cycle Transfer of electrons to carrier molecules from the citric acid cycle intermediate molecules forms the reduced coenzymes NADH and FADH 2

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Section 9.2: Citric Acid Cycle Citric acid cycle intermediates also play an important role in a variety of biosynthetic reactions A variety of coenzymes play important roles in the citric acid cycle

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Section 9.2: Citric Acid Cycle Conversion of Pyruvate to Acetyl- CoA Pyruvate dehydrogenase complex converts pyruvate to acetyl- CoA Large complex multienzyme structure Highly exergonic ( D Gº′ = -33.5 kJ/mol)

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Figure 9.11 Lipoamide Thiamine pyrophosphate (TPP) coenzyme helps decarboxylate pyruvate Lipoic acid helps convert an intermediate (HETPP) into acetyl-CoA Section 9.2: Citric Acid Cycle

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Section 9.2: Citric Acid Cycle Figure 9.10 Reactions of Pyruvate Dehydrogenase Complex Decarboxylation Action of lipoic acid Pyruvate decarboxylase Dihydrolipoyl dehydrogenase

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Figure 9.10 Reactions of Pyruvate Dehydrogenase Complex Section 9.2: Citric Acid Cycle Action of TPP Formation of Acetyl-CoA Pyruvate decarboxylase Dihydrolipoyl transacetylase

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Figure 9.12 Citrate Synthesis Reactions of the Citric Acid Cycle Eight reactions in two stages: 1. Liberation of two CO 2 from acetyl- CoA 2. Regeneration of oxaloacetate Section 9.2: Citric Acid Cycle

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Reactions of Citric Acid Cycle Continued 1. Introduction of two carbons as acetyl- CoA -forming citrate 2. Citrate isomerization 3. Isocitrate is oxidized to form NADH and CO 2 4. a - Ketoglutarate is oxidized; forms NADH and CO 2 Reactions 3. and 4. are oxidative decarboxylation reactions Section 9.2: Citric Acid Cycle Figure 9.12 Citrate Synthesis

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Reactions of Citric Acid Cycle Continued 5. Succinyl-CoA cleaved and ATP/GTP is formed 6. Succinate oxidized forms fumarate and FADH 2 7. Fumarate is hydrated and forms malate 8. Malate oxidized forms oxaloacetate and NADH Section 9.2: Citric Acid Cycle

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Figure 9.13 Amphibolic Citric Acid Cycle The Amphibolic Citric Acid Cycle Citric acid cycle involved in anabolic as well as catabolic processes Anabolic reactions lead to the formation of many important biomolecules Section 9.2: Citric Acid Cycle

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press The Amphibolic Citric Acid Cycle continued Anaplerotic reactions also contribute intermediates into the cycle Oxaloacetate from pyruvate or aspartate Succinyl-CoA from fatty acids Section 9.2: Citric Acid Cycle Figure 9.13 Amphibolic Citric Acid Cycle

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Citric Acid Cycle Regulation Regulation controlled by three irreversible enzymes Citrate synthase regulated by substrate levels, ATP/ADP ratio, and NADH/NAD + ratio Section 9.2: Citric Acid Cycle Figure 9.14 Control of Citric Acid Cycle

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Isocitrate dehydrogenase regulated by substrate levels, ATP/ADP ratio, and NADH/NAD + ratio a -Ketoglutarate dehydrogenase regulated by substrate levels, AMP, and NADH levels Section 9.2: Citric Acid Cycle Figure 9.14 Control of Citric Acid Cycle

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Figure 9.15 Citrate Metabolism Section 9.2: Citric Acid Cycle

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Section 9.2: Citric Acid Cycle Figure 9.15 Citrate Metabolism

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Section 9.2: Citric Acid Cycle Citrate Metabolism: Citrate plays a role in oxaloacetate, malate, and pyruvate formation Can also lead to NADPH production used for fatty acid biosynthesis Citrate in the cytoplasm can also inhibit glycolysis

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Figure 9.16 The Glyoxylate Cycle Section 9.2: Citric Acid Cycle

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press The Glyoxylate Cycle Occurs in plants and some fungi, algae, protozoans , and bacteria Modified citric acid cycle Five reactions use two-carbon compounds Section 9.2: Citric Acid Cycle

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Figure 9.17 Role of Glyoxylate in Gluconeogenesis Glyoxylate cycle produces two molecules: succinate and oxaloacetate Succinate can be used to make metabolically important molecules like glucose Section 9.2: Citric Acid Cycle

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Evolutionary History of the Citric Acid Cycle First originated as two pathways: 1. The reductive pathway provided free electron acceptors 2. The oxidative pathway generated a -ketoglutarate, a biosynthetic precursor molecule Biochemistry in Perspective

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