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12.3 The Citric Acid Cycle Oxidizes AcetylCoA
• Table 12.2
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Summary of the citric acid cycle
• For each acetyl CoA which enters the cycle:
(1) Two molecules of CO2 are released
(2) Coenzymes NAD+ and Q are reduced
(3) One GDP (or ADP) is phosphorylated
(4) The initial acceptor molecule (oxaloacetate) is reformed
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• Citric acid cycle (Four slides)
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• Fates of carbon atoms in the cycle
• Carbon atoms from acetyl CoA (red) are not lost in the first turn of the cycle
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Energy conservation by the cycle
• Energy is conserved in the reduced coenzymes NADH, QH2 and one GTP
• NADH, QH2 can be oxidized to produce ATP by oxidative phosphorylation
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The Citric Acid Cycle Can Be a Multistep Catalyst
• Oxaloacetate is regenerated
• The cycle is a mechanism for oxidizing acetyl CoA to CO2 by NAD+ and Q
• The cycle itself is not a pathway for a net degradation of any cycle intermediates
• Cycle intermediates can be shared with other pathways, which may lead to a resupply or net decrease in cycle intermediates
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1. Citrate Synthase
• Citrate formed from acetyl CoA and oxaloacetate
• Only cycle reaction with C-C bond formation
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Proposed mechanism of citrate synthase
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Stereo views of citrate synthase
(a) Open conformation
(b) Closed conformation
Product citrate (red)
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2. Aconitase
• Elimination of H2O from citrate to form C=C bond of cis-aconitate
• Stereospecific addition of H2O to cis-aconitate to form 2R,3S-Isocitrate
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Reaction of Aconitase
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Three point attachment of prochiral substrates to
enzymes • Chemically identical groups a1 and a2 of a prochiral
molecule can be distinguished by the enzyme
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3. Isocitrate Dehydrogenase
• Oxidative decarboxylation of isocitrate to-ketoglutarate (-kg) (a metabolically irreversible reaction)
• One of four oxidation-reduction reactions of the cycle
• Hydride ion from the C-2 of isocitrate is transferred to NAD(P)+ to form NAD(P)H
• Oxalosuccinate is decarboxylated to -kg
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Isocitrate dehydrogenase reaction
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4. The -Ketoglutarate Dehydrogenase Complex
• Second decarboxylation (CO2 released)
• Energy stored as reduced coenzyme, NADH
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Structure of -Ketoglutarate dehydrogenase complex
• Similar to pyruvate dehydrogenase complex
• Same coenzymes, identical mechanisms
E1 - -ketoglutarate dehydrogenase (with TPP)
E2 - succinyltransferase (with flexible lipoamide prosthetic group)
E3 - dihydrolipoamide dehydrogenase (with FAD)
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5. Succinyl-CoA Synthetase
• Free energy in thioester bond of succinyl CoA is conserved as GTP (or ATP in plants, some bacteria)
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Fig 12.9
• Mechanism of succinyl-CoA synthetase (continued on next slide)
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(continued)
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6. The Succinate Dehydrogenase (SDH) Complex
• Located on the inner mitochondrial membrane (other components are dissolved in the matrix)
• Dehydrogenation is stereospecific; only the trans isomer is formed
• Substrate analog malonate is a competitive inhibitor of the SDH complex
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Reaction of the succinate dehydrogenase complex
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Succinate and malonate
• Malonate is a structural analog of succinate
• Malonate binds to the enzyme active site, and is a competitive inhibitor
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Structure of the SDH complex
• Complex of several polypeptides, a covalently bound FAD prosthetic group and 3 iron-sulfur clusters
• Electrons are transferred from succinate to ubiquinone (Q), a lipid-soluble mobile carrier of reducing power
• FADH2 generated is reoxidized by Q
• QH2 is released as a mobile product
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7. Fumarase
• Stereospecific trans addition of water to the double bond of fumarate to form L-malate
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8. Malate Dehydrogenase
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Reduced Coenzymes Fuel the Production of ATP
• Each acetyl CoA entering the cycle nets:
(1) 3 NADH
(2) 1 QH2
(3) 1 GTP (or 1 ATP)
• Oxidation of each NADH yields 2.5 ATP
• Oxidation of each QH2 yields 1.5 ATP
• Complete oxidation of 1 acetyl CoA = 10 ATP
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Glucose degradation via glycolysis, citric acid cycle, and oxidative phosphorylation
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NADH fate in anaerobic glycolysis
Anaerobic glycolysis
• NADH produced by G3PDH reaction is reoxidized to NAD+ in the pyruvate to lactate reaction
• NAD+ recycling allows G3PDH reaction (and glycolysis) to continue anaerobically
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NADH fate in aerobic glycolysis
• Glycolytic NADH is not reoxidized via pyruvate reduction but is available to fuel ATP formation
• Glycolytic NADH (cytosol) must be transferred to mitochondria (electron transport chain location)
• Two NADH shuttles are available (next slide)
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NADH shuttles
• Malate-aspartate shuttle (most common)
One cytosolic NADH yields ~ 2.5 ATP (total 32 ATP/glucose)
• Glycerol phosphate shuttle
One cytosolic NADH yields ~1.5ATP (total 30 ATP/glucose)
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12.6 Regulation of the Citric Acid Cycle
• Pathway controlled by:
(1) Allosteric modulators
(2) Covalent modification of cycle enzymes
(3) Supply of acetyl CoA
(4) Regulation of pyruvate dehydrogenase complex controls acetyl CoA supply
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Fig 12.12 Regulation of the PDH complex
• Increased levels of acetyl CoA and NADH inhibit E2, E3 in mammals and E. coli