BISC221 Lecture11b - The citric acid cycle •The citric...

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The citric acid cycle The citric acid cycle completes the energy- yielding oxidation of organic molecules • The citric acid cycle takes place in the matrix of the mitochondrion Before the citric acid cycle can begin, pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis CYTOSOL MIT NADH + NAD + 2 3 1 CO 2 Coenzyme A Pyruvate Transport protein O O O C C CH 3 Figure 9.10
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The citric acid cycle, also called the Krebs cycle, takes place within the mitochondrial matrix The cycle oxidizes organic fuel derived from pyruvate, generating one ATP, 3 NADH, and 1 FADH 2 per turn
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An overview of the citric acid cycle ATP 2 CO 2 3 NAD + 3 NADH + 3 H + ADP + P i FAD FADH 2 Citric acid cycle CoA CoA Acetyl CoA NADH + 3 H + CoA CO 2 Pyruvate (from glycolysis, 2 molecules per glucose) ATP ATP ATP Glycolysis Citric acid cycle Oxidative phosphorylation Figure 9.11 For each turn of the cycle: - 2 carbons enter as acetyl groups and 2 leave as CO2. - 3 NAD + are reduced to NADH - 1 FAD is reduced to FADH2 - 1 ATP molecule is produced
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The citric acid cycle has eight steps, each catalyzed by a specific enzyme The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle The NADH and FADH 2 produced by the cycle relay electrons extracted from food to the electron transport chain
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Acetyl CoA NADH Oxaloacetate Citrate Malate Fumarate Succinate Succinyl CoA α -Ketoglutarate Isocitrate Citric acid cycle S CoA CoA SH NADH NADH FADH 2 FAD GTP GDP NAD + ADP P i NAD + CO 2 CO 2 CoA SH CoA SH CoA S H 2 O + H + + H + H 2 O C CH 3 O O C COO CH 2 COO COO CH 2 HO C COO CH 2 COO COO COO CH 2 HC COO HO CH COO CH CH 2 COO HO COO CH HC COO COO CH 2 CH 2 COO COO CH 2 CH 2 C O COO CH 2 CH 2 C O COO 1 2 3 4 5 6 7 8 Glycolysis Oxidative phosphorylation NAD + + H + ATP Citric acid cycle Figure 9.12 The citric acid cycle 1. Acetyl group from acetyl-CoA is added to oxaloacetate to form citrate (citric cycle). 2. Citrate is converted to isocitrate 3. Isocitrate loses a CO 2 and NAD+ is reduced to NADH. 4. Another CO2 is lost and another NADH is produced. Attachment of product to CoA. 5. Formation of GTP from GDP + Pi. GTP is converted to ATP: substrate-level Phosphorylation. 6. Two H + and 1 electron are transferred to FAD, forming FADH2 and oxidizing succinate . 7. Addition of 1 H 2 O to fumarate producing malate. 8. Oxidation of malate and reduction of NAD+ to NADH .
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During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis Following glycolysis and the citric acid cycle, NADH and FADH 2 account for most of the energy extracted from food These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation
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The Pathway of Electron Transport The electron transport chain is in the cristae of the mitochondrion
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