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Download Panel 13–2 The complete citric acid cycle
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Transcript
Panel 13–2 The complete citric acid cycle NAD+ NADH + H + HS CoA O CH3 C The complete citric acid cycle. The two carbons from acetyl CoA that enter this turn of the cycle (shadowed in red ) will be converted to CO2 in subsequent turns of the cycle: it is the two carbons shadowed in blue that are converted to CO2 in this cycle. COO– CO2 pyruvate O acetyl CoA (2C) CH3 C S CoA NADH + H COO– COO– H2O COO– CH2 HO C COO– Step 1 oxaloacetate (4C) Step 2 CH2 COO– COO– C O CH2 COO– Step 8 H C OH CH2 malate (4C) COO– C O CH2 COO– + NAD+ HS CoA COO– next cycle HC COO– oxaloacetate (4C) HO CH COO– CITRIC ACID CYCLE NAD+ Step 3 H2O α-ketoglutarate (5C) COO– CH CH COO– COO– H2O Step 5 CH2 CH2 Step 4 GTP HS CoA CO2 COO– NAD+ C O GDP + Pi CH2 C O CH2 CH2 COO– FAD COO– NADH + H CH2 succinyl CoA (4C) succinate (4C) Step 6 FADH2 + COO– fumarate (4C) Step 7 isocitrate (6C) CH2 citrate (6C) HS CoA S CoA NADH + H + CO2 Details of the eight steps are shown below. For each step, the part of the molecule that undergoes a change is shadowed in blue, and the name of the enzyme that catalyzes the reaction is in a yellow box. Step 1 After the enzyme removes a proton from the CH3 group on acetyl CoA, the negatively O C S CoA charged CH2– forms a bond to a carbonyl carbon CH3 of oxaloacetate. The subsequent loss by hydrolysis of the coenzyme A (CoA) drives the reaction strongly acetyl CoA forward. Step 2 An isomerization reaction, in which water is first removed and then added back, moves the hydroxyl group from one carbon atom to its neighbor. COO– C + O O citrate synthase COO– oxaloacetate COO– C H HO C COO– H C H H COO– citrate S CoA COO– H2O CH2 CH2 HO CH2 C C COO– HO CH2 COO– COO– COO– H aconitase H2O + HS CoA + H+ COO– CH2 S-citryl-CoA intermediate H2O C citrate COO– H2O C H H C H C COO– H C COO– C H HO C H COO– cis-aconitate intermediate H2O COO– isocitrate COO– Step 3 In the first of four oxidation steps in the cycle, the carbon carrying the hydroxyl group is converted to a carbonyl group. The immediate product is unstable, losing CO2 while still bound to the enzyme. COO– isocitrate dehydrogenase C H H C COO– HO C H H H H NAD+ NADH + H COO– + H H C H C COO– H C H C O C O H + H H a-ketoglutarate C COO– H C H C O a-ketoglutarate dehydrogenase complex + HS CoA NAD+ NADH + H C H H C H C O S CoA CO2 a-ketoglutarate succinyl-CoA COO– H C H H C H C O COO– succinyl-CoA synthetase H C H H C H COO– GDP Pi COO– H C H COO– C H FAD COO– fumarate COO– fumarase HO C H H C H COO– HO C H H C H COO– H2O fumarate COO– H C FADH2 C malate H succinate dehydrogenase H COO– Step 8 H GTP succinate C C succinate COO– Step 7 H COO– H2O S CoA Step 6 In the last of four oxidation steps in the cycle, the carbon carrying the hydroxyl group is converted to a carbonyl group, regenerating the oxaloacetate needed for step 1. H + succinyl-CoA The addition of water to fumarate places a hydroxyl group next to a carbonyl carbon. COO– oxalosuccinate intermediate COO– Step 5 In the third oxidation step in the cycle, FAD removes two hydrogen atoms from succinate. CO2 COO– Step 4 A phosphate molecule from solution displaces the CoA, forming a high-energy phosphate linkage to succinate. This phosphate is then passed to GDP to form GTP. (In bacteria and plants, ATP is formed instead.) C COO– isocitrate The a-ketoglutarate dehydrogenase complex closely resembles the large enzyme complex that converts pyruvate to acetyl CoA (pyruvate dehydrogenase). It likewise catalyzes an oxidation that produces NADH, CO2, and a high-energy thioester bond to coenzyme A (CoA). COO– malate COO– malate dehydrogenase C O CH2 + NAD NADH + H + COO– oxaloacetate + HS CoA