TCA CYCLE (Citric Acid Cycle) - Purdue Chemistry

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CHM333 LECTURE 32: 11/23 – 30/09FALL 2009Professor Christine HrycynaTCA CYCLE (Citric Acid Cycle) The Citric Acid Cycle is also knownas:– Kreb’s cycle Sir Hans Krebs Nobel prize, 1953– TCA (tricarboxylic acid) cycle The citric acid cycle requires aerobicconditions!!!!TCA CYCLE–Cells have evolved to useoxygen–Oxygen serves as the finalelectron acceptor as pyruvate(from glycolysis) is converted(oxidized) completely to CO2and H2O If cell is under anaerobicconditions energy productionis not too efficient - 10% ofenergy possible is generated Pyruvate converted toAcetyl-CoA by PDH andthen Acetyl-CoA enters theTCA cycleEnergy in the citric acid cycle Energy of the oxidationreactions is largely conservedas reducing powerCoenzymes reduced:- NAD /NADH- FAD/FADH2 Reduced coenzymes used byelectron transport chain andoxidative phosphorylation tomake ATP221

CHM333 LECTURE 32: 11/23 – 30/09FALL 2009Professor Christine HrycynaThe Tricarboxylic acid (TCA) cycle (citric acid cycle) is amphibolic (both catabolic and anabolic)The TCA Cycle Serves Two Purposes:1. Oxidize Acetyl-CoA to CO2 to produce energy- ATP (GTP)- Reducing power of NADH and FADH2- The cycle is involved in the aerobic catabolism ofcarbohydrates, lipids and amino acids2.Supply precursors for biosynthesis ofcarbohydrates, lipids, amino acids, nucleotides andporphyrins Intermediates of the cycle are starting pointsfor many biosynthetic reactions The cycle itself is not a pathway for a netdegradation of any cycle intermediatesCycle intermediates can be shared with otherpathways, which may lead to a re-supply ornet decrease in cycle intermediatesReactions feeding into the cycle replenish thepool of cycle intermediates Fundamental Differences between Glycolysis and TCA Cycle:1.2.3.Glycolysis is a linear pathway; TCA cycle is cyclicGlycolysis occurs in the cytosol and TCA is in the mitochondrialmatrixGlycolysis does not require oxygen; TCA requires oxygen(aerobic)Summary of the citric acid cycleFor each acetyl-CoA that enters the cycle:(1) Two molecules of CO2 are released(2) Coenzymes NAD and FAD arereduced(3) One GDP (or ADP) is phosphorylated(4) The initial acceptor molecule oxaloacetate is reformedFADH2H2Energy conservation by the cycle Energy is conserved in the reduced coenzymes NADH, FADH2 and one GTP222

CHM333 LECTURE 32: 11/23 – 30/09FALL 2009Professor Christine Hrycyna NADH, FADH2 can be oxidized to produce ATP by oxidative phosphorylation Energy is also conserved in either ATP or GTP- produced by substrate-level phosphorylation(from the thioester bond in succinyl-CoA.) The use of many steps in the oxidation of acetyl CoA to CO2 enables conservation of most of theenergy as work with little lost as heat223

CHM333 LECTURE 32: 11/23 – 30/09FALL 2009Professor Christine Hrycyna8 REACTIONS OF THE TCA CYCLE:1. Formation of Citrate Citrate formed from condensation of acetyl CoA and oxaloacetateAddition of acetyl to the keto double bond of OAA aldol condensationOnly cycle reaction with C-C bond formationNo energy of ATP hydrolysis neededSynthase is an enzyme that catalyzes addition to a double bond or elimination to form a doublebond without needing ATP hydrolysisBoth Hydrolysis Reaction and Non-hydrolytic cleavage (addition or elimination)224

CHM333 LECTURE 32: 11/23 – 30/09 FALL 2009Professor Christine HrycynaLocoweed is toxic because it accumulates fluoroacetate2. Aconitase Isomerization of citrate (3 alcohol) to isocitrate (2 alcohol)Aconitase contains an iron-sulfur center as a prosthetic groupCatalyzes a lyase reaction that results in rearrangement of citrate with a tertiary alcohol toisocitrate with a secondary alcoholNon-hydrolytic cleavage (addition or elimination)Goes through an enzyme bound cis-aconitate intermediateElimination of H2O from citrate to form C C bond of cis-aconitateRearrangement allows the further oxidation of the molecule225

CHM333 LECTURE 32: 11/23 – 30/09FALL 2009Professor Christine Hrycyna3. Isocitrate Dehydrogenase First oxidative decarboxylation of isocitrate to α-ketoglutarate (α-kg) Metabolically irreversible reaction One of four oxidation-reduction reactions of the cycle Also a Non-hydrolytic cleavage reaction (addition or elimination) Hydride ion from the C-2 of isocitrate is transferred to NAD to form NADH Oxalosuccinate is decarboxylated to α-ketoglutarate4. α-Ketoglutarate Dehydrogenase Complex-Second oxidative decarboxylation reaction-Also a Non-hydrolytic cleavage reaction (addition or elimination)-α-Ketoglutarate converted to Succinyl-CoA-Similar to pyruvate dehydrogenase complex except a succinyl group is activated, not acetyl Same coenzymes, identical mechanisms-Succinyl-CoA thioester is VERY high energy-Generates NADH226

CHM333 LECTURE 32: 11/23 – 30/09-FALL 2009Professor Christine HrycynaPurpose of step: Collect energy from α-ketoglutarate decarboxylation into the high energysuccinyl-CoA molecule5. Succinyl-CoA Synthetase (Formation of succinate) Free energy in thioester bond of succinyl CoA is conserved as GTP (or ATP in plants, somebacteria)Enzyme: Succinyl-CoA Synthetase Two forms in higher animals: One prefers ADP the other GDP) SUBSTRATE-LEVEL PHOSPHORYLATION Formation of ATP directly coupledto the reaction (group transfer reaction) Only step where ATP (GTP) is formed directly in the TCA cycle All other ATP is produced by oxidative phosphorylation Oxidative phosphorylation is the oxidation of reduced co-factors NADH andFADH2 to O2 – release of energy drives ATP formation from ADP PiGTP227

CHM333 LECTURE 32: 11/23 – 30/09FALL 2009Professor Christine Hrycyna6. The Succinate Dehydrogenase (SDH) Complex Located on the inner mitochondrial membrane (other components are in the matrix)Oxidation-reduction reaction that forms a carbon-carbon double bondSuccinate is oxidized to fumarate, while FAD is reduced to FADH2 NAD functions in reactions that interconvert hydroxyl and carbonyl groupsDehydrogenation is stereospecific; only the trans isomer is formedAlso known as Complex II of the electron transport chain – direct feed of electrons fromFADH2 into the electron transport chain.Substrate analog malonate is a competitive inhibitor of the SDH complexMalonate is a structural analog of succinateMalonate binds to the enzyme active site, and is a competitive inhibitor228

CHM333 LECTURE 32: 11/23 – 30/09FALL 2009Professor Christine Hrycyna7. Fumarase Stereospecific trans addition of water to the double bond of fumarate to form L-malate-Only forms the L-isomerNon-hydrolytic cleavage reaction8. Malate Dehydrogenase-Regeneration ofoxaloacetate from Lmalate-Enzyme: malatedehydrogenase(oxidation – reductionreaction)-Generates NADHOVERALL SUMMARY OF TCACYCLE:1. Oxidation of Acetyl-CoA to CO2- CO2 leaves at steps 3 and 42. 3 NAD are reduced to NADH bydehydrogenase reactions-Steps 3, 4, and 8-isocitrate dehydrogenase-α ketoglutarate dehydrogenase-malate dehydrogenase3. 1 molecule of FAD reduced to FADH2-Step 6 – Succinate dehydrogenase4. 1 phosphoanhydride bond formed in ATPor GTP-Substrate level phosphorylation at step 5: Succinyl-CoA Synthetase-Generated from energy stored in CoA thioester229

CHM333 LECTURE 32: 11/23 – 30/09FALL 2009Professor Christine HrycynaSo, per pyruvate:4 NADH (one from pyruvate dehydrogenase complex 3 TCA)1 ATP or GTP1 im/Chapter 16 Fig. 16-2 -- The Reactions of the Citric Acid CycleENERGY FROM THE TCA CYCLE:Reduced Coenzymes Fuel the Production of ATP Each acetyl CoA entering the cycle nets:(1) 3 NADH(2) 1 FADH2(3) 1 GTP (or 1 ATP) Oxidation of each NADH yields 2.5 ATP Oxidation of each FADH2 yields 1.5 ATP Complete oxidation of 1 acetyl CoA 10 ATP230

CHM333 LECTURE 32: 11/23 – 30/09FALL 2009Professor Christine HrycynaGlucose degradation via glycolysis, citric acid cycle, and oxidative phosphorylationAEROBICTOTAL/glucose 32 ATPIf anerobic – Lactate is formed from pyruvate after glycolysis by lactate dehydrogenase and the NADHformed is USED. Therefore, net gain of 2 ATP/glucose, not 32! (Hence 5-10% efficiency)-Occurs in muscles during exercise because they go into oxygen debt.Soreness due to H from lactic acidMetabolism in muscles returns to normal when oxygen replenished**Should be able to determine #ATP produced given a starting and stopping point in glycolysis or citricacid cycle. (i.e. know where NADH/FADH2 or ATP are made and how many.Regulation of the Citric Acid Cycle Regulation depends on the ENERGY LEVEL of cells – key to keep energy level constantWhen cells have lots of energy (ATP, NADH), the reactions involved in making more are slowedThe reverse is also true.Pathway controlled by:(1) Small molecule modulators (products of the cycle can inhibit)(2) Covalent modification of cycle enzymes(3) Supply of acetyl CoARegulation of the PDH complex-Highly regulated231

CHM333 LECTURE 32: 11/23 – 30/09FALL 2009Professor Christine Hrycyna-Regulation of pyruvate dehydrogenase complex controls acetyl CoA supply-Gatekeeper to aerobic metabolism-Represents the committed step because pyruvatecan still go back to glucose (gluconeogenesis)but acetyl-CoA cannot go back to glucose.-Inhibitors: Indicators of high energy statusCitrate Synthase isalso regulated:( ) ADP(-) NADH,succinyl-CoA,citrate, ATPo NADH, ATP, Acetyl-CoA, Fatty acids(degraded to form acetyl-CoA)-Stimulators: Indicators of low energy statuso AMP, NAD , Coenzyme A (CoA-SH)Regulation of citrate synthase- Inhibitors: NADH, ATP, succinyl-CoA, citrate- Stimulators: ADPRegulation of isocitrate dehydrogenase (ICDH)- Inhibitors: NADH and ATP- Stimulators: NAD , ADP and Ca 2POINTS OF REGULATIONRegulation of α-ketoglutarate dehydrogenase complex- Inhibitors: NADH, ATP and succinyl-CoA- Stimulators: NAD , ADP, / Chapter 16Fig. 16-14 -- Regulation of the Citric Acid Cycle232

Cycle intermediates can be shared with other pathways, which may lead to a re-supply or net decrease in cycle intermediates Reactions feeding into the cycle replenish the pool of cycle intermediates Funda

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