Kreb's Cycle (aka, tricarboxylic acid cycle, citric acid cycle)

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PROF. SYLVESTER L.B. KAJUNAKreb’s Cycle Tricarboxylic Acid (TCA) Cycle, citric acid cycle)

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Overall goal Makes ATP Makes NADH Makes FADH2 Requires some carbohydrate to run Reaction coupling

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LocationGlycolysis in the cytosol Krebs in mitochondrial matrix Mitochondrion Outer membrane very permeable Space between membranes called intermembrane space Inner membrane (cristae) Permeable to pyruvate, Impermeable to fatty acids, NAD, etc Matrix is inside inner membrane

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Conversion of pyruvate to Acetyl CoA2 per glucose (all of Kreb’s) Oxidative decarboxylation Makes NADH

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Fates of Acetyl CoAIn the presence of CHO and using energy Metabolized to CO2, NADH, FADH2,GTP and, ultimately, ATP If energy not being used (Lots of ATP present) Made into fat If energy being used, but no CHO present Starvation Forms ketone bodies (see fat metabolism slides) Danger!

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Kreb’s Cycle

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Net From Kreb’sOxidative process 3 NADH FADH2 GTP X 2 per glucose 6 NADH 2 FADH2 2 GTP All ultimately turned into ATP (oxidative phosphorylation…later)

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Citrate Synthase Reaction (First)Claisen condensation

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Aconitase ReactionForms isocitrate Goes through alkene intermediate (cis-aconitate) elimination then addition Hydroxyl moved and changed from tertiary to secondary (can be oxidized)

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Isocitrate DehydrogenaseAll dehydrogenase reactions make NADH or FADH2 Oxidative decarboxylation Energy from increased entropy in gas formation

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α-ketoglutarate dehydrogenaseSame as pyruvate dehydrogenase reaction Formation of thioester endergonic driven by loss of CO2 increases entropy Exergonic

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Succinyl CoA synthetaseHydrolysis of thioester Releases CoASH Exergonic Coupled to synthesis of GTP Endergonic GTP very similar to ATP and interconverted later -2.9kJ

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Succinate dehydrogenaseDehydrogenation Uses FAD NAD used to oxidize oxygen-containing groups Aldehydes alcohols FAD used to oxidize C-C bonds

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FumaraseAddition of water to a double bond

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Malate DehydrogenaseOxidation of secondary alcohol to ketone Makes NADH Regenerates oxaloacetate for another round

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Net From Kreb’sOxidative process 3 NADH FADH2 GTP X 2 per glucose 6 NADH 2 FADH2 2 GTP All ultimately turned into ATP (oxidative phosphorylation…later)

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Total Energy per glucoseCytosol Glycolysis 2 NADH 2 ATP Mitochondrion Pyruvate dehydrogenase 2 NADH Krebs 6 NADH 2 FADH2 2 GTP

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Total Energy/glucoseIn mitochondrion: Each NADH makes 2.5 ATP Each FADH2 makes 1.5 ATP GTP makes ATP So… From in mitochondrion 8 NADH X 2.5 ATP/NADH = 20 ATP 2 FADH2 X 1.5 ATP/FADH2= 3 ATP 2 GTP X 1 ATP / GTP = 2 ATP TOTAL in mitochondrion 25 ATP

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Total Energy/ glucoseCytosol 2 ATP 2 NADH NADH can’t get into mitochondrion In eukaryotes two pathways, transferred to FADH2 get 1.5 ATP/ FADH2 Or transferred to NADH Get 2.5 ATP/ NADH (Not a problem in prokaryotes (why?)) 2 NADH X 1.5 ATP = 3 ATP Or 2 NADH X 2.5 ATP = 5 ATP + =2 ATP Total 3+ 2 or 5 + 2 so either 5 or 7

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ATP/glucoseEukaryotes Mitochondrial: 25 ATP Cytosolic: 5 or 7 ATP Total 30 or 32 ATP/glucose 30 ATP X 7.3kcal X 4.18 kJ = 915 kJ ATP kcal If 32 ATP = 976 kJ Prokaryotes 32 ATP X 7.3kcal X 4.18 kJ = 976 kJ ATP kcal

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Last Updated: 8th March 2018

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