Citric Acid Cycle

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• Citric Acid Cycle

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Gylcolysis
Electron Transport and Oxidative phosphorylation
TCA Cycle
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The TCA Cycle

• (aka Citric Acid Cycle, Krebs Cycle)
• Pyruvate (actually acetate) from glycolysis is
  degraded to CO2
• Some ATP is produced
• More NADH is made
• NADH goes on to make more ATP in electron
  transport and oxidative phosphorylation

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Entry into the TCA Cycle

• Pyruvate is translocated from the cytosol to the
  mitochondria
• Pyruvate is oxidatively decarboxylated to form
  acetyl-CoA
• Pyruvate dehydrogenase uses TPP, CoASH, lipoic
  acid, FAD and NAD
• Acetyl-CoA then enters TCA cycle thru citrate
  synthase

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Pyruvate Dehydrogenase Complex

• Composed of three enzymes
• pyruvate dehydrogenase (E1) (cofactor TPP)
• Dihydrolipoamide acetyltransferase (E2) (cofactor
  Lipoamide, CoA)
• Dihydrolipoamide dehydrogenase (E3) (cofactor
  FAD, NAD)

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Pyruvate Dehydrogenase
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Citrate Synthase

• Only step in TCA cycle that involves the
  formation of a C-C bond

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Aconitase

• Isomerization of Citrate to Isocitrate
• Citrate is a poor substrate for oxidation
• So aconitase isomerizes citrate to yield
  isocitrate which has a secondary -OH, which can
  be oxidized
• Aconitase uses an iron-sulfur cluster to position
  citrate (binds OH and carboxyl of central carbon)

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Isocitrate Dehydrogenase

• Oxidative decarboxylation of isocitrate to yield
  ? -ketoglutarate
• Classic NAD chemistry (hydride removal) followed
  by a decarboxylation
• Isocitrate dehydrogenase is a link to the
  electron transport pathway because it makes NADH
• Rxn is metabolically irreversible

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? -Ketoglutarate Dehydrogenase

• A second oxidative decarboxylation
• This enzyme is nearly identical to pyruvate
  dehydrogenase - structurally and mechanistically
• Five coenzymes used - TPP, CoASH, Lipoic acid,
  NAD, FAD

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Succinyl-CoA Synthetase

• A substrate-level phosphorylation
• A nucleoside triphosphate is made (ATP in
  plants/bacteria and GTP in animals)
• Its synthesis is driven by hydrolysis of a CoA
  ester

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Succinate Dehydrogenase

• An oxidation involving FAD
• Mechanism involves hydride removal by FAD and a
  deprotonation
• This enzyme is actually part of the electron
  transport pathway in the inner mitochondrial
  membrane
• The electrons transferred from succinate to FAD
  (to form FADH2) are passed directly to ubiquinone
  (UQ) in the electron transport pathway
• Enzyme inhibited by malonate

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Fumarase

• Hydration across the double bond
• trans-addition of the elements of water across
  the double bond
• Stereospecific reaction

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Malate Dehydrogenase

• An NAD-dependent oxidation
• The carbon that gets oxidized is the one that
  received the -OH in the previous reaction
• This reaction is energetically expensive
• ?Go' 30 kJ/mol

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Reduced Coenzymes Fuel ATP Production

• Acetyl-CoA 3 NAD Q GDP Pi 2 H20 ?
  HS-CoA 3NADH QH2 GTP 2 CO2 2 H
• Isocitrate Dehydrogenase 1 NADH2.5 ATP
• a-ketoglutarate dehydrogenase 1 NADH2.5 ATP
• Succinyl-CoA synthetase 1 GTP1 ATP
• Sunccinate dehydrogenase 1 QH21.5 ATP
• Malate Dehydrogenase 1 NADH2.5 ATP
• Total of 10 ATPs gained from oxidation of 1
  Acetyl-CoA

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Regulation of TCA Cycle
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TCA Cycle provides intermediates for many
biosynthetic processes
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The Anaplerotic Reactions

• The "filling up" reactions
• PEP carboxylase - converts PEP to oxaloacetate
• Pyruvate carboxylase - converts pyruvate to
  oxaloacetate
• Malic enzyme converts pyruvate into malate