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Lecture 24 ½

Lecture 24 ½. Krebs Cycle. Overview. Condensation. Formation of citrate One of the methyl-Hs can easily come off acetyl CoA Gives a very reactive species that reacts with oxaloacetate Citrate Contains 6 carbons 3 carboxylic groups  Tricarboxylic acid (TCA)

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Lecture 24 ½

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  1. Lecture 24½ Krebs Cycle

  2. Overview

  3. Condensation • Formation of citrate • One of the methyl-Hs can easily come off acetyl CoA • Gives a very reactive species that reacts with oxaloacetate • Citrate • Contains 6 carbons • 3 carboxylic groups  Tricarboxylic acid (TCA) • Very symmetrical, but the enzyme can still distinguish which carboxylic group comes from acetyl CoA & oxaloacetate  prochiral • Citrate can leave the mitochondria or be oxidised • Depending on whether the cell is doing lipogenesis or needs energy

  4. Regulation • Krebs cycle activity is controlled early on • At isocitrate dehydrogenase (ICDH) • alpha-ketoglutarate dehydrogenase (aKGDH) • ICDH and OGDH are stimulated by rise in Ca2+ • Such as is found during exercise • ICDH & OGDH are also sensitive to NAD levels • Activity is dependent on availability of NAD

  5. Important Features of Krebs Cycle • During the cycle, 2 carbon atoms come in, 2 carbon atoms has gone • but on each cycle only 1 carbon atom from acetyl CoA gets released as carbon dioxide • The other carbon dioxide comes from oxaloacetate • Generates 3 NADH, 1 reduced FAD, i.e. FADH2 plus a GTP • Each NAD gives 2.5 ATP in oxidative phosphorylation • Oxaloacetate is not ‘net’ consumed in the cycle • acts as carrier • Fluoroacetate is a strong inhibitor of the cycle • Creates fluorocitrate which inhibits citrate utilisation • Very toxic and used as poison(1080)

  6. Carriers • Krebs cycle can only go faster if we provide enough carrier • need more oxaloacetate during exercise • In muscle pyruvate carboxylase supplies extra oxaloacetate • Anaplerotic reaction - “filling” reaction • So not only the liver has pyruvate carboxylase • Krebs cycle regenerates the carrier CoA • Enables fatty acids to be oxidised faster • Sequence of reactions from succinate to oxaloacetate have similar ‘strategy’ to oxidation of fatty acids • –CH2- going to –C=O • Oxidation with FAD  hydration  oxidation with NAD

  7. Summary Acetyl-CoA Regenerates CoA Citrate could leave to cytoplasm in liver and WAT oxaloacetate citrate isocitrate Identical sequence of reactions to FA oxidation:FAD, water, NAD Controlled by calcium & NAD, loss of CO2 ketoglutarate Controlled by calcium & NAD, loss of CO2 Succinyl-CoA succinate Generates GTP

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