Lecture 24½ 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) –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

Regulation Krebs cycle activity is controlled early on –At isocitrate dehydrogenase (ICDH) –alpha-ketoglutarate dehydrogenase (aKGDH) ICDH and OGDH are stimulated by rise in Ca 2+ –Such as is found during exercise ICDH & OGDH are also sensitive to NAD levels –Activity is dependent on availability of NAD

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. FADH 2 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)

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 ––CH 2 - going to –C=O –Oxidation with FAD  hydration  oxidation with NAD

Acetyl-CoA oxaloacetatecitrate ketoglutarate Succinyl-CoA succinate isocitrate Regenerates CoA Citrate could leave to cytoplasm in liver and WAT Controlled by calcium & NAD, loss of CO 2 Generates GTP Identical sequence of reactions to FA oxidation: FAD, water, NAD Summary