Chapter 17 Kreb·s cycle y y y In mitochondria Oxidize the acetyl moiety of acetyl CoA Reduces coenzymes reoxidized through ETC, linked to formation of ATP Final common pathway for oxidation of CHO, Lipid and CHON because glucose, FA and AA are metabolized to Acetyl CoA or intermediates Central role in glycogenesis, lipogenesis and interconversion of AA
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CAC provides substrate in resp chain Acetyl CoA + oxaloacetate = citrate y 2 CoA released y Oxaloacetate regenerated - Oxidation of acetyl CoA = coenzyme are reduced & reoxidized in resp chain, linked to formation of ATP - Aerobic - O2 final oxidant - Enzymes = mitochondrial matrix, free or attached y Reactions of the CAC liberate reducing equivalents & CO2
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Acetyl CoA + oxaloacetate = citrate Enzyme: citrate synthase C-C bond between methyl C of acetyl CoA & Carbonyl C of oxaloacetate Citrate dehydrated = cis aconitate Enzyme: aconitase Fluoroacetate : toxic, condense with oxaloacetate to form fluocitrate which inhibits aconitase = Citrate accumulate Citrate Rehydrated =isocitrate (aconitase) Isocitrate dehydrogenated= oxalosuccinate Enzyme: isocitrate dehydrogenase Oxalosuccinate decarboxylated = ketoglutarate -requires Mg++ or Mn++ Enzyme: isocitrate dehydrogenase 3 isoenzymes of isocitrate dehydrogenase ²uses NAD+ only in mitochondria other 2 uses NADP+ in mitochondria and cytosol ketoglutarate = oxidative carboxylation = succinyl CoA Enzyme : ketoglutarate dehydrogenase complex Cofactors : thiamin diphosphate, lipoate, NAD+, FAD+ & CoA Physiologically unidirectional
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In pyruvate oxidation, arsenite inhibits the reaction, causing ketoglutarate to accumulate Succinyl CoA ² succinate Enzyme : succinate thiokinase (succinyl CoA synthetase) Liver & Kidney: 2 isoenzymes of succinate thiokinase, 1 specific for GDP, 1 for ADP GTP formed: used in decarboxylation of oxaloacetate to phosphoenolpyruvate in glucogenesis Nongluconeogenic tissues : isoenzyme that uses ADP When ketone bodies are being metabolized in extrahepatic tissues : alternative reaction catalyzed by succinyl CoAAcetoacetate- CoA transferase(thiophorase) CoA from succinyl CoA to acetoacetate = aceto acetyl CoA Onward metabolism of succinate = regeneration of oxaloacetate Dehydregeneration to form C-C double bond, + of H2O to form hydroxyl group , further dehydrogenation to yield the oxo group of oxalacetate 1st dehydrogenation = fumarate
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Enzyme : succinate dehydrogenase (inner surface of inner mitoc membrane Contains FAD & Fe : S protein Reduce Obiquinone in ETC Double bond of fumarate + H2O = Malate Enzyme : Fumarase (Fumarate Hydratase) Malate ² Oxaloacetate Enzyme : Malate Dehydrogenase Requires NAD+
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Twelve ATP are formed/turn of CAC - 3 NADH & 1 FADH2 for each acetyl CoA catabolized in one turn - Reoxidation of each NADH = 2.5 ATP - Reoxidation of each FADH2 = 1.5 ATP - 1 ATP (or GTP) formed by substrate ² level phosphorylation catalyzed by succinate thiokinase Vitamins play key roles in the CAC Vitamin B y Riboflavin = FAD- cofactor for succinate dehydrogenase y Niacin = NAD- electron acceptor for isocitrate dehydrogenase, ketoglutarate dehydrogenase & malate dehydrogenase
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Thiamin (B1) = thiamine diphosphate- coenzyme for decarboxylation in the ketoglutarate dehydrogenase reaction Pantothenic Acid = part of coenzyme A- cofactor attached to ´activeµ carboxylic acid residues
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pyruvate(pyruvate carboxylase) Aminotransferase(transamin ase) produce pyruvate from alanine, oxaloacetate from aspartate, ketoglutarate fromglutamate
CAC takes part in fatty acids synthesis Acethyl CoA formed from pyruvate by pyruvate dehydrogenase y Pyruvate dehydrogenase ² mitochondrial enzyme y Extramitochondrial ²fatty acid synthesis y Acetyl CoA form to citrate to transfer out from mito membrane ²Acetyl CoA now available in cytosol - Catalyzed enzyme : ATP citrate lyase
The CAC plays a pivotal role in metabolism Pathways concern the process of y Glucogenesis y Transamination y Deamination y Fatty acid synthesis Therefore CAC play roles in y Oxidative y Synthetic processes The CACtakes part in glucogenesis transamination and deamination Key enzyme facilitates net transfer out of the cycle of gluconeogenesis y Phosphoenolpyruvate carboxikinase ²catalizes decarboxylation of oxaloacetate to phosphoenolpyruvate y GTP acting as source high energy y Most significant ² oxaloacetate by the carboxylation of