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Transcript
Citric Acid Cycle General Considerations What is the importance of citric acid cycle? final common pathway for oxidation of fuel molecules provides intermediates for biosynthesis amino acids nucleotide bases porphyrin General Considerations Where in the cell does this cycle occur? General Considerations What basically occurs during this cycle? Formation of Acetyl Coenzyme A Importance? links glycolysis to citric acid cycle What’s involved? oxidative decarboxylation catalyzed by pyruvate dehydogenase complex three enzymes five cofactors Formation of Acetyl Coenzyme A Reaction consists of three steps Formation of Acetyl Coenzyme A Pyruvate dehydrogenase catalyzes the first step uses thiamine pyrophosphate (TPP) as coenzyme Formation of Acetyl Coenzyme A Step 1 - Pyruvate combines with TPP and is decarboxylated Formation of Acetyl Coenzyme A Next step also catalyzed by pyruvate decarboxylase and involves lipoamide Formation of Acetyl Coenzyme A Step 2 – hydroxyethyl group is oxidized and acetyl group is transferred to lipoic acid Formation of Acetyl Coenzyme A Step 3 – acetyl group is transferred to coenzyme A reaction catalyzed by dihydrolipoyl transacetylase Formation of Acetyl Coenzyme A Step 4 – lipoamide is regenerated and electrons are transferred to FAD and NAD+ catalyzed by dihyrolipoyl dehdrogenase Formation of Acetyl Coenzyme A Formation of Acetyl Coenzyme A Model of pyruvate dehydrogenase complex Formation of Acetyl Coenzyme A Synthesis of Citrate How is citrate formed? What kind of reaction is this? Synthesis of Citrate How does citrate synthase catalyze this reaction? binding of oxaloacetate causes structural rearrangement binding site for acetyl CoA forms catalysis via proximity of substrates Synthesis of Citrate Synthesis of Citrate Formation of Isocitrate Isomerization of citrate occurs by a dehydration followed by a hydration catalyzed by aconitase Formation of Isocitrate Aconitase is an iron-sulfur protein Formation of -Ketoglutarate Isocitrate is oxidized and decarboxylated to ketoglutarate isocitrate dehydrogenase Formation of Succinyl CoA Oxidative decarboxylation of -ketoglutarate uses same mechanism as conversion of pyruvate to acetyl CoA -ketoglutarate dehdrogenase complex Formation of Succinate Cleavage of thioester bond coupled to formation of GTP substrate level phosphorylation succinyl CoA synthetase Regeneration of Oxaloacetate oxidation of succinate – succinate dehydrogenase iron-sulfur protein hydration of fumarate – fumarase oxidation of malate – malate dehydrogenase Summary of Citric Acid Cycle Regulation of Pyruvate Dehdrogenase Complex end-product inhibition covalent modification energy charge hormones & 1adrenergic agonists via Ca++ Control of Citric Acid Cycle allostric enzymes are at primary control points -ketoglutarate dehydrogenase isocitrate dehydrogenase pyruvate dehydrogenase Intermediates for Biosynthesis Citric Acid Cycle How is oxaloacetate replenished to keep CAC going? carboxylation of pyruvate energy charge influences use of OAA high – converted to glucose low – converted to citrate Clinical Applications What is beriberi and what causes it? nutritional deficiency of thiamine leading to neurological and cardiovascular problems? What specifically causes the problems? Why does arsenic or mercury poisoning cause similar symptoms? binds lipoamide Glyoxylate Cycle What is this cycle and who uses it? metabolic cycle for utilization of acetate plants and bacteria