Citric acid cycle © Department of Biochemistry J.D.+E.T.) 2009 Three phases of nutrient catabolism • I.Hydrolysis of biopolymers to smaller units in digestion tract – no yield of energy II. Metabolism of glucose ® acetylCoA – small amount of ATP + reduced cofactors, metabolism of amino acids ® pyruvate, acetylCoA or some intermediates of TCA – some reduced cofactors beta oxidation of FA – acetyl-CoA + reduced cofactors III. Oxidation of acetyl-CoA in citric acid cycle – GTP + reduced cofactors oxidation of reduced cofactors in respiratory chain – ATP (highest yield of energy) Formation of acetyl-CoA • oxidative decarboxylation of pyruvate • β-oxidation of fatty acids • catabolism of some amino acids • Keton bodies ® acetoacetylCoA  acetylCoA (in extrahepatal tissues) • metabolism of ethanol Citric acid cycle • Products of TCA: CO[2] ® is expired four oxidative steps ® reduced cofactors ® respiratory chain GTP ® ATP (1) Oxalacetate + Acetyl-CoA (2) Citrate ® Isocitrate (2a) Dehydratation of citrate (2b) Hydratation of cis-aconitate Aconitase is inhibited by fluoracetate (3) Isocitrate ® 2-oxoglutarate (4) 2-Oxoglutarate ® succinyl-CoA (5) Succinyl-CoA + GDP + P[i] GTP i formed in three-steps reaction Chemical energy of macroergic succinyl-CoA is gradually transformed into two macroergic intermediates and in the end to macroergic GTP (Passing a hot potato) (5a) Addition of phosphate to succinyl-CoA (5b) Phosforylation of His in active center of the enzyme (5c) Phosforylation of GDP GTP is quickly converted to ATP (6) Succinate ® fumarate Succinate dehydrogenase is a component of respiratory chain in the inner mitochondrial membrane Malonate is competitive inhibitor of succinate dehydrogenase (7) Fumarate ® L-malate Compare: Hydration of fumarate (8) L-malate ® oxalacetate The net equation of TCA The energetic yield Products of TCA 1 × GTP 3 × NADH + H^+ 1 × FADH[2] Equivalent to ATP (resp.chain) 1 9 2 Total: 12 ATP Factors affecting TCA • Energy charge of the cell • NADH+H^+/NAD^+ ratio • Allosteric inhibition • Inhibition by products • Supply of oxygen -TCA can proceed only at aerobic conditions (reduced cofactors must be reoxidize in respiratory chain) Key enzymes for regulation Anaplerotic reactions of TCA • Reaction that fill up intermediates of TCA: • Carboxylation of pyruvate → oxalacetate • (reductive carboxylation of pyruvate → malate) • Transamination of aspartate → oxalacetate • Catabolismus of Phe, Tyr → fumarate • Asp (synt. Of urea, purines → fumarate • catabolisms of Val, Ile, Met → succinyl-CoA • Transamination of glutamate → 2-oxoglutarate Carboxylation of pyruvate (biotin) Reductive carboxylation of pyruvate Amphibolic character of TCA TCA provides important metabolic intermediates for anabolic processes: gluconeogenesis, transamination etc. Catabolic processes - entries into the cycle: Anabolic processes – intermediates for synthesis Interrelations among the metabolism pathways Interrelations among the metabolism of nutrients TCA and transamination TCA and synthesis of lipids Vitamins necessary for TCA Recommended intake of nutrients