Biochemistry TCA Cycle PDF

1A BIOCHEMISTRY TRICARBOXYLIC ACID CYCLE DR. JANDOC...

1A BIOCHEMISTRY TRICARBOXYLIC ACID CYCLE DR. JANDOC  Transported to mitochondria to enter TCA cycle OUTLINE  Pyruvate dehydrogenase complex I. Overview  Pyruvate  acetyl CoA II. Reactions in the TCA cycle  Component enzymes  Oxidative decarboxylation of pyruvate  Pyruvate dehydrogenase complex  Component enzymes  E1 – Pyruvate Dehydrogenase or Decarboxylase  Coenzymes  Thiamine pyrophosphate  Regulation of the pyruvate dehydrogenase  E2 – Dihydrolipoyl Dehydrogenase complex  Lipoic acid  Pyruvate dehydrogenase deficiency  CoA  Mechanism of Arsenic poisoning  E3 – Dihydrolipoyl dehydrogenase  Synthesis of citrate  FAD  Isomerization of citrate  NAD+  Oxidation and decarboxylation of isocitrate  Regulatory enzymes  Cleavage of succinyl CoA  Pyruvate dehydrogenase kinase  Oxidation of the succinate  Pyruvate dehydrogenase phosphatase  Hydration of the fumarate  Deficiency of thiamine or niacin  Oxidation of the malate  Wernicke-Korsakoff III. Energy produced by the TCA cycle  Encephalopathy-psychosis syndrome IV. Regulation of the TCA cycle  Thiamine deficiency  Alcohol abuse  CNS problems I. OVERVIEW  Brain cells unable to produce enough ATP, if the  Tricarboxylic acid cycle PDH complex is inactive  Kreb’s cycle or Citric acid cycle  FINAL PATHWAY where metabolism of carbohydrate, amino acids and fatty acids converge  Carbon skeleton converted to CO2  Production of the majority of ATP  Cycle occurs in mitochondria, in close proximity to the reactions of electron transport, which oxidize the reduced coenzymes  Aerobic pathway, O2 as final electron acceptor  Regulation of the pyruvate dehydrogenase complex  Regulatory enzymes  Activate and inactivate E1  Pyruvate dehydrogenase kinase  Phosphorylates and inhibits E1  TCA cycle does not proceed  Activator (turn off PDH complex)  ATP  acetyl CoA  NADH  Inhibitor  Pyruvate  Pyruvate dehydrogenase phosphatase  Dephosphorylates and activates E1  TCA cycle proceeds  Activator  Calcium – released during contraction, which the stimulate the PDH complex for II. REACTIONS OF THE TCA CYCLE energy production A. OXIDATIVE DECARBOXYLATION OF PYRUVATE  PYRUVATE  Endproduct of aerobic glycolysis  Specific pyruvate transporter Trans 1 | Raff 1 of 5 BIOCHEMISTRY TCA CYCLE C. ISOMERIZATION OF CITRATE  Citrate  isocitrate  Via Acotinase  Inhibited by Fluoroacetate (rat poison)  Fluoroacetyl CoA + OAA  Fluorocitrate  Pyruvate dehydrogenase deficiency  Citrate accumulation  most common cause of Congenital Lactic Acidosis  X-linked dominant  unable to convert pyruvate to acetyl CoA  shunted to lactic acid via LDH  neurodegeneration, muscle spasticity, neonatal death  Treatment: none  Dietary restriction of carbohydrate  Supplementation with TPP may reduce symptoms  Mutation in PDH complex, electron transport chain or ATP synthase  Leigh syndrome  Subacute necrotizing encephalomyelopathy  Progressive neurological disorder  Defect in mitochondrial ATP production  Mechanism of Arsenic poisoning  Forms stable complex with the thiol (-SH) group of lipoic acid D. OXIDATION AND DECARBOXYLATION OF ISOCITRATE  Inhibit enzymes that require lipoic acid as a coenzyme  Isocitrate dehydrogenase  E2 of the PDH complex  Irreversible oxidative decarboxylation of isocitrate  Aα-ketoglutarate dehydrogenase  Yield 3 NADH and CO2  Aα-keto acid dehydrogenase  Activator  Accumulation of pyruvate  lactate  ADP  Affect brain  Inhibitor  Neurologic disturbance and death  ATP  NADH B. SYNTHESIS OF CITRATE  Rate limiting step in TCA cycle  Acetyl CoA + Oxaloacetate  Citrate  Catalysed by Citrate synthase  Not an allosteric enzyme  CITRATE  Inhibit citrate synthase and phosphofructokinase  Activates acetyl CoA carboxylase RAFF 2 of 5 BIOCHEMISTRY TCA CYCLE  Cleaves the high-energy thioester bond  Coupled to phosphorylation of GDP to GTP  GTP and ATP are energetically interconvertible  GTP + ADP GDP + ATP  Succinyl CoA  Can be produced from propionyl CoA derived from fatty acids with odd number of C, and from several AA E. OXIDATICE DECARBOXYLATION OF α-ketoglutarate  α-ketoglutarate to succinyl CoA  α-ketoglutarate dehydrogenase complex nd  Yield 2 CO2 and NADH  Coenzymes required  TPP  Lipoic acid  FAD  NAD G. OXIDATION OF THE SUCCINATE  CoA  Succinate dehydrogenase  Inhibitor  Succinate  fumarate  NADH  FAD  FADH2 (succinate is not sufficient to reduce NAD)  Succinyl CoA  Only enzyme embedded in the inner mitochondrial  Ca++ membrane  α-ketoglutarate  Function as Complex II of the electron transport chain  Also produced from oxidative deamination or transamination of glutamate H. HYDRATION OF THE FUMARATE  Fumarase (Fumarate hydratase)  Fumarate  L-Malate  Fumarate  Also produced by urea cycle, purine synthesis and F. CLEAVAGE OF SUCCINYL CoA catabolism of phenylalanine and tyrosine  Succinate thiokinase (succinyl CoA synthetase – for reverse reaction) RAFF 3 of 5 BIOCHEMISTRY TCA CYCLE  2 C atoms  acetyl CoA and leave as CO2  No net consumption or production of oxaloacetate and its intermediate  4 pairs of electrons are transferred  3 from NAD  NADH  From FAD  FADH2  Oxidation of NADH  3 ATP  Oxidation of FADH2  2 ATP I. OXIDATION OF MALATE  Malate dehydrogenase  Malate  Oxaloacetate rd  3 NADH 0  ∆G is positive, but reaction is driven in the direction of oxaloacetate by the highly exergonic citrate synthase  Oxaloacetate  Transamination of aspartic acid IV. REGULATION OF THE TCA CYCLE III. ENERGY PRODUCED BY THE TCA CYCLE RAFF 4 of 5 BIOCHEMISTRY TCA CYCLE RAFF 5 of 5