Tricarboxylic Acid Cycle (TCA) Oct 2024 PDF

Tricarboxylic Acid Cycle Oct 2024 Dr Lynn O’Connor  TCA cycle Citric acid cycle Krebs cycle  What is it? A metabolic series of reactions functioning like a traffic roundabout –molecule move on and off to and from various different roads The final pathway where the oxidative...

Tricarboxylic Acid Cycle Oct 2024 Dr Lynn O’Connor  TCA cycle Citric acid cycle Krebs cycle  What is it? A metabolic series of reactions functioning like a traffic roundabout –molecule move on and off to and from various different roads The final pathway where the oxidative catabolism of CHO, AAs and FAs converge and their carbon skeletons converted to CO2  What is the function of TCA? A pathway for the oxidation of CHO, AAs and FAs which provides energy for the production of the majority of ATP in most animals Supplies intermediates for a number of synthetic rxns e.g Generation of glucose from the c- skeleton of some AAs Provides building blocks for the synthesis of Aas and heme  Other important Features of TCA Anaplerotic reactions help replenish intermediates in the pathway (filling up – from catabolism of aas for example ) Located in the mitochondria – close to the location of ETC O2 requiring pathway REACTIONS OF THE CYCLE 1. Oxidative decarboxylation of pyruvate Acetyl CoA is the 2-C substrate for the cycle Acetyl CoA is derived from oxidative decarboxylation of pyruvate Pyruvate must be transported from cytosol into the mitochondrion Pyruvate is converted to acetyl CoA by pyruvate dehydrogenase complex- a multienzyme complex  Pyruvate Dehydrogenase Complex Requires 5 coenzymes Thiamine pyrophosphate (TPP) Lipoic acid CoA Flavin adenine dinucleotide (FAD) Nicotinamide adenine dinucleotide (NAD) Note: Deficiencies of thiamine or niacin can cause serious CNS problems. Brain cells unable to produce sufficient ATP if PDH complex is inactive Wernicke-Korsakoff – an encephalopathy-psychosis syndrome due to Thiamine deficiency is often seen in alcoholics  Regulation of PDH Covalently modified by phosphorylation/dephosphorylation Allosterically modified by ATP, acetyl CoA and NADH (all high energy signals) –product inhibition Ca2+ activates enzyme – particularly important in skeletal muscle High energy signals switch the enzyme off specifically ATP/ADP, NADH/NAD+, acetyl CoA/CoA ratios  PDH Deficiency Case study for PDH deficiency https://www.youtube.com/watch?v=mrFb3v1SUSQ RARE The most common biochemical cause of congenital lactic acidosis Symptoms – Neurodegeneration, muscle spasticity and in the neonatal onset form , early death – No proven treatment – dietary restriction of CHO and supplementation with thiamine helpful with some patients – Leigh Syndrome: Neurodegenerative disorder mutation resulting in either Mit or Nuclear DNA impairing some aspect of ATP production Arsenic poisoning: inhibition of enzymes that require lipoic acid as a coenzyme Alex EMG -Peripheral Nerve Damage in arms and legs  Alex – development milestones delayed Lacking in energy unsteady on his feet Electromyograph – peripheral nerve damage MRI – clear Another infection – Alex ICU on a glucose drip Lactic acid in his blood rises to dangerous level PDH deficiency Glucose drip replaced with proteins and lactate levels drop Peripheral nerve damage incurred as an infant – permanent Life long ketogenic diet  Deficiency of enzymes of the TCA cycle is rare This indicates importance of this pathway for survival Some cases of Fumarase deficiency have been reported – extremely rare Result – encephalopathy , severe intellectual disabilities, unusual facial features, brain malformation and epileptic seizures- due to the low levels of fumarase 2. Synthesis of Citrate from Acetyl CoA and oxaloacetate By Citrate Synthase Enzyme is inhibited by citrate/substrate availability Note 1: citrate provides a source of Acetyl CoA for the synthesis of Fatty acids (interconversion of metabolites) Note 2: Citrate inhibits PFK -1 in glycolysis 3. Isomerization of Citrate By aconitase Enzyme is an Fe-S protein Inhibited by fluoracetate, a plant toxin used as a pesticide 4. Oxidative decarboxylation of isocitrate By isocitrate dehydrogenase - Yields the 1st of 3 NADHs - Yields the first of 2 CO2 This is one of the rate limiting steps of TCA Enzyme allosterically activated by ADP and Ca2+ Inhibited by ATP and NADH 5. Oxidative decarboxylation of -ketoglutarate By -ketoglutarate dehydrogenase rxn releases the 2nd CO2 produces the 2nd NADH of TCA Coenzymes are TPP, Lipoic acid, FAD, NAD+ and CoA -KGDH is inhibited by its products, NADH and succinyl CoA and activated by Ca2+ Not regulated by phosphorylation -KG is also produced from breakdown of amino acid glutamate (note: anaplerotic pathway/ interconversion of metabolism of various molecules) 6. Cleavage of succinyl coenzyme A By Succinate thiokinase -generated GTP – substrate-level phosphorylation Succinyl CoA is also produced from propionyl CoA (FA metabolism) and from the metabolism of several amino acids 7. Oxidation of Succinate By Succinate dehydrogenase FAD is coenzyme and is reduced to FADH2 The only TCA enzyme embedded in the inner mit membrane – functions as Complex 11 of the ETC 8. Hydration of Fumarate By Fumarate hydratase (fumarase) Fumarate is also produced by the urea cycle, in purine synthesis and during catabolism of the amino acids phenylalanine and tyrosine 9. Oxidation of Malate By Malate Dehydrogenase Produces the 3rd NADH: Note OAA also produced by transamination of the amino acid aspartate  Energy produced by TCA Acetyl CoA is a 2-C compound One turn of the cycle -2 CO2 leave No net C change No net consumption of OAA or any other intermediate 4 pairs of electrons are transferred during one turn of the cycle 3 NAD 1 FAD  Regulation of TCA Most important a) Citrate synthase All have highly negative G o b) Isocitrate dehydrogenase c) -Ketoglutarate dehydrogenase Reducing equivalents for ox phos – generated by PDH and the TCA - Both upregulated by a low ATP to ADP ratio Role of TCA in anabolism  Defects in the citric acid cycle contribute to the development of cancer See p 513 Stryer 8th edition  Where do the reactions of TCA occur in the cell? Identify the positive and negative regulators of the regulatory enzymes? How is this cycle linked to glycolysis and to ETC? Identify links with fatty acid and protein metabolism What are the important cofactors for the enzymes of TCA – significant in the event of a vit deficiency Which reactions require oxygen? If any What role does TCA have in anabolism? What role does TCA have in catabolism?