Lecture 21 TCA Cycle, Regulation (2) PDF

Respiration: Stage 2 Acetyl-CoA Oxidation Generates more NADH, FADH2, and one GTP Remaining carbon atoms from carbohydrates, amino acids, and fatty acids are released during Stage 2. 17 Chemical Logic of the Citric Acid Cycle “Acetyl-CoA” must be oxi...

Respiration: Stage 2 Acetyl-CoA Oxidation Generates more NADH, FADH2, and one GTP Remaining carbon atoms from carbohydrates, amino acids, and fatty acids are released during Stage 2. 17 Chemical Logic of the Citric Acid Cycle “Acetyl-CoA” must be oxidized to CO2 to extract the maximum amount of potential energy (ATP) Simple decarboxylation of Acetyl-CoA would yield CO2 and methane (CH4) - majority of organisms can’t oxidize. Methylene groups (-CH2-) are readily metabolized by enzyme systems present in most organisms The Citric Acid Cycle (CAC) Key enzymes for regulation of PDH and Citric Acid Cycle: irreversible reactions _ _ C-C Bond Formation by Condensation of Acetyl- CoA and Oxaloacetate – Citrate Synthase The only reaction involving C- C bond formation Activity largely depends on [oxaloacetate]. Highly thermodynamically favorable/irreversible – regulated by substrate availability and product inhibition  Induced Fit in Citrate Synthase Conformational change occurs upon binding oxaloacetate. a) Open conformation: Free enzyme does not have a binding site for acetyl-CoA. b) Closed conformation: Binding of OAA creates binding for acetyl-CoA. Reactive carbanion is protected. Citrate Synthase (homodimer)  Citrate synthase regulation Allosteric inhibition by succinyl-CoA Allosteric inhibition by ATP, succinyl-CoA, NADH Product inhibition by citrate  Aconitase Water removal from citrate and subsequent addition to cis- aconitate are catalyzed by the iron-sulfur center. Citrate, tertiary alcohol = poor substrate for oxidation. Isocitrate, secondary alcohol = good substrate for oxidation HO - C - H Cytosolic Aconitase moonlights as an Iron Response Protein – regulates mRNA translation Ferritin = iron-storage Transferrin receptor – protein. Chelates Fe. uptake of Fe into cells  Final Oxidative Decarboxylation - α-Ketoglutarate Dehydrogenase (αKGDH) 10MDa 3MDa  Reaction mechanism of α-ketoglutarate dehydrogenase is analogous to PDH Succinyl- lipoyllysine Hydroxysuccinyl- TPP Succinate Dehydrogenase Bound to mitochondrial inner membrane = Complex II in electron- transport chain FAD is covalently bound  Origin of C-Atoms in CO2 H2C COOH H2C COOH H2C COOH H2C COOH HO C COOH HC COOH CH2 CH2 H2C COOH HO C COOH O C COOH O C SCoA H Citrate Isocitrate -ketoglutarate Succinyl-CoA Carbons from acetate (red). All CO2 (2 per acetyl-CoA = 4) generated - produced before succinyl-CoA is made. In one turn of the citric acid cycle, neither of the red carbons is lost. Both CO2 molecules lost were present on the oxaloacetate used to begin the cycle.  Citrate is symmetric molecule, why is decarboxylation asymmetric? - reacts with aconitase asymmetrically Citrate  One Turn of the Citric Acid Cycle Net oxidation of two carbons to CO2 – equivalent to two carbons of acetyl-CoA – but NOT the exact same carbons Energy captured by electron transfer to NADH and FADH2 Generates 1 GTP, which can be converted to ATP Completion of cycle Acetyl-CoA + 3NAD+ + FAD + GDP + Pi + 2 H2O → 2CO2 + 3NADH + FADH2 + GTP + CoA + 3H+