The Citric Acid Cycle and Pyruvate Dehydrogenase Complex Quiz

The Citric Acid Cycle and Pyruvate Dehydrogenase Complex

• The citric acid cycle (CAC) is a series of oxidation-reduction reactions that oxidize an acetyl group to two molecules of CO2, generating high-energy electrons for ATP synthesis and biosynthesis.
• Acetyl CoA is the main entry point for fuel molecules into the citric acid cycle.
• The pyruvate dehydrogenase complex oxidatively decarboxylates pyruvate to acetyl CoA, and the reactions take place in the mitochondrial matrix.
• The citric acid cycle removes electrons from acetyl CoA to produce NADH and FADH2, which then enter the electron-transport chain to generate a proton gradient for ATP synthesis.
• The pyruvate dehydrogenase complex links glycolysis to the citric acid cycle and consists of three distinct enzymes.
• The conversion of pyruvate into acetyl CoA involves three steps and five coenzymes, including thiamine pyrophosphate (TPP), lipoic acid, and FAD.
• The decarboxylation step of pyruvate is the rate-limiting step in acetyl CoA synthesis and is catalyzed by the pyruvate dehydrogenase component (E1) of the complex.
• The oxidation step of the pyruvate dehydrogenase complex involves the transfer of the hydroxyethyl group to lipoamide, forming acetyllipoamide, and the subsequent formation of acetyl CoA.
• The regeneration of oxidized lipoamide in the complex involves the transfer of two electrons to an FAD prosthetic group and then to NAD+.
• Lipoamide, a derivative of lipoic acid, plays a crucial role in the pyruvate dehydrogenase complex by carrying substrates between active sites and increasing the overall reaction rate.
• The core of the pyruvate dehydrogenase complex is formed by 60 molecules of the transacetylase subunit, which consists of 20 catalytic trimers assembled to form a hollow cube.
• In mammals, the pyruvate dehydrogenase complex consists of E1 as an α2β2 tetramer, E3 as an αβ dimer, and E3-binding protein (E3-BP) to facilitate the interaction between E2 and E3.

Pyruvate Dehydrogenase Complex Regulation and Dysfunction

• Pyruvate dehydrogenase complex phosphorylation and inactivation is promoted by PDK activation, while high ADP and pyruvate activate the complex by inhibiting the kinase.
• Ca2+ stimulates the phosphatase, enhancing pyruvate dehydrogenase activity.
• In liver tissue, epinephrine and insulin regulate the phosphatase, affecting Ca2+ concentration and pyruvate dehydrogenase activity.
• Individuals with pyruvate dehydrogenase phosphatase deficiency have inactive pyruvate dehydrogenase complex, leading to lactic acidosis and tissue malfunction.
• Diabetic neuropathy, a common complication of diabetes, may be due to inhibition of the pyruvate dehydrogenase complex, resulting in numbness, tingling, or pain.
• Hyperglycemia increases PDK activity, leading to inhibition of the pyruvate dehydrogenase complex and subsequent lactic acid overproduction in diabetic neuropathy.
• The citric acid cycle is regulated by several allosteric enzymes, including isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.
• The citric acid cycle integrates the cell's metabolic pathways and plays an important role in biosynthesis of key biomolecules.
• Citric acid cycle intermediates must be rapidly replenished through anaplerotic reactions, as mammals lack enzymes for net conversion of acetyl CoA into cycle intermediates.
• Beriberi, caused by thiamine deficiency, disrupts pyruvate metabolism and leads to higher levels of pyruvate and α-ketoglutarate in the blood.
• Mercury and arsenite inhibit the pyruvate dehydrogenase complex by binding to the E3 component, leading to central nervous system pathologies.
• The citric acid cycle likely evolved from preexisting pathways, with compounds like pyruvate and α-ketoglutarate present early in evolution for biosynthetic purposes.

Citric Acid Cycle and Pyruvate Dehydrogenase Mechanism Explained

• Pyruvate Dehydrogenase Complex involves 6 steps: E2 inserts lipoamide arm into E1, E1 transfers acetyl group to lipoamide, acetyl moiety transfers to CoA, reduced lipoamide arm swings to E3 flavoprotein, lipoamide is oxidized by coenzyme FAD, NADH is produced.
• Citrate synthase catalyzes addition of acetyl CoA and oxaloacetate, yielding citrate and CoA through aldol addition and hydrolysis, proceeds through energy-rich citryl CoA.
• Citrate synthase minimizes hydrolysis of acetyl CoA to acetate and CoA, exhibits sequential, ordered kinetics, and has active sites in a cleft between subunit domains.
• Citrate synthase mechanism involves conformational changes upon binding oxaloacetate, with His 274 promoting enol intermediate formation and hydrolysis of thioester to yield citrate and CoA.
• Aconitase catalyzes isomerization of citrate into isocitrate through dehydration and hydration steps, with citrate binding directly to iron-sulfur complex.
• Isocitrate dehydrogenase catalyzes oxidative decarboxylation of isocitrate to form α-ketoglutarate and NADH, proceeds through unstable oxalosuccinate and releases CO2 to yield α-ketoglutarate.
• α-Ketoglutarate dehydrogenase complex catalyzes oxidative decarboxylation of α-ketoglutarate to succinyl CoA, yielding NADH, and has entirely analogous mechanisms to pyruvate dehydrogenase complex.
• Succinyl CoA synthetase catalyzes cleavage of thioester linkage of succinyl CoA to yield succinate, coupled to phosphorylation of ADP or GDP, with readily reversible reaction.
• Mammals have two isozymic forms of succinyl CoA synthetase: GDP-requiring enzyme in anabolic tissues and ADP-requiring enzyme in tissues performing cellular respiration.
• Nucleoside diphosphokinase allows adjustment of nucleotide triphosphate and diphosphate concentrations to meet cell's needs and maintain equilibrium.
• Succinyl CoA synthetase transforms energy inherent in thioester molecule into phosphoryl-group-transfer potential, with a ∆G°′ of −3.4 kJ mol−1, and allows formation of ATP through substrate-level phosphorylation.
• Succinyl CoA synthetase mechanism involves orthophosphate attacking succinyl CoA to displace coenzyme A and generate succinyl phosphate, an energy-rich compound.