TCA Cycle

• The tricarboxylic acid (TCA) cycle, also known as the Krebs cycle, is a common pathway for the final oxidation of all metabolic fuels.
• The TCA cycle has two major functions: energy production and biosynthesis.
• The TCA cycle is involved in catabolic processes and anabolic processes such as gluconeogenesis, lipogenesis, and synthesis of amino acids.
• Acetyl-CoA is a common product of many catabolic pathways and is the final destination for metabolism of fuel molecules.
• Most fuel molecules enter the pathway as acetyl CoA, but the carbon skeletons of amino acids may also enter the TCA cycle at various points.
• The TCA cycle consists of 8 steps: acetyl CoA is converted to CO2, electrons are released to NADH and FADH2, and PDH is important.
• In aerobic organisms, the citric acid cycle is an essential metabolic pathway along with glycolysis, the pyruvate dehydrogenase reaction, and oxidative phosphorylation.
• Four oxidative steps in the TCA cycle provide free energy for ATP synthesis.
• Acetyl-CoA is oxidized in the TCA cycle to produce reduced coenzymes by four redox reactions per turn of the cycle.
• Four different B vitamins play important roles in the functioning of the TCA cycle: Riboflavin (Vit B2), Niacin (Vitamin B3), Thiamine (Vitamin B1), and Pantothenic acid.
• Thiamine, also known as Vitamin B1, exists in Thiamine pyrophosphate (TPP) and is a coenzyme of alfa ketoglutarate dehydrogenase enzyme system.
• Niacin, also known as Vitamin B3, exists in NAD and is a coenzyme of the alfa ketoglutarate dehydrogenase enzyme system, isocitrate dehydrogenase, and malate dehydrogenase.
• Pantothenic acid is a component of Coenzyme A (CoASH) and CoASH is the cofactor of active carboxylic acid residues, such as acetyl-CoA and succinyl-CoA.
• The TCA cycle is controlled by the regulation of several enzyme activities.
• Inhibitors of the TCA cycle include fluoroacetate, Arsenite, and Malonate.
• The most important of these regulated enzymes are those that catalyze reactions with highly negative ΔG0: citrate synthase, isocitrate dehydrogenase (rate limiting step), α-ketoglutarate dehydrogenase complex.
• Riboflavin, also known as Vitamin B2, exists in FAD and is the prosthetic group in alfa ketoglutarate Dehydrogenase enzyme system.
• Electrons derived from the carbon skeletons are captured and transferred by the electron transport chain to oxygen, driving the generation of ATP.
• The reactions of the TCA cycle occur entirely within the mitochondrial matrix.
• The TCA cycle provides a common ground for interconversion of fuels and metabolites.
• Biosynthetic reactions proceeding from the TCA cycle require the input of carbons from intermediates other than acetyl-CoA.
• E2: dihydrolipoyl transacetylase transfers the acetyl CoA to its lipoic acids coenzyme with a reduction of the lipoic acid.
• E3: dihydrolipoyl dehydrogenase transfers electrons from reduced lipoic acid to produce NADH and regenerate the oxidized form of lipoic acid.
• The PDH Complex is a multi-enzyme complex with three enzymes and five co-enzymes, allowing for efficient direct transfer of product from one enzyme to the next.
• Oxaloacetate is first condensed with an acetyl group from acetyl CoA, and then oxaloacetate is regenerated as the cycle is completed.
• The complex functions as a unit consisting of three principal enzymes: pyruvate dehydrogenase, dihydrolipoyl transacetylase, and dihydrolipoyl dehydrogenase.
• The PDH Reaction is highly regulated and is activated by pyruvate, ADP and Ca ions, and inhibited by increases in the ratio (NADH/NAD+) and by the product acetyl - CoA.
• Citrate rearranges to isocitrate in a reaction catalysed by acotinase.
• Genetic defects in PDH can result in decrease or complete loss of activity, causing symptoms such as lactic acidosis, neurological disorders, and early death.
• PDH regulation involves product inhibition, availability of substrate, and covalent modification.
• Deficiencies of thiamine or niacin can cause serious central nervous system problems because brain cells are unable to produce sufficient ATP for proper function if pyruvate dehydrogenase is inactive.
• The TCA Cycle begins with Acetyl CoA entering the cycle by condensing with OAA to form citrate, stimulated by citrate synthase.
• The PDH Enzyme Complex links glycolysis and the TCA cycle, oxidizing pyruvate to CO2 and acetyl CoA, which is the substrate for the TCA cycle.
• Succinyl CoA is hydrolyzed to succinate and CoA, a reaction catalyzed by Succinate thiokinase.
• The PDH Reaction involves E1: pyruvate dehydrogenase, which removes CO2 and transfers the remaining acetyl group to the enzyme bound coenzyme thiamine pyrophosphate.
• Succinate is converted to fumarate with the transfer of electrons to FAD to form FADH2, a reaction catalyzed by Succinate dehydogenase (SDH), tightly associated with inner mitochondrial membrane reduction.
• This reaction involves simultaneous coupling of GDP and Pi to form GTP, which is substrate-level phosphorylation with energy being conserved in the form of GTP.
• Conversion of alfa-ketoglutarate to succinyl CoA, CO2, and NADH is catalyzed by an analog to PDH complex, which is made up of three enzyme activities with similar array of activities and coenzymes requirements.
• Fumarate to malate which is converted to OAA, completing the cycle, is catalyzed by fumarase.
• The net reaction of the TCA cycle is: Acetyl-CoA + 3NAD+ + FAD + GDP + Pi + 2H2O → 2CO2 + 3NADH + FADH2 + GTP + 2H + + HSCoA.