Free energy tricarboxylic acid cycle

K. Burton and H. A. Krebs, The free energy changes associated with the individual steps of the tricarboxylic acid cycle, glycolysis, alcoholic fermentation, and with the hydrolysis of the pyrophosphate group of adensosine triphosphate, Biochem.. /. 54, 94-107 (1953). 

Fig. 1 Oxidative metabolism and energy production by mitochondria. The oxidation of pyruvate and free fatty acids (FFA) inside mitochondria produces NADH and FADH2, which transfer their electrons to the mitochondrial respiratory chain. The flow of electrons in mitochondrial complexes I, III, and IV is coupled with the extrusion of protons from the mitochondrial matrix into the intermembrane space. When energy is needed, these protons reenter the matrix through ATP synthase, to generate ATP from ADP. The adenine nucleotide translocator (ANT) then exchanges the formed ATP for cytosolic ADP. G-6-P Glucose 6-phosphate, PDH pyruvate dehydrogenase, LCFA-CoA long-chain fatty acyl-CoA, CPTI carnitine palmitoyltransferase I, TCA cycle tricarboxylic acid cycle, c cytochrome c...

Biochemical pathways may be described as catabolic, anabolic (biosynthetic), amphibolic or anaplerotic. The principal function of a catabolic sequence is to degrade (usually by an oxidative process) simple organic molecules derived from the breakdown of polymers (e.g. amino acids from proteins) and retain some of the free energy released in a biologically useful form. Anabolic pathways consume energy and synthesize (usually by a reductive process) the simple molecules which are assembled into proteins, nucleic acids, carbohydrate polymers and lipids. Amphibolic pathways, such as the tricarboxylic acid cycle, have both catabolic and anabolic properties. They are central metabolic pathways which furnish, from catabolic sequences, the intermediates which form the substrates of anabolic processes. The... 

This reaction is essentially irreversible and the thioester bond of acetyl-S-CoA (CHgCO-SCoA) has a high free energy of hydrolysis (AG° = —31 kJ —7 5 kcal). This energy is utilized in a condensation reaction of acetyl CoA with the enol form of oxaloacetic acid to produce citric acid and CoASH is liberated. The enzyme mediating this reaction, citrate synthase (condensing enzyme), is the first enzyme of the tricarboxylic acid cycle (Krebs cycle) (Fig. 17.4). 

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