The end products of glycolysis still contain much of the energy of the glucose molecule, therefore, these products enter the Krebs cycle or tricarboxylic acid cycle so that they may be oxidized through the oxidative reactions of the Krebs cycle to liberate energy.
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In this cycle NADH formed in the oxidation of the organic acid is oxidized through a series of steps involving the cytochrome system as the final link with oxygen. Each of these steps represents free energy decrease.
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The energy which is liberated during Krebs cycle is conserved m the same way as it is conserved in glycolysis, e. g., by conversion to high energy phosphate compounds.
The pyruvic acid formed during the process of glycolysis, undergoes oxidative decarboxilation in the presence of pyruvic dehydrogenase and coenzyme thiamine pyrophosphate (co-carboxylase) to form acetyl COA.
The acetyl CoA in the presence of citrate synthetase reacts with oxaloacetic acid to form citric acid.
In this reaction no oxidation or decarboxilation is involved but a molecule of water is required to hydrolase the linkage between the acetyl group and co-enzyme A.
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In this way the Co-enzyme A, of which only a small amount is present in the tissue cells, is liberated and can react with more pyruvic acid.
In the mean time, the citric acid undergoes an internal rearrangement in the presence of enzyme aconitate hydratase, in which the hydroxyl group migrates to an adjacent carbon atom to give isocitric acid.
The isocitric acid is then oxidized (two hydrogens removed) in the presence of isocitrate dehydrogenase to form the oxalosuccinic acid, which then decarboxylates (loses CO2) to form a-ketoglutaric acid.
The a-ketoglutaric acid then undergoes an oxidative decarboxylation (i. e., it loses CO2 and essentially adds oxygen) to form succinic acid and free Co-enzyme A.
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During this reaction energy is released which is utilized indirectly to form ATP from ADP and P1.
The succinic acid is oxidized to yield fumaric acid. The succinic acid is first dehydrogenated in the presence of flavoprotein and succinate dehydrogenase to give fumaric acid.
Fumaric acid in the presence of the enzyme fumarate hydratase (fumarase) takes up water molecule to give malic acid which is dehydrogenated by malate dehydrogenase and NAD to give oxaloacetic acid which is ready to react with more acetyl CoA to repeat the cycle.
The citric acid cycle is repeated for the oxidation of other molecule of pyruvic acid. During Krebs cycle for one molecule of pyruvic acid, two molecules of NADH, one of NADPH, one molecule of GTP and one molecule of succinic acid are produced.
AH are utilized as energy carriers. Ultimately the energy is located in ATP. For example, GTP converts ADP to ATP by transfer of phosphate.