Oxalacetate citrate synthesis from

The synthesis of citrate from oxalacetate and acetate is endergonic AF = +8.26 Kcal.), whereas the synthesis from oxalacetate and acetyl CoA is exergonic (AF = —7.8 Kcal.). This implies that the S-acetyl bond of acetyl CoA must be energy-rich, AF of the hydrolysis being about —16 Kcal. at pH 7. This is borne out by independent observations. 

The basic starting substrate for fatty acid synthesis is acetyl-CoA (see below). In ruminants, the provision of this substrate is straightfoward. Acetate from blood (+ CoA + ATP) is converted by the cytosolic acetyl-CoA synthase (EC 2.3.1.169) to AMP and acetyl-CoA, which can then be used for fatty acid synthesis. In non-ruminants, glucose is converted via the glycolytic pathway to pyruvate, which is, in turn, converted to acetyl-CoA in mitochondria. Acetyl-CoA thus formed is converted to citrate which passes out to the cytosol where it is cleaved by ATP-citrate lyase (EC 2.3.3.8) to acetyl-CoA + oxalacetate (OAA). This transport of acetyl-CoA from... 

Studies of the enzymic mechanism of the citric acid synthesis by Stern and Ochoa have directly shown that citric acid, and not aconitic acid, is the primary product. It had earlier been thought that the mechanism of citric acid synthesis might be similar to that of the reaction leading in vitro to the formation of citric acid from oxalacetic and pyruvic acid in the presence of hydrogen peroxide, where oxalocitramalic acid is an intermediate. Martins, however, found this substance to be metabolically inert in animal tissue. Stern and Ochoa found that aqueous extracts of acetone-dried pigeon liver formed citrate when acetate, oxalacetate, ATP, coenzyme A, and Mg or Mn ions were present. Thus the condensation reaction is preceded by the decarboxylation of pyruvic acid and the formation of an active form of acetate. This active acetate, as discussed below, is acetyl coenzyme A. 

In the presence of suitable enzymes, which can be prepared from animal tissues or bacteria, acetate becomes reactive, provided that coenzyme A and ATP are present. With these two cofactors and the appropriate enzymes acetate can acetylate sulfanilamide, undergo esterification with choline, or combinefwith oxalacetate to form citrate. Since acetyl coenzyme A is an intermediary in these reactions, one of the main problems concerning the enzymic mechanisms is that of the role of ATP in the synthesis of acetyl coenzyme A. Lynen and Reichert pointed out that it is very unlikely that ATP, acetate, and coenzyme A react together simultaneously, and that the most probable sequence is a primary phosphorylation of coenzyme A by ATP... 

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