Acetyl phosphate exchange reaction

The oxidation to form acyl phosphates is reversible. Acyl groups may also be transferred by this enzyme. Acetyl groups are transferred from acetyl phosphate to the sulfhydryl groups of compounds such as glutathione and CoA acetyl phosphate is hydrolyzed in the presence of arsenate and the phosphate of acetyl phosphate is exchanged with inorganic phosphate (measured by labeling with These reactions... 

Hydroxy- 9-methylglutaryl CoA further yields acetyl CoA and acetoacetic acid, as was shown earlier by Coon et cU. (I48). In biotin deficiency the carboxylation reaction does not occur. It was shown by Lynen et al. that the actual carboxylation is preceded by the enzymic dehydration (rf jS-hydroxyisovaleryl CoA to /8-methylcrotonyl CoA, which is the true substrate for the entry of CO2. TTiis occurs at the expense of the hydrolysis of the terminal P04 of ATP. The unsaturated intermediate is then saturated by the addition of H2O to yield the final product. The critical step of this carboxylation is the conversion of CO2 to a reactive form. The analogy of the biochemical activation of other substances through an acyl adenylate type of compound did not fit CO2 activation. The final mechanism of the activation of CO2 was derived from the discovery that the carboxylase enzyme was a biotin-protein. This observation explains earlier work 149) which indicated that biotin is a cofactor of the fatty acid-synthesizing enzyme system. When the purified carboxylase was incubated with P and ATP an exchange reaction of phosphate occurred, which was inhibited by avidin, a protein which specifically binds biotin. This indicated that the primary reaction in CO2 fixation is the combination of ATP with the biotin-protein enzyme to yield ADP biotin-protein -f P. The active CO2 is then the product of an exchange reaction between ADP and C02 which is finally attached to the biotin-protein complex. 

In 1945 Lipmann found that a novel coenz3mae—coenzyme A— is required for the enzymic acetylation of sulfanilamide in pigeon liver preparations. Soon afterwards Nachmannsohn and Berman (see also ) found that a coenzyme is also required for the synthe of acetyl choline from choline and acetate in brain tissue, and this was found to be identical with the coenzyme of the acetylation of sulfanilamide, i Subsequently, three other reactions of acetate were found to involve coenzyme A the formation of acetoacetic acid from acetate, the s3mthesis of citrate from oxalacetate and acetate, - and the exchange reaction between acetyl phosphate and inorganic phosphate in bacterial extracts. " Thus, coenzyme A was shown to be a general coenzyme of acetylations, and... 

The evidence supporting this concept is derived from experiments on yeast extracts and pigeon liver preparations but further joint studies by Lipmann and Lynen make the formation of phosphoryl-ated or pyrophosphorylated coenzyme A improbable and suggest the formation of enzyme-bound adenosine phosphate and coenzyme A. In the presence of purified yeast enzymes isotopic pyrophosphate was found to exchange the isotope with ATP in the absence of coenzyme A. This excludes the latter as an obligatory participant. Isotopic acetate also exchanged readily with acetyl coenzyme A in the presence of this yeast enzyme. The following scheme of 3 reactions fits the facts ... 

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