Acetate, active from acetyl phosphate

The activity of carbamoyl phosphate synthase I is determined by A -acetylglutamate, whose steady-state level is dictated by its rate of synthesis from acetyl-CoA and glutamate and its rate of hydrolysis to acetate and glutamate. These reactions are catalyzed by A -acetylglu-tamate synthase and A -acetylglutamate hydrolase, respectively. Major changes in diet can increase the concentrations of individual urea cycle enzymes 10-fold to 20-fold. Starvation, for example, elevates enzyme levels, presumably to cope with the increased production... 

This phosphotransferase [EC 2.7.2.1] catalyzes the thermodynamically favored phosphorylation of ADP to form ATP Aeq = [ATP][acetate]/ [acetyl phosphate] [ADP] = 3000). GDP is also an effective phosphoryl group acceptor. This enzyme is easily cold-denatured, and one must use glycerol to maintain full catalytic activity. Initial kinetic evidence, as well as borohydride reduction experiments, suggested the formation of an enzyme-bound acyl-phosphate intermediate, but later kinetic and stereochemicaT data indicate that the kinetic mechanism is sequential and that there is direct in-line phosphoryl transfer. Incidental generation of a metaphosphate anion during catalysis may explain the formation of an enzyme-bound acyl-phosphate. Acetate kinase is ideally suited for the regeneration of ATP or GTP from ADP or GDP, respectively. 

D-Glucosamine 6-phosphate can be readily acetylated by a AT-acetylase obtained from a preparation of yeast hexokinase. The resulting iV-acetyl-D-glucosamine 6-phosphate is identified by its Morgan-Elson reaction. The acetyl-coenzyme A which appears to be required for this reaction may be generated by acetate, adenosine-5-triphosphoric acid, and coenzyme A (in the presence of an acetate-activating enzyme). 

It was evident from early studies that in the presence of CoASH, ATP could in some way be used to activate acetate so that it will acetylate sulfanilamide (7) and choline (8). Chou and Lipmann (9) succeeded in partially purifying the enzyme(s) responsible for this activation from pigeon-liver extracts and they concluded that the phosphate bond energy of ATP is utilized to bring about the synthesis of acetyl coenzyme A (acetyl-SCoA) however, the mechanism of this activation remained obscure. The nature of the over-all process was further elucidated through the experiments of Lipmann et al. (10) who demonstrated that ATP, CoASH, and acetate react to form acetyl-SCoA, AMP, and inorganic pyrophosphate (P-P) in stoichiometric amounts [reaction (4)]. They also demonstrated that the reaction is freely reversible. More recently, the studies of Jones et al. (11) have indicated that the mechanism of this conversion is as follows ... 

Some phosphate esters of m /o-inositol have been prepared by the action of phytase on phytic acid 80 H6)y in order to arrive at a better understanding of the structure of the biologically important phytic acid. The structures of these esters are not known. Iselin 117) has prepared myoinositol 2-phosphate and scyZZo-inositol monophosphate via the penta-acetates derived from the reduction of penta-O-acetyl-myo-inosose-2. The myo-inositol derivative does not have the biological activity toward microorganisms that the free inositol has. 

As the study of CoA developed, it became apparent that the coenzyme was involved in reactions whereby acetate was activated by ATP and subsequently transferred to various acetyl acceptors. In pigeon liver extracts it was shown that acetate could be activated by ATP in the presence of CoA to acetylate sulfanilamide, PABA, histamine, glucosamine, to synthesize acetoacetic acid and citrate. Acetyl phosphate, which has been demonstrated to be a product of pyruvate metabolism in several bacteria and could theoretically be considered to be an intermediate in these reactions, was found to be unable to replace acetate and ATP in animal tissues. Eventually it was shown that there is present in certain bacteria an enzyme, phosphotransacetylase, which could convert acetyl phosphate to a reactive product which was thought to be acetyl-CoA.i 194 isolation of acetyl-CoA from yeast extract by Lynen and Reichert confirmed the idea that acetyl-CA is the reactive 2-carbon unit in these reactions. Stadtman has demonstrated that acetyl-CoA is indeed the product of the action of phosphotransacetylase. Lipmann has recently... 

This preparation differs from the previously considered pyruvate system of bacteria in that free acetate is a product of the reaction and acetyl-CoA is not an intermediate. However, an activated acetyl derivative is certainly involved, since it is reported that, upon the addition of CoA and a fraction from pigeon liver extracts, the acetylation of sulfanilamide could be effected. Also the addition of CoA and bacterial phosphotransacetylase led to the formation of acetyl phosphate. These observations are summarized in the diagram. 

Reference has already been made to an active form of acetic acid which arises from carbohydrate, fatty acids, ketone bodies, and indirectly also from certain amino acids, and combines with oxalacetate to form citrate. It was at one time suspected that this active acetic acid might be identical with acetyl phosphate ... 

PTA and AK activities have been measured in cell-free extracts of solvent-producing clostridia, and these activities can be separated from those attributable to PTB and BK, respectively (Chen 1993). PTA has only been partially purified from C. beijerinckii (Chen 1993), whereas AK has been purified from C. acetobutylicum DSM 1731 (Winzer et al. 1997). AK of C. acetobutylicum DSM 1731 has a native MW of 87-94 kDa and a measured subunit MW of 43 kDa. The K values for acetyl phosphate, Mg-ADP, acetate, and Mg-ATP are, respectively, 0.58, 0.71, 73, and 0.37 mM. Results of northern blot analysis show that there is no significant difference in the transcription of the acetate kinase gene (ack) in cells of C. acetobutylicum DSM 1731 under acid- and solvent-producing conditions (Winzer et al. 1997). 

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