Acetyl phosphate synthesis

Priiss, B.M. and Wolfe, A.J. (1994). Regulation of acetyl phosphate synthesis and degradation, and the control of flagellar expression in Escherichia coli. Mol. Microbiol. 12, 973-984. 

Wanner, B.L. and Wilmes-Riesenberg, M.R. (1992). Involvement of phospho-transacetylase, acetate kinase, and acetyl phosphate synthesis in control of the phosphate regulon in Escherichia coli.J. Bacteriol. 174, 2124 2130. 

S ATP -P acetate <1-18> (<8> acetate kinase/phosphotransacetylase, major role of this two-enzyme sequence is to provide acetyl coenzyme A which may participate in fatty acid synthesis, citrate formation and subsequent oxidation [1] <3> function in the metabolism of pyruvate or synthesis of acetyl-CoA coupling with phosphoacetyltransacetylase [15] <11> function in the initial activation of acetate for conversion to methane and CO2 [19] <10> key enzyme and responsible for dephosphorylation of acetyl phosphate with the concomitant production of acetate and ATP [30]) (Reversibility r <1-18> [1, 2, 5-21, 24-27, 29-33]) [1, 2, 5-21, 24-27, 29-33]... 

A reaction that is related to that of transketolase but is likely to function via acetyl-TDP is phosphoketolase, whose action is required in the energy metabolism of some bacteria (Eq. 14-23). A product of phosphoketolase is acetyl phosphate, whose cleavage can be coupled to synthesis of ATP. Phosphoketolase presumably catalyzes an a cleavage to the thiamin-containing enamine shown in Fig. 14-3. A possible mechanism of formation of acetyl phosphate is elimination of HzO from this enamine, tautomerization to 2-acetylthiamin, and reaction of the latter with inorganic phosphate. 

Animal and bacterial enzymes that utilize or synthesize carbamyl phosphate have activity with acetyl phosphate. Acyl phosphatase hydrolyzes both substrates, and maybe involved in the specific dynamic action of proteins. Ornithine and aspartic transcarbamylases also synthesize acetylornithine and acetyl aspartate. Finally, bacterial carbamate kinase and animal carbamyl phosphate synthetase utilize acetyl phosphate as well as carbamyl phosphate in the synthesis of adenosine triphosphate. The synthesis of acetyl phosphate and of formyl phosphate by carbamyl phosphate synthetases is described. The mechanism of carbon dioxide activation by animal carbamyl phosphate synthetase is reviewed on the basis of the findings concerning acetate and formate activation. 

Mg 2 were required for the reaction to proceed. The acetyl phosphate is utilized for the synthesis of acetyl-CoA, required for lysine degradation. The importance of Stadtman s finding becomes apparent upon close examination of the experimental data. 

Methoxycarbonyl phosphate/acetate kinase. Methoxycarbonyl phosphate (MCP 3) was designed to replace AcP as phosphoryl donor1181. It is comparable to PEP in its high phosphorylating strength (see Table 13-2), but resembles acetyl phosphate in its ease of synthesis. Aqueous solutions of MCP are prepared from aqueous phosphate and methyl chloroformate and used in ATP regeneration without purification. The reaction product after phosphoryl transfer is methyl carbonate, which hydrolyses rapidly to form C02 and MeOH. Product isolation is simple and bicarbonate inhibition can be avoided by purging the reaction mixture. 

With respect to mechanism of action, the most extensive kinetic and equilibrium exchange studies have been carried out on monofunctional 10-formyl-H4-folate synthetase from Cl. cylindrosporum [84]. The data support a random sequential mechanism that does not involve the formation of freely dissociable intermediates. The most likely mechanism, however, is not concerted but probably involves the formation of a formyl phosphate intermediate, since the synthetase catalyzes phosphate transfer from carbamyl phosphate but not acetyl phosphate to ADP with H 4-folate serving as an activator. Carbamyl phosphate is an inhibitor of 10-formyl-H 4-folate synthesis - an inhibition that can be eliminated only when both ATP and formate are present in accord with the concept that it spans both sites [85]. It would be of considerable interest to attempt to demonstrate positional isotope exchange employing [, y- 0]ATP for this enzyme in order to further implicate an enzyme-bound formyl phosphate species [86]. 

With such an extensive knowledge base, what is the present state of our understanding of the mechanisms of this disorder Not unexpectedly, initial studies, primarily in experimental animal models, focused on the known metabolic pathways which involve thiamine. Indeed, the classical studies of Peters in 1930 (Peters, 1969) showed lactate accumulation in the brainstem of thiamine deficient birds with normalization of this in vitro when thiamine was added to the tissue. This led to the concept of the biochemical lesion of the brain in thiamine deficiency. The enzymes which depend on thiamine are shown in Fig. 14.1. They are transketolase, pyruvate and a-ketoglutarate dehydrogenase. Transketolase is involved in the pentose phosphate pathway needed to form nucleic acids and membrane lipids, including myelin. The ketoacid dehydrogenases are key enzymes of the Krebs cycle needed for energy (ATP) synthesis and also to form acetylcholine via Acetyl CoA synthesis. Decrease in activity of this cycle would result in anaerobic metabolism and lead to lactate formation (i.e., tissue acidosis) (Fig. 14.1). 

In 1951, Feodor Lynen (1911-1979) and his coworker E. Reichert demon-started that S-acetyl coenzyme A is a more generally implicated form of active acetate than acetyl phosphate that was recognized in this role by Fritz Lipmann in 1940. The thiol ester character of 5-acetyl Co A called the attention of Th. Wieland to energy-rich S-acyl compounds as promising intermediates for the formation of the peptide bond. In 1951, the same year when the isolation of 5-acetyl CoA was published [3], Wieland and his coworkers described [4] the preparation of thiophenyl esters of benzyloxycarbonyl-amino acids and benzyloxycarbonyl-peptides and their application in the synthesis of blocked peptides ... 

Stadtman et have studied the synthesis of acetoacetate by an enzyme from liver in the presence of added phosphotransacetylase of bacterial origin, using isotopically labeled acetyl phosphate. These workers found that 2 moles of acetyl phosphate were used up for each mole of acetoacetate synthesized. 

Reaction 41 as written is highly exergonic, since only 16,000 cal. are required for the synthesis of acetoacetate. One mole of acetyl phosphate should furnish sufficient energy to allow the reaction to proceed. Here two high-energy bonds are dissipated, which may seem wasteful from a teleological point of view. However, it is possible that mechanisms exist in the intact tissues for the recovery of this energy by some type of feedback mechanism. 

Acetyl phosphate is formed here due to the presence of phosphotransacetyl-ase in these extracts. In the absence of inorganic phosphate the reaction is greatly diminished and no acetyl phosphate is formed. However, if in the absence of phosphate the system is coupled with crystalline-condensing enzyme and oxaloacetate, a rapid synthesis of citrate is observed. This indicates that acetyl-CoA is formed as an intermediate in the absence of acetyl acceptors, such as phosphate or oxaloacetate, the reaction is limited by the CoA, since free CoA cannot be regenerated. In the presence of appropriate acetyl acceptors, free CoA is regenerated and allowed to act catalytically. 

Stadtman, E. R. and Barker, H. A. (1949) Fatty acid synthesis by enzyme preparations of Clostridium kluyveri. II. The aerobic oxidation of ethanol and butyrate with the formation of acetyl phosphate. J. Biol. Chem. 180, 1095-1116 Chantrenne, H. and Lipmann, F. (1950) Coenzyme A dependence and acetyl donor function of the pyruvate-formed exchange system, ibid. 187, 757. 

The precursor for ornithine synthesis is N-acetylglutamate, which is also an obligatory activator of carbamyl phosphate synthetase. This provides a regulatory mechanism — if N-acetylglutamate is not available for ornithine synthesis (and hence there would be impaired activity of the urea synthesis cycle), then ammonium is not incorporated into carbamyl phosphate. This can be a cause of hyperammonaemia in a variety of metabolic disturbances that lead to either a lack of acetyl CoA for N-acetyl glutamate synthesis or an accumulation of propionyl CoA, which is a poor substrate for, and hence an inhibitor of, N-acetylglutamate synthetase. 

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