Acetyl phosphate, reaction

A coupled process links a spontaneous reaction with a nonspontaneous one. In this case, the negative free energy change of the acetyl phosphate reaction drives the conversion of ADP to ATP. 

The negative value of A Gjjg shows that the free energy released in the acetyl phosphate reaction is more than enough to drive the conversion of ADP to ATP. 

C14-0083. Although the ATP-ADP reaction is the principal energy shuttle in metabolic pathways, many other examples of coupled reactions exist. For example, the glutamic acid-glutamine reaction discussed in the text can couple with the acetyl phosphate reaction shown in Example 14-10. Write the balanced equation for the coupled reaction operating in the direction of overall spontaneity and calculate A G ° for the overall process. 

FIGURE 3.12 The hydrolysis reactions of acetyl phosphate and 1,3-bisphospho-glycerate. 

One biochemical reaction that produces ATP involves the conversion of acetyl phosphate to acetic acid and ... 

As for the acetyl phosphate monoanion, a metaphosphate mechanism has also been proposed 78) for the carbamoyl phosphate monoanion 119. Once again, an intramolecular proton transfer to the carbonyl group is feasible. The dianion likewise decomposes in a unimolecular reaction but not with spontaneous formation of POf as does the acetyl phosphate dianion, but to HPOj and cyanic acid. Support for this mechanism comes from isotopic labeling proof of C—O bond cleavage and from the formation of carbamoyl azide in the presence of azide ions. 

The highly electrophilic character of the POf ion would suggest a very unselective phosphorylation behavior. For example, the ratio of alkyl phosphate to inorganic phosphate obtained in hydrolyses of phosphoric esters in water/alcohol mixtures should reflect the molar ratio of water and alcohol. This is indeed found in numerous cases, e.g. in the hydrolysis of phenyl and 4-nitrophenyl phosphate monoanions 97) or of 4-nitrophenyl phosphate dianions 65) at 100 °C in methanol/ water mixtures of various compositions, as also in the solvolysis of the acetyl phosphate dianion at 37 °C 97) or of phosphoenol pyruvate monoanions 82). Calculations of the free energy of the addition reactions of water and ethanol to the POf ion support the energetic similarity of the two reactions 98) (Table 4). 

The critical discovery that acetyl phosphate is generated and the information gained from several studies of each of the components of GR allowed an enzyme mechanism to be proposed (Arkowitz and Abeles 1991). However, with the current knowledge that one of the subunits of protein B also contains selenium, further work is needed to characterize the intermediates of the reaction and to explain the role of an additional selenocysteine residue. Whether this additional selenocysteine residue in protein B might serve as a direct reductant of the postulated thioselenide derivative of selenoprotein A and possibly serve as a link to the Trx-TrxR system is unknown. It should also be noted that the selenium-limited cultures that were initially studied during analysis of selenoprotein A (Turner and Stadtman 1973) apparently contained active fractions of proteins B and C, suggesting the role for selenium in protein B may not prove to be absolutely necessary for enzyme catalysis. 

An enzymatic reaction intermediate formed by phospho-ryl transfer to a carboxyl group on an enzyme. Acyl-phosphates are structurally analogous to acid anhydrides (R—CO —O —CO—R ), and they are thermodynamically less stable than either of the two phosphoanhydride bonds in ATP. This is evident by the fact that the acetate kinase reaction (ADP + acetyl-phosphate = ATP + acetate) favors ATP formation with an equilibrium constant of about 3,000. Acetyl-phosphate can be chemically synthesized by reacting orthophosphate with acetic anhydride. 

Occasionally, one may also wish to use an auxiliary enzyme not as an assay system but strictly as a means for maintaining the steady-state concentration of a primary reactant in a multisubstrate reaction system. For instance, acetate kinase (and its substrate acetyl phosphate) or creatine kinase (and its substrate creatine phos-... 

This enzyme catalyzes the reaction of glycine with orthophosphate to produce acetyl phosphate and ammonium ion. The oxidized form of the enzyme is subsequently reduced to its dithiol form in a NADH-lmked step. 

For most reactions, one can use 1-2 mM acetyl phosphate and 1-2 international units of acetate kinase. The enzyme is usually supplied as a crystalhne suspension in 3 M ammonium sulfate, and 10 mM ammonium ion is inhibitory. Therefore, a useful practice is to snip off 0.5 cm from a disposable Eppendorf micropipette tip to facih-tate removal of 10-20 microliters of the crystalline suspension then spin down the enzyme in a 1.5 ml disposable conical plastic centrifuge tube, and remove the ammonium sulfate solution with a wick of twisted Kim-wipe. The enzyme precipitate can now be taken up directly into your working buffer. Note Acetate kinase is inactivated by cold exposure, but incubation with 10 M ATP or GTP reactivates the enzyme if warmed to room temperature for 5-10 min. 

This enzyme [EC 2.3.1.8], perhaps more widely known as phosphotransacetylase and less so as phosphoacylase, catalyzes the reaction of acetyl-CoA with orthophosphate to produce coenzyme A and acetyl phosphate. 

This thiamin pyrophosphate-dependent enzyme [EC 4.1.2.9] catalyzes the reaction of D-xylulose 5-phosphate with orthophosphate to produce acetyl phosphate, d-glyceraldehyde 3-phosphate, and water. 

This enzyme [EC 1.2.3.3], which requires thiamin pyrophosphate and FAD, catalyzes the reaction of pyruvate with orthophosphate, dioxygen, and water to produce acetyl phosphate, carbon dioxide, and hydrogen peroxide. 

For ATP regeneration, a similar concept has been used (Berke et al, 1988). ATP can be regenerated from ADP using acetyl phosphate and the enzyme acetate kinase, upon release of acetate. The reaction is irreversible and acetyl phosphate is a relatively cheap phosphate donor. Thus, in an enzyme membrane reactor, PEGderivatized ATP was consumed by a phosphorylase or a synthetase in a reaction leading to a product of interest, and the ATP was regenerated by the acetate kinase (Figure 10.9). 

The nucleophilic reactions of acetyl phosphate are catalyzed by cations such as magnesium (26), calcium (15, 32), and even lithium (30). Most of the reactions involve catalysis of hydrolysis. Calcium ion catalyzes a neutral as well as a... 

Harting, J., Velick, S. F., Transfer Reactions of Acetyl Phosphate Cata-... 

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. 

In both cases it has been proposed that the pyruvoyl group forms a positively charged Schiff base with the substrate. The bond-breaking mechanisms are not obvious but some ideas have been proposed.281-2833 The ATP may arise from reaction with an intermediate acetyl phosphate.282 SeeEq. 15-61. 

Pyruvate oxidase. The soluble flavoprotein pyruvate oxidase, which was discussed briefly in Chapter 14 (Fig. 14-2, Eq. 14-22), acts together with a membrane-bound electron transport system to convert pyruvate to acetyl phosphate and C02.319 Thiamin diphosphate is needed by this enzyme but lipoic acid is not. The flavin probably dehydrogenates the thiamin-bound intermediate to 2-acetylthiamin as shown in Eq. 15-34. The electron acceptor is the bound FAD and the reaction may occur in two steps as shown with a thiamin diphosphate radical intermediate.3193 Reaction with inorganic phosphate generates the energy storage metabolite acetyl phosphate. 

The reaction has sometimes been called the "phosphoro-clastic reaction" because the product acetyl-CoA usually reacts further with inorganic phosphate to form acetyl phosphate (see Fig. 15-16). The latter can then transfer its phospho group to ADP to form ATP. 

Some lactic acid bacteria of the genus Lactobacillus, as well as Leuconostoc mesenteroides and Zymomonas mobilis, carry out the heterolactic fermentation (Eq. 17-33) which is based on the reactions of the pentose phosphate pathway. These organisms lack aldolase, the key enzyme necessary for cleavage of fructose 1,6-bisphosphate to the triose phosphates. Glucose is converted to ribulose 5-P using the oxidative reactions of the pentose phosphate pathway. The ribulose-phosphate is cleaved by phosphoketolase (Eq. 14-23) to acetyl-phosphate and glyceraldehyde 3-phosphate, which are converted to ethanol and lactate, respectively. The overall yield is only one ATP per glucose fermented. 

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