Acetyl phosphate, hydrolysis

Calculate the free energy change for acetyl phosphate hydrolysis in a solution of 2 mM acetate, 2 mM phosphate, and 3 iiM acetyl phosphate. 

Figure 2 Proposed mechanism for acetyl phosphate hydrolysis by Gutsche s metallomi-celle 4 (a) acetyl transfer mechanism, (b) general base mechanism.

The catalysis of hydrolysis of carboxylic acid derivatives by weak bases has not been carefully studied until relatively recently. Koshland reported in 1952 the catalysis of acetyl phosphate hydrolysis by pyridine Bafna and Gold (1953) reported the pyridine-catalyzed hydrolysis of acetic anhydride. A short time later the catalysis of aromatic ester hydrolysis by imidazole was demonstrated (Bender and Turnquest, 1957 a, b Bruice and Schmir, 1957). Since that time a large amount of work has been devoted to the understanding of catalyzed ester reactions. Much of the work in this area has been carried out with the purpose of inquiry into the mode of action of hydrolytic enzymes. These enzymes contain on their backbone weak potential catalytic bases or acids, such as imidazole in the form of histidine, carboxylate in the form of aspartate and glutamate, etc. As a result of the enormous effort put into the study of nucleophilic displacements at the carbonyl carbon, a fair understanding of these reactions has resulted. An excellent review is available for work up to 1960 (Bender, 1960). In addition, this subject has been... 

Berman MC, King SB. 1990. Stoichiometries of calcium and strontium transport coupled to ATP and acetyl phosphate hydrolysis by skeletal sarcoplasmic reticulum. Biochim Biophys Acta 1029 235-240. 

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

The standard-state free energy of hydrolysis for acetyl phosphate is AG° = —42.3 kj/mol. 

For the hydrolysis of acetyl phosphate under the conditions prevailing in the body, AG = —41 kj-mol. If the phosphorylation of acetic acid (the reverse of the hydrolysis of acetyl phosphate) was driven by coupling to the hydrolysis of ATP at pH = 7, what is the minimum amount of ATP molecules (in moles) that would have to be hydrolyzed to form... 

Attention has been drawn to the potential of phosphoric acid anhydrides of nucleoside 5 -carboxylic acids (14) as specific reagents for investigating the binding sites of enzymes. For example, (14 B = adenosine) inactivates adenylosuccinate lyase from E. coli almost completely, but has little effect on rabbit muscle AMP deaminase. The rate of hydrolysis of (14) is considerably faster than that of acetyl phosphate, suggesting intramolecular assistance by the 3 -hydroxyl group or the 3-nitrogen atom. 

Metal ions have a profound influence on the hydrolysis of acetyl phosphate, Thus, in the magnesium(ii)-catalysed system P—O bond flssion... 

Acetate kinase is phosphorylated by acetyl phosphate and it has been shown that the phosphoenzyme can synthesise ATP from ADP, and acetyl phosphate from acetate. The mode of decomposition of carbamyl phosphate in aqueous solution is pH dependent and can proceed with either the production of ammonia and carbon dioxide (equation 1), or cyanate (equation 2). No cyanate could be detected during the hydrolysis... 

A situation similar to that in acetyl phosphate is also encountered in benzoyl phosphate76 . Electron-attracting substituents on the phenyl ring accelerate the hydrolysis of the dianion (a linear relationship exists between log khydrol and the Hammett a constants with q = 1.2 and the linear log ki,j,drol./pKa relationship is the same as for the phosphoric monoaryl ester dianions65 . On the other hand, hydrolysis of the monoanion is influenced only slightly by substituents in the phenyl ring. These observations can also be rationalized in terms of the decomposition mechanism to the POf ion formulated for 116 and 117. 

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). 

Figure 1. Acid- and base-catalyzed hydrolysis of acetyl-phosphate. From G. Di Sabato Jencks (1961) J. Am. Chem. Soc. 83, 4400 reproduced with permission of the authors and the American Chemical Society.

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... 

CaOH1 and MgOH+ on free acyl phosphate is a likely reaction pathway for Ca11 and Mg11 catalysis of the base hydrolysis of acetyl phosphate. 

Gutsche et al. observed that a lipophilic dioxime (the ligand in 4) accelerates by a factor of 60 to 140 the hydrolysis of acetyl phosphate in the presence of an equimolar amount of metal (M = Cu, Zn, Ni) under comicellar conditions with cetyl trimethylammonium bromide (CTAB) at pH 11.5 [22], The hydrolysis reaction was followed by pseudo-first-order kinetics at [ligand] = [M] = 2.5 mM and [CTAB] = 250 mM. Although a possible complex 4 was proposed to exist in the hydroxamate form, there were no experimental data for the stability and structure of metal complexes at pH 11.5 (e.g., it is not known whether the... 

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