Acetyl-phosphate

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. 

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

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... 

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

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. 

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... 

Arkowitz RA, RH Abeles (1989) Identification of acetyl phosphate as the product of clostridial glycine reductase evidence for an acyl enzyme intermediate. Biochemistry 28 4639-4644. 

A strain of Rhodopseudomonas palustris, which was isolated by enrichment with taurine, could use this as electron source, and as a source of sulfur and nitrogen during photo-autotrophic growth with CO2. Taurine was metabolized to sulfoacetaldehyde and acetyl phosphate by a pathway, which has already been noted (Novak et al. 2004). 

Ruff J, K Denger, AM Cook (2003) Sulphoacetaldehyde acetyltransferase yields acetyl phosphate purification from Alcaligenes defragrans and gene clusters in taurine degradation. Biochem J 369 275-285. 

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. 

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). 

Die Hydrolyse von Acetyl-phosphat verlauft dagegen nach zwei ver-schiedenen Wegen, je nachdem, ob man im alkalischen oder saueren Medium hydrolysiert [Bentley (IT)]. In alkalischem Medium wird die C—O-Bindung gelost, in sauerem dagegen die P—O-Bindung. 

To render this system more effective, it must be coupled with the regeneration of acetyl-SCoA from HSCoA and acetyl phosphate under catalysis by phosphotransacetylase (PTA) (Fig. 19). 

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. 

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