Phosphate-oxygen exchange

Fig. 21. Time-course for phosphate-oxygen exchange catalyzed by glutamine synthetase. The fractional concentrations of the species P[lsO]4 ( ), P[ia0]3[l60]1 (O), P[180]2[,60]2 ( ), P[180]i[I60]3 ( ), and P[160]4 (A) are plotted vs. time for the scheme given in Balakrish-nan et al. (90). Reprinted with permission of Academic Press.

A determination of the pH dependence of the lanthanum hydroxide gel-promoted hydrolysis of /3-glyceryl phosphate revealed that the two maxima exist in the pH-rate profile, one at pH 8.6 which presumably involves the species La (OH)+2 and another (smaller) maximum at pH 10.4 which involves the species La (OH) 2+ (4). Presumably the same kind of catalytic mechanism is operative in both cases. These reactions may serve as models for the metal ion-promoted alkaline phosphatases which have been shown to proceed with P—O cleavage (and with no oxygen exchange). 

The hydrolysis in acid or base of five-membered cyclic esters of phosphoric acid proceed some 10 times faster than their acyclic analogs or the six- and seven-membered cyclic phosphates 55 56. Unlike that of dimethyl phosphate, the hydrolysis of hydrogen ethylene phosphate is accompanied by rapid oxygen exchange into the unreacted substrate57, viz-... 

FIGURE 30 Local environment of the Eu cation and encrypted water molecule in [(EUOH2) C P5W30O110] 2-. Protonation of the indicated phosphate oxygen atom (one of five in the complete anion) results in slow intermolecular proton exchange, but rapid H/D exchange with the internal water. 

Mildred Cohn, 1991. She pioneered research on use of the 0, which led to the discovery that mitochondria catalyzed a rapid exchange of phosphate oxygens with water. Measurement of the exchange reactions played a crucial... 

Kinetic studies with calcineurin yielded a modest solvent isotope effect of 1.35, and a proton inventory and fractionation factor data that were most consistent with a mechanism involving a single proton transfer from a water molecule coordinated to a metal ion.136 No transphosphorylation products were found in the presence of alternate nucleophiles, consistent with direct phosphoryl transfer to a metal-coordinated water.137 No calcineurin-catalyzed oxygen exchange of 180 labeled water with phosphate could be detected.138 In a study using / NPP as the substrate, product inhibition studies found that both phosphate and p-nitrophenol are... 

Kaiser and Kezdy have pointed out that the failure to observe oxygen exchange in the hydrolysis of sultones can be understood if the preference rules which apply to the pseudorotation of intermediates in phosphate ester hydrolysis can be applied to pentacoordinate sulphur intermediates142. Since on this basis negatively charged groups would be expected to occupy equatorial positions, equilibration of oxygen atoms in a... 

---