Phosphate dianions, hydrolysis mechanism

The iron(II)-iron(III) form of purple acid phosphatase (from porcine uteri) was kinetically studied by Aquino et al. (28). From the hydrolysis of a-naphthyl phosphate (with the maximum rate at pH 4.9) and phosphate binding studies, a mechanism was proposed as shown in Scheme 6. At lower pH (ca. 3), iron(III)-bound water is displaced for bridging phosphate dianion, but little or no hydrolysis occurs. At higher pH, the iron(III)-bound OH substitutes into the phosphorus coordination sphere with displacement of naphthoxide anion (i.e., phosphate hydrolysis). The competing affinity of a phosphomonoester anion and hydroxide to iron(III) in purple acid phosphatase reminds us of a similar competing anion affinity to zinc(II) ion in carbonic anhydrase (12a, 12b). 

The pH-rate profile for unbuffered hydrolysis of glyceraldehyde-3-phosphate (6-3-P) has been attributed to hydrolysis of the monoanion of the phosphate monoester at pH < 4, spontaneous formation of glyceraldehyde from the phosphate dianion at pH 7-8, and, at higher pH, hydroxide-catalysed methylglyoxal formation. Reaction of the dianion is not subject to a solvent isotope effect and is believed to occur by the irreversible ElcB mechanism whereby an enediolate intermediate, formed on rate-determining C(2) deprotonation, subsequently expels phosphate trianion by C—0 bond breaking. The diethylacetal and 2-methyl-G-3-P do not hydrolyse under the same conditions.5... 

The data and mechanistic conclusions summarized above come from work with aryl phosphomonoesters as predicted by the steep jSlg value, alkyl ester dianions react at very slow rates. A recent study of methyl phosphate found the rate of the dianion hydrolysis to be below the threshold of detectability, with an estimated rate constant of 2 x 10 20 s 1 at 25 °C.3 Since this value is close to the rate predicted from an extrapolation of the Bronsted plot of aryl phosphomonoester dianions, a similar mechanism is likely to be followed for alkyl and aryl esters. 

Klahn M, Rosta E, Warshel A. On the mechanism of hydrolysis of phosphate monoesters dianions in solutions and proteins. J Am Chem Soc. 2006 128 15310-15323. Florian J, Warshel A. A fundamentA assumption about OH- attack in phosphate ester hydrolysis is not fuUy justified. J Am Chem Soc. 1997 119 5473-5474. 

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 mechanism of hydrolysis of o-carboxyaryl phosphates, whose dianions also hydrolyze much faster then, e.g., the phenyl phosphate monoanion 79,80) (maximum rate at about pH 4.8 and 25 °C 81)), was long a point of mechanistic contention. Thorough investigations81 led to proposal of a fast initial transprotonation... 

A mechanism proposed 87) for the alkaline hydrolysis of tetraethyl pyrophosphate, which is markedly accelerated by HPO e ions, has been substantiated by isotopic labeling 88). The nucleophilic attack by HPOJp on the symmetrical pyrophosphate 131 is considered to lead initially to the unsymmetrical P P1-diethyl pyrophosphate dianion 132 which decomposes spontaneously under the conditions of reaction to give the diethyl phosphate anion and POf 102. The latter reacts with water to form inorganic phosphate and with alcohols suclj as methanol and ethylene glycol to produce alkyl phosphates. 

Two protein tyrosine phosphatases (PTPases) have been studied with hybrid potentials — the catalytic domain of human PTPIB [89] and the bovine PTPase (BPTP) [90]. These proteins have similar active centers and there is an invariant catalytic cysteine amino acid residue. Hillier et al characterized the transition state for the phosphate hydrolysis by PTPIB (with a dianion phosphate) using a PM3/MM potential but keeping the protein matrix and some of the QM atoms fixed. They found a dissociative mechanism in which the cleavage of the P-0 bond occurred before the formation of the S-P bond. The breaking of the P-O bond was determined to be the rate limiting step in agreement with kinetic... 

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