Sulfate monoester, hydrolysis mechanisms

Figure 20 Possible mechanisms for sulfate ester hydrolysis catalyzed by members of the AS family. Formation of the sulfoenzyme (a) nucleophilic attack of the hydrated FGly oxygen on the substrate (b) nucleophilic attack of the sulfate oxygen of the substrate on the aldehyde FGly yields a diester sulfoenzyme, which hydrolyzes into the sulfoenzyme monoester. Enzyme desulfurylation (c) elimination of inorganic sulfate leaves an aldehyde moiety, which is hydrated to give a gerr7-diol (d) nucleophilic attack of a nucleophile, such as water, yields the hydrated gerr7-diol FGly.

In contrast, the acid-catalyzed hydrolysis of alkyl selenates is A-2158. The actual species which undergoes decomposition to alcohol and sulfur trioxide is probably the zwitterion as in the case of phosphate monoester monoanions. Evidence for sulfur trioxide as the reactive initial product of the A-1 solvolysis is obtained from the product compositions arising with mixed alcohol-water solvents. The product distribution is identical to that found for sulfur trioxide solvolysis, with the latter exhibiting a three-fold selectivity for methanol. Although the above entropies of activation and solvent deuterium isotope effects do not distinguish between the conventional A-l mechanism and one involving rate-limiting proton transfer, a simple calculation, based on the pKa of the sulfate moiety and the fact that its deprotonation is diffusion controlled. 

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