Proton-in-Flight Mechanism for the Spontaneous Hydrolysis of N-Methyl O-Phenyl Sulfamate: Implications for the Design of Steroid Sulfatase Inhibitors

The hydrolysis of N-methyl O-phenyl sulfamate (1) has been studied as a model for steroid sulfatase inhibitors such as Coumate, 667 Coumate, and EMATE. At neutral pH, simulating physiological conditions, hydrolysis of 1 involves an intramolecular proton transfer from nitrogen to the bridging oxygen atom of the leaving group. Remarkably, this proton transfer is estimated to accelerate the decomposition of 1 by a factor of 10¹¹. Examination of existing kinetic data reveals that the sulfatase PaAstA catalyzes the hydrolysis of sulfamate esters with catalytic rate accelerations of ∼10⁴, whereas the catalytic rate acceleration generated by the enzyme for its cognate substrate is on the order of ∼10¹⁵. Rate constants for hydrolysis of a wide range of sulfuryl esters, ArOSO₂X^–, are shown to be correlated by a two-parameter equation based on pK_a^ArOH and pK_a^ArOSO2XH.