Thioester hydrolysis reactivity of an Fe(III)Zn(II) complex

Glyoxalase II enzymes catalyze the hydrolysis of a thioester substrate and have been found to coordinate a variety of dimetal combinations, including Fe(III)Zn(II), within the enzyme active site. Of relevance to these enzymes, the thioester hydrolysis reactivity of the Fe(III)Zn(II) compound [(BPBPMP)Fe(III)Zn(II)(mu-OAc)(2)]ClO(4) (1) was evaluated in CH(3)CN/H(2)O (50:50; buffered) at 26.5 degrees C. Thioester hydrolysis in the absence and presence of 1 was monitored using (2)H NMR by following the loss of the thioester -SCD(3) signal. Two products are generated in the reaction involving the metal complex, D(3)CSSCD(3) and CD(3)SH. Kinetic studies of this reaction as a function of pH revealed maximum rate above the pK(a) of a Zn-OH(2) moiety of [(BPBPMP)Fe(III)(OH)(mu-OH)Zn(II)(OH(2))](+), which forms from 1 in CH(3)CN/H(2)O (50:50). UV-vis and electron paramagnetic resonance (EPR) studies of a single turnover thioester hydrolysis reaction in the presence of 1 equiv of 1 at pH = 9.0 suggest that the thioester does not initially interact with the Fe(III) center, but that changes occur at this site over the course of the reaction. The formation of a Fe(III)-SCD(3) moiety is proposed based on the observed D(3)CSSCD(3) formation, which likely results from redox activity involving a iron(III) thiolate species. A mechanism for thioester hydrolysis is proposed involving initial coordination of the deprotonated alpha-hydroxy thioester to the zinc center followed by nucleophilic attack by a terminal Fe(III)-OH moiety and thiolate leaving group stabilization by the Fe(III) center. Overall, this study outlines a novel approach of using an aliphatic thioester substrate and (2)H NMR to provide mechanistic insight into thioester hydrolysis involving an Fe(III)Zn(II) complex of relevance to glyoxalase II.