Biodesulfurization is an attractive option for enzymatically removing sulfur from the recalcitrant thiophenic derivatives that comprise the majority of organosulfur compounds remaining in hydrotreated petroleum products. Desulfurization in the bacteria Rhodococcus erythropolis follows a four-step pathway culminating in C–S bond cleavage in the 2′-hydroxybiphenyl-2-sulfinate (HBPS) intermediate to yield 2-hydroxybiphenyl and bisulfite. The reaction, catalyzed by 2′-hydroxybiphenyl-2-sulfinate desulfinase (DszB), is the rate-limiting step and also the least understood, as experimental evidence points to a mechanism unlike that of other desulfinases. On the basis of structural and biochemical evidence, two possible mechanisms have been proposed: nucleophilic addition and electrophilic aromatic substitution. Density functional theory calculations showed that electrophilic substitution by a proton is the lower energy pathway and is consistent with previous kinetic and site-directed mutagenesis studies. C27 transfers its proton to HBPS, leading directly to the release of SO2 without the formation of a carbocation intermediate. The H60–S25 dyad stabilizes the transition state by withdrawing the developing negative charge on cysteine. Establishing the desulfination mechanism and specific role of active site residues, accomplished in this study, is essential to protein engineering efforts to increase DszB catalytic activity, which is currently too low for industrial-scale application.
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Acknowledgement is made to the Donors of the American Chemical Society Petroleum Research Fund for support of this research (53861-DNI4). Computing resources were provided by the University of Kentucky (DLX cluster) and the NSF Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by NSF grant no. ACI-1053575 (Gordon cluster under allocation MCB090159).
Electronic supplementary information (ESI) available: Molecular dynamics simulations method, additional data, equations, and Cartesian coordinates of active site cluster models. See DOI: 10.1039/c7sc00496f
Geronimo, Inacrist; Nigam, Shawn R.; and Payne, Christina M., "Desulfination by 2′-Hydroxybiphenyl-2-Sulfinate Desulfinase Proceeds Via Electrophilic Aromatic Substitution by the Cysteine-27 Proton" (2017). Chemical and Materials Engineering Faculty Publications. 33.