Enzymatic C[sbnd]S bond cleavage: Mechanism for the conversion of methanesulfonate to sulfite by flavin-dependent MsuD monooxygenase

Abstract

MsuD, a flavin-dependent monooxygenase, catalyzes the conversion of methanesulfonate (MS−) into formaldehyde and sulfite, playing a crucial role in the organic sulfur cycle. Using density functional calculations, we have demonstrated that the MsuD reaction uses mechanism A involving a FlN5-OO− intermediate. It mainly includes electron transfer from the N5 atom of deprotonated reduced flavin to O2, NO bond formation yielding FlN5-OO−, deprotonation of MS− methyl by FlN5-OO− forming FlN5-OOH and MS2− carbanion, and hydroxy rebound from FlN5-OOH to MS2− accompanying with immediate CS bond dissociation producing sulfite and formaldehyde. In contrast, mechanism B containing FlC4-OO− is ruled out due to the inaccessible hydroxy rebound from FlC4-OOH to MS2−, although the FlC4-OO− species, a common intermediate in the family of flavin-dependent monooxygenases, may also appear in MsuD. Our research provides insights into the enzymatic CS bond cleavage and confirms the potential of the flavin N5 site to facilitate various chemical reactions.