Dismantling and rebuilding the trisulfide cofactor demonstrates its essential role in human sulfide quinone oxidoreductase

Sulfide quinone oxidoreductase (SQR) catalyzes the first step in sulfide clearance, coupling H2S oxidation to coenzyme Q reduction. Recent structures of human SQR revealed a sulfur atom bridging the SQR active site cysteines in a trisulfide configuration. Here, we assessed the importance of this cofactor using kinetic, crystallographic and computational modeling approaches. Cyanolysis of SQR proceeds via formation of an intense charge transfer complex that subsequently decays to eliminate thiocyanate. Cyanolysis leads to reversible loss of SQR activity, which is restored in the presence of sulfide. We captured a crystallographic intermediate in SQR that provides clues as to how the oxidized state of the cysteines is preserved. Computational modeling and MD simulations revealed an ~105-fold rate enhancement for nucleophilic addition of sulfide into the trisulfide versus a disulfide cofactor. The cysteine trisulfide in SQR is thus critical for activity and provides a significant catalytic advantage over a cysteine disulfide.