A putative Fe2+-bound persulfenate intermediate in cysteine dioxygenase

Abstract

The common reactions of dioxygen, superoxide, and hydroperoxides with thiolates are thought to proceed via persulfenate intermediates, yet these have never been visualized. Here we report a 1.4 A resolution crystal structure of the Fe(2+)-dependent enzyme cysteine dioxygenase (CDO) containing this putative intermediate trapped in its active site pocket. The complex raises the possibility that, distinct from known dioxygenases and proposed CDO mechanisms, the Fe-proximal oxygen atom may be involved in the primary oxidation event yielding a unique three-membered Fe-S-O cyclic intermediate. A nonpolar environment of the distal oxygen would facilitate isomerization of the persulfenate to the sulfinate product.

Figure 1

The trapped persulfenate intermediate in the active site of CDO. (a) Stereoview of the active site of CDO with electron density for the ligand shown at 0.8*ρ_rms. Bonds to the iron (orange dashed lines) and potential hydrogen bonds (red dashed lines) are shown. See Figure S1 for a close-up of omit density for the ligand. (b) Schematic of the active site interactions and metallocenter geometry based on the same view shown in panel (a). Close non-polar approaches surrounding the ligand (double arcs) and potential hydrogen bonds have been inferred from the environment. The geometry is such that one of the two protons of the sp3 hybridized α-amino group points toward a bound water (Wat156) and the other points toward where it can make a bifurcated hydrogen bond to both Tyr157-OH and the α-carboxylate oxygen. This is the main interaction allowing us to propose that the Tyr157 hydroxyl donates a hydrogen bond to atom O₁.

Scheme 1

Proposed CDO mechanism