Structural and electronic properties of the active site of [ZnFe] SulE

The function of the recently isolated sulerythrin (SulE) has been investigated using a combination of structural and electronic analyses based on quantum mechanical calculations. In the SulE structure of Fushinobu et al. (2003), isolated from a strictly aerobic archaeon, Sulfolobus tokadaii, a dioxygen-containing species was tentatively included at the active site during crystallographic refinement although the substrate specificity of SulE remains unclear. Studies have suggested that a structurally related enzyme, rubrerythrin, functions as a hydrogen peroxide reductase. Since SulE is a truncated version of rubrerythrin, the enzymes are hypothesized to function similarly. Hence, using available X-ray crystallography data (1.7 Å), we constructed various models of SulE containing a ZnII-Fe active site, differing in the nature of the substrate specificity (O₂, H₂O₂), the oxidation level and the spin state of the iron ion, and the protonation states of the coordinating glutamate residues. Also, the substrate H₂O₂ is modeled in two possible configurations, differing in the orientation of the hydrogen atoms. Overall, the optimized geometries with an O₂ substrate do not show good agreement with the experimentally resolved geometry. In contrast, excellent agreement between crystal structure arrangement and optimized geometries is achieved considering a H₂O₂ substrate and FeII in both spin states, when Glu92 is protonated. These results suggest that the dioxo species detected at the [ZnFe] active site of sulerythrin is H₂O₂, rather than an O₂ molecule in agreement with experimental data indicating that only the diferrous oxidation state of the dimetal site in rubrerythrin reacts rapidly with H₂O₂. Based on our computations, we proposed a possible reaction pathway for substrate binding at the ZnFeII site of SulE with a H₂O₂ substrate. In this reaction pathway, Fe or another electron donor, such as NAD(P)H, catalyzes the reduction of H₂O₂ to water at the zinc-iron site.