Electronic State of the His/Tyr-Ligated Heme of BthA by Mössbauer and DFT Analysis.

The BthA protein from the microorganism contains two hemes with axial His/OH and His/Tyr coordinations separated by the closest interheme distance of 14 Å. BthA has a similar structure and belongs to the same family of multiheme cytochrome peroxidases as MauG, which performs long-range oxidation of the partner protein methylamine dehydrogenase. Magnetic Mössbauer spectroscopy of the diferric state of BthA corroborates previous structural work identifying a high-spin (His/OH) peroxidatic heme and a low-spin (His/Tyr) electron transfer heme. Unlike MauG, addition of HO fully converts the diferric form of BthA to a stable 2e oxidized state, allowing a new assessment of this state. The peroxidatic heme is found to be oxidized to a canonical compound II, S = 1 oxoiron(IV) heme. In contrast, the electronic properties of the oxidized His/Tyr heme are puzzling. The isomer shift of the His/Tyr heme (0.17 mm/s) is close to that of the precursor S = 1/2 Fe heme (0.21 mm/s) which suggests oxidation of the Tyr. However, the spin-dipolar hyperfine coupling constants are found here to be the same as those for the ferryl peroxidatic heme, indicating that the His/Tyr heme is also a compound II, S = 1 Fe heme and ruling out oxidation of the Tyr. DFT calculations indicate that the unusually high isomer shift is not attributable to the rare axial His/Tyr heme coordination. The calculations are only compatible with spectroscopy for an unusually long Fe-O distance, which is presumably under the influence of the protein environment of the His/Tyr heme moiety in the HO oxidized state of the protein. The results offer new insights into how high valence intermediates can be tuned by the protein environment for performing long-range oxidation.