Spectroscopic Investigations Of Catalase Compound Ii: Characterization Of An Iron(Iv) Hydroxide Intermediate In A Non-Thiolate-Ligated Heme Enzyme

We report on the protonation state of Helicobacter pylon catalase compound II. UV/visible, Mossbauer, and X-ray absorption spectroscopies have been used to examine the intermediate from pH 5 to 14. We have determined that HPC-II exists in an iron(IV) hydroxide state up to pH 11. Above this pH, the iron(IV) hydroxide complex transitions to a new species (pK(a) = 13.1) with Mossbauer parameters that are indicative of an iron(IV)-oxo intermediate. Recently, we discussed a role for an elevated compound II pK(a) in diminishing the compound I reduction potential. This has the effect of shifting the thermodynamic landscape toward the two-electron chemistry that is critical for catalase function. In catalase, a diminished potential would increase the selectivity for peroxide disproportionation over off-pathway one-electron chemistry, reducing the buildup of the inactive compound II state and reducing the need for energetically expensive electron donor molecules.