Spectroscopic Investigations of Catalase Compound Ⅱ:Characterization of an Iron(Ⅳ) Hydroxide Intermediate in a Non-thiolate-Ligated Heme Enzyme

摘要

We report on the protonation state of Helicobacter pylori catalase compound Ⅱ. UV/visible, Moessba- uer, 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, 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.