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Setting an Upper Limit on the Myoglobin Iron(IV)Hydroxide pKa: Insight into Axial Ligand Tuning in Heme Protein Catalysis
- Yosca, Timothy
H., Behan, Rachel K., Krest, Courtney
M., Onderko, Elizabeth L., Langston, Matthew C., Green, Michael T.
- Journal of the American Chemical Society 2014 v.136 no.25 pp. 9124-9131
- Raman spectroscopy, catalytic activity, cytochrome P-450, heme, histidine, iron, ligands, myoglobin, pH
- To provide insight into the iron(IV)hydroxide pKₐ of histidine ligated heme proteins, we have probed the active site of myoglobin compound II over the pH range of 3.9–9.5, using EXAFS, Mössbauer, and resonance Raman spectroscopies. We find no indication of ferryl protonation over this pH range, allowing us to set an upper limit of 2.7 on the iron(IV)hydroxide pKₐ in myoglobin. Together with the recent determination of an iron(IV)hydroxide pKₐ ∼ 12 in the thiolate-ligated heme enzyme cytochrome P450, this result provides insight into Nature’s ability to tune catalytic function through its choice of axial ligand.