Setting an Upper Limit on the Myoglobin Iron(IV)Hydroxide pKa: Insight into Axial Ligand Tuning in Heme Protein Catalysis

ABSTRACT: 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. A fundamental understanding of protein structure−function relationships is key to harnessing the catalytic power of enzymes for chemical applications. For some time now, we have been interested in the factors that govern C−H bon