Research Article

Iron(IV)hydroxide pKa and the Role of Thiolate Ligation in C–H Bond Activation by Cytochrome P450

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Science  15 Nov 2013:
Vol. 342, Issue 6160, pp. 825-829
DOI: 10.1126/science.1244373

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The pKa of P450

Cytochrome P450 enzymes oxidize hydrocarbons through activation of oxygen at heme iron centers. However, the protein backbone has various sites (particularly tyrosine residues) that are also sensitive to oxidation, so how can the enzyme rapidly transform substrates without attacking itself? Yosca et al. (p. 825) explored the energetics of the competition between substrate and self-oxidation by measuring the pKa of the enzyme's iron(IV)hydroxide motif. Cysteine thiolate coordination to iron in the P450 structure raised the pKa almost to 12—rendering the iron oxo far more basic than analogous motifs in other heme environments. Correspondingly, the electronic environment for H-atom transfer from the substrate was relatively favorable, compared to electron transfer from a backbone residue.

Abstract

Cytochrome P450 enzymes activate oxygen at heme iron centers to oxidize relatively inert substrate carbon-hydrogen bonds. Cysteine thiolate coordination to iron is posited to increase the pKa (where Ka is the acid dissociation constant) of compound II, an iron(IV)hydroxide complex, correspondingly lowering the one-electron reduction potential of compound I, the active catalytic intermediate, and decreasing the driving force for deleterious auto-oxidation of tyrosine and tryptophan residues in the enzyme’s framework. Here, we report on the preparation of an iron(IV)hydroxide complex in a P450 enzyme (CYP158) in ≥90% yield. Using rapid mixing technologies in conjunction with Mössbauer, ultraviolet/visible, and x-ray absorption spectroscopies, we determine a pKa value for this compound of 11.9. Marcus theory analysis indicates that this elevated pKa results in a >10,000-fold reduction in the rate constant for oxidations of the protein framework, making these processes noncompetitive with substrate oxidation.

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