Iron(IV)hydroxide pK(a) and the role of thiolate ligation in C-H bond activation by cytochrome P450

• Published in: Science (American Association for the Advancement of Science) 2013-11, Vol.342 (6160), p.825-829
• Main Authors: Yosca, Timothy H, Rittle, Jonathan, Krest, Courtney M, Onderko, Elizabeth L, Silakov, Alexey, Calixto, Julio C, Behan, Rachel K, Green, Michael T
• Format: Article
• Language: English
• Subjects: Carbon - chemistry; Catalysis; Cysteine - analogs & derivatives; Cysteine - chemistry; Cytochrome P-450 Enzyme System - chemistry; Enzyme Activation; Hydrogen Bonding; Hydroxides - chemistry; Oxidation-Reduction; Tryptophan - chemistry; Tyrosine - chemistry
• ISSN: 1095-9203
• DOI: 10.1126/science.1244373

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 pK(a) (where K(a) 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 pK(a) value for this compound of 11.9. Marcus theory analysis indicates that this elevated pK(a) results in a >10,000-fold reduction in the rate constant for oxidations of the protein framework, making these processes noncompetitive with substrate oxidation.