Iron(IV)hydroxide pK(a) and the role of thiolate ligation in C-H bond activation by cytochrome P450.
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.
Department of Chemistry, Pennsylvania State University, University Park, PA 16802, USA.
Timothy H
TH
Yosca
Jonathan
J
Rittle
Courtney M
CM
Krest
Elizabeth L
EL
Onderko
Alexey
A
Silakov
Julio C
JC
Calixto
Rachel K
RK
Behan
Michael T
MT
Green
D002244:Carbon / Q000737:chemistry; D002384:Catalysis; D003545:Cysteine / Q000031:analogs & derivatives* / Q000737:chemistry; D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry*; D004789:Enzyme Activation; D006860:Hydrogen Bonding; D006878:Hydroxides / Q000737:chemistry*; D010084:Oxidation-Reduction; D014364:Tryptophan / Q000737:chemistry; D014443:Tyrosine / Q000737:chemistry
D006878:Hydroxides; C545158:cysteine thiolate; D014443:Tyrosine; D002244:Carbon; D014364:Tryptophan; D003577:Cytochrome P-450 Enzyme System; C031356:hydroxide ion; D003545:Cysteine