2 research outputs found
C(sp<sup>3</sup>)–H Bond Hydroxylation Catalyzed by Myoglobin Reconstituted with Manganese Porphycene
Myoglobin reconstituted with manganese
porphycene was prepared
in an effort to generate a new biocatalyst and was characterized by
spectroscopic techniques. The X-ray crystal structure of the reconstituted
protein reveals that the artificial cofactor is located in the intrinsic
heme-binding site with weak ligation by His93. Interestingly, the
reconstituted protein catalyzes the H<sub>2</sub>O<sub>2</sub>-dependent
hydroxylation of ethylbenzene to yield 1-phenylethanol as a single
product with a turnover number of 13 at 25 °C and pH 8.5. Native
myoglobin and other modified myoglobins do not catalyze C–H
hydroxylation of alkanes. Isotope effect experiments yield KIE values
of 2.4 and 6.1 for ethylbenzene and toluene, respectively. Kinetic
data, log <i>k</i><sub>obs</sub> versus BDEÂ(CÂ(sp<sup>3</sup>)–H) for ethylbenzene, toluene, and cyclohexane, indicate
a linear relationship with a negative slope. These findings clearly
indicate that the reaction occurs via a rate-determining step that
involves hydrogen-atom abstraction by a MnÂ(O) species and a subsequent
rebound hydroxylation process which is similar to the reaction mechanism
of cytochrome P450
Manganese(V) Porphycene Complex Responsible for Inert C–H Bond Hydroxylation in a Myoglobin Matrix
A mechanistic study
of H<sub>2</sub>O<sub>2</sub>-dependent C–H
bond hydroxylation by myoglobin reconstituted with a manganese porphycene
was carried out. The X-ray crystal structure of the reconstituted
protein obtained at 1.5 Ã… resolution reveals tight incorporation
of the complex into the myoglobin matrix at pH 8.5, the optimized
pH value for the highest turnover number of hydroxylation of ethylbenzene.
The protein generates a spectroscopically detectable two-electron
oxidative intermediate in a reaction with peracid, which has a half-life
up to 38 s at 10 °C. Electron paramagnetic resonance spectra
of the intermediate with perpendicular and parallel modes are silent,
indicating formation of a low-spin Mn<sup>V</sup>-oxo species. In
addition, the Mn<sup>V</sup>-oxo species is capable of promoting the
hydroxylation of sodium 4-ethylbenzenesulfonate under single turnover
conditions with an apparent second-order rate constant of 2.0 M<sup>–1</sup> s<sup>–1</sup> at 25 °C. Furthermore,
the higher bond dissociation enthalpy of the substrate decreases the
rate constant, in support of the proposal that the H-abstraction is
one of the rate-limiting steps. The present engineered myoglobin serves
as an artificial metalloenzyme for inert C–H bond activation
via a high-valent metal species similar to the species employed by
native monooxygenases such as cytochrome P450