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A Highly Reactive P450 Model Compound I

2009; American Chemical Society; Volume: 131; Issue: 28 Linguagem: Inglês

10.1021/ja903394s

1943-2984

Seth R. Bell, John T. Groves,

Pharmacogenetics and Drug Metabolism

The detection and kinetic characterization of a cytochrome P450 model compound I, [OFeIV−4-TMPyP]+ (1), in aqueous solution shows extraordinary reaction rates for C−H hydroxylations. Stopped-flow spectrophotometric monitoring of the oxidation of FeIII−4-TMPyP with mCPBA revealed the intermediate 1, which displays a weak, blue-shifted Soret band at 402 nm and an absorbance at 673 nm, typical of a porphyrin π-radical cation. This intermediate was subsequently transformed into the well-characterized OFeIV−4-TMPyP. Global analysis afforded a second-order rate constant k1 = (1.59 ± 0.06) × 107 M−1 s−1 for the formation of 1 followed by a first-order decay with k2 = 8.8 ± 0.1 s−1. 1H and 13C NMR determined 9-xanthydrol to be the major product (∼90% yield) of xanthene oxidation by 1. Electrospray ionization mass spectrometry carried out in 47.5% 18OH2 indicated 21% 18O incorporation, consistent with an oxygen-rebound reaction scenario. Xanthene/xanthene-d2 revealed a modest kinetic isotope effect, kH/kD = 2.1. Xanthene hydroxylation by 1 occurred with a very large second-order rate constant k3 = (3.6 ± 0.3) × 106 M−1 s−1. Similar reactions of fluorene-4-carboxylic acid and 4-isopropyl- and 4-ethylbenzoic acid also gave high rates for C−H hydroxylation that correlated well with the scissile C−H bond energy, indicating a homolytic hydrogen abstraction transition state. Mapping the observed rate constants for C−H bond cleavage onto the Brønsted−Evans−Polanyi relationship for similar substrates determined the H−OFeIV−4-TMPyP bond dissociation energy to be ∼100 kcal/mol. The high kinetic reactivity observed for 1 is suggested to result from a high porphyrin redox potential and spin-state-crossing phenomena. More generally, subtle charge modulation at the active site may result in high reactivity of a cytochrome P450 compound I.