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Mccubbin Stepanic, O.
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article
Determination of the iron(IV) local spin states of the Q intermediate of soluble methane monooxygenase by Kβ X-ray emission spectroscopy.
Abstract
Soluble methane monooxygenase (sMMO) facilitates the conversion of methane to methanol at a non-heme Fe<sup>IV</sup><sub>2</sub> intermediate MMOH<sub>Q</sub>, which is formed in the active site of the sMMO hydroxylase component (MMOH) during the catalytic cycle. Other biological systems also employ high-valent Fe<sup>IV</sup> sites in catalysis; however, MMOH<sub>Q</sub> is unique as Nature's only identified Fe<sup>IV</sup><sub>2</sub> intermediate. Previous <sup>57</sup>Fe Mössbauer spectroscopic studies have shown that MMOH<sub>Q</sub> employs antiferromagnetic coupling of the two Fe<sup>IV</sup> sites to yield a diamagnetic cluster. Unfortunately, this lack of net spin prevents the determination of the local spin state (S<sub>loc</sub>) of each of the irons by most spectroscopic techniques. Here, we use Fe Kβ X-ray emission spectroscopy (XES) to characterize the local spin states of the key intermediates of the sMMO catalytic cycle, including MMOH<sub>Q</sub> trapped by rapid-freeze-quench techniques. A pure XES spectrum of MMOH<sub>Q</sub> is obtained by subtraction of the contributions from other reaction cycle intermediates with the aid of Mössbauer quantification. Comparisons of the MMOH<sub>Q</sub> spectrum with those of known S<sub>loc</sub> = 1 and S<sub>loc</sub> = 2 Fe<sup>IV</sup> sites in chemical and biological models reveal that MMOH<sub>Q</sub> possesses S<sub>loc</sub> = 2 iron sites. This experimental determination of the local spin state will help guide future computational and mechanistic studies of sMMO catalysis.