Solvent tuning of electrochemical potentials in the active sites of HiPIP versus ferredoxin |
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Authors: | Dey Abhishek Jenney Francis E Adams Michael W W Babini Elena Takahashi Yasuhiro Fukuyama Keiichi Hodgson Keith O Hedman Britt Solomon Edward I |
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Institution: | Department of Chemistry, Stanford University, Stanford, CA 94305, USA. |
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Abstract: | A persistent puzzle in the field of biological electron transfer is the conserved iron-sulfur cluster motif in both high potential iron-sulfur protein (HiPIP) and ferredoxin (Fd) active sites. Despite this structural similarity, HiPIPs react oxidatively at physiological potentials, whereas Fds are reduced. Sulfur K-edge x-ray absorption spectroscopy uncovers the substantial influence of hydration on this variation in reactivity. Fe-S covalency is much lower in natively hydrated Fd active sites than in HiPIPs but increases upon water removal; similarly, HiPIP covalency decreases when unfolding exposes an otherwise hydrophobically shielded active site to water. Studies on model compounds and accompanying density functional theory calculations support a correlation of Fe-S covalency with ease of oxidation and therefore suggest that hydration accounts for most of the difference between Fd and HiPIP reduction potentials. |
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