Solvent Tuning of Electrochemical Potentials in the Active Sites of HiPIP Versus Ferredoxin
Abhishek Dey,1
Francis E. Jenney, Jr.,2
Michael W. W. Adams,2
Elena Babini,3
Yasuhiro Takahashi,4
Keiichi Fukuyama,4
Keith O. Hodgson,1,5*
Britt Hedman,5*
Edward I. Solomon1,5*
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.
1 Department of Chemistry, Stanford University, Stanford, CA 94305, USA.
2 Department of Biochemistry and Molecular Biology, University of Georgia, Athens, GA 30602, USA.
3 University of Bologna, Campus Universitario di Scienze degli Alimenti Via Ravennate, 1020, 47023 Cesena, Italy.
4 Department of Biological Sciences, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan.
5 Stanford Synchrotron Radiation Laboratory (SSRL), Stanford Linear Accelerator Center (SLAC), Stanford University, Menlo Park, CA 94025, USA.
* To whom correspondence should be addressed. E-mail: hodgson{at}ssrl.slac.stanford.edu (K.O.H.); hedman{at}ssrl.slac.stanford.edu (B.H.); edward.solomon{at}stanford.edu (E.I.S.)
