Solvent Tuning of Electrochemical Potentials in the Active Sites of HiPIP Versus Ferredoxin

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.

Characterization of a unique [FeS] cluster in the electron transfer chain of the oxygen tolerant [NiFe] hydrogenase from Aquifex aeolicus

“…Such high coordination leads to a large quadrupole splitting for the distorted Fe subsite and shifts the 3 þ ∕2þ reduction potential of the HiPIP-like core to lower values, as has been shown for synthetic five-coordinated [FeS] cubanes (24). The latter originates from the increased negative charge of the cluster, which in biological systems, however, may be partially counterbalanced by additional hydrogen bonds leading overall to less covalent Fe-S bonds (30,36). In addition, synthetic five-coordinated clusters can also carry out two redox steps in a close range of potentials.…”

“…This high redox compliance is proposed to be related to an unusual ligation by five or six cysteines, as may be inferred from homology modeling. Such additional thiol ligand(s) can easily alter the cluster geometry as well as the redox properties (24,26,30).…”

Synthetic analogues of [Fe 4 S 4 (Cys) 3 (His)] in hydrogenases and [Fe 4 S 4 (Cys) 4 ] in HiPIP derived from all-ferric [Fe 4 S 4 {N(SiMe 3 ) 2 } Ohki1, Tanifuji2, Yamada3 et al. 2011Proc. Natl. Acad. Sci. U.S.A.421280View full textAdd to dashboardCite

“…Even the removal of a small amount of negative charge due to the formation of hydrogen bonds appears to destabilize the all-ferric oxidation state. As for the HiPIP ox cluster, the ½Fe 4 S 4 3þ core is buried in a hydrophobic pocket of the protein, while the ½Fe 4 S 4 2þ∕1þ core of Fd is exposed to hydrogen bonding by water (13). In this context, the unique ability of higher oxidation states of 3 may arise from the environment provided by the bulky hydrocarbon groups of the -SDmp ligands, which mimics a hydrophobic pocket of the protein.…”

“…For Fd and HiPIP the formation of hydrogen bonds with water has been suggested to account for most of the difference between the redox potentials of HiPIP and Fd (13), indicating the importance of hydrophobic shielding of the ½Fe 4 S 4 clusters in the more oxidized form. Synthetic analogues of HiPIP ox with thiolates derived from bulky hydrocarbon groups are valuable for the investigation of the dependency of the redox potentials on hydrogen bonding.…”

Electronic structure of the unique [4Fe-3S] cluster in O 2 -tolerant hydrogenases characterized by 57 Fe Mössbauer and EPR spectroscopy

“…Both forms share the same isoelectronic 2+ oxidation state with spin S = 0, albeit with some minor differences in structures and electron delocalization (10,11). In contrast, the proximal [4Fe-3S] cluster in hydrogenase (Hase) I from A. aeolicus can attain three redox states within a redox potential span of only 150 mV, two of which are paramagnetic with S = 1/2, as we could show by EPR-detected potentiometric titrations (3).…”