Bond-Mediated Electron Tunneling in Ruthenium-Modified High-Potential Iron−Sulfur Protein

Abstract

High-potential iron-sulfur proteins (HiPIPs) 2 are found in photosynthetic purple nonsulfur bacteria. 3 The three-dimensional structure of Chromatium Vinosum HiPIP features two short segments of R-helix, three strands of antiparallel β-pleated sheet, and a small helix near the N-terminus. 4 The cubane [Fe 4 S 4 ] cluster is attached covalently to the polypeptide matrix through Fe-S γ bonds to cysteines 43, 46, 63, and 77. The side chains of Tyr19, Phe48, Trp60, Phe66, Trp76, Trp80, and other nonpolar residues encapsulate the cluster in a hydrophobic cavity that is inaccessible to solvent. 5 Tyr19, which contacts the [Fe 4 S 4 ] core, has been suggested to play a particularly important structural role. 6 In both oxidation states, the cysteinyl and core inorganic sulfur atoms are involved in H-bonding interactions with peptide NH protons. 7 We have studied intramolecular electron transfer in C. V. HiPIPs in which surface histidines at positions 18, 42, 50, and 81 (42 is wild type; 18, 50, and 81 are mutants in which 42 is Gln) 8 were modified by coordination of Ru(bpy) 2 (im) 2+ (Figure 1A). 9,10 The calculated [Fe 4 S 4 ] 2+ :[Ru(HisX)] 3+ electronic couplings in these four Ru-modified proteins vary dramatically, even though the closest Fe-Ru ET distances fall in a narrow range (Figure 1B-E). Thus these Ru-HiPIPs provide a particularly rigorous test of the tunneling-pathway model of ET in proteins. 11,12 Kinetics of ET from [Fe 4 S 4 ] 2+ to Ru(HisX) 3+ (X ) 18, 42, 50, 81) were measured by transient absorption changes following intramolecular quenching of electronically excited Ru-HiPIPs. 13 Rate constants (k ET ) and driving forces (-∆G°) are set out in Table 1. It is likely that the reorganization energy (λ) is below 1 eV for these reactions, since NMR work has established that the solution structures of oxidized and reduced C. V. HiPIP 14 are similar. 15 A value in the 0.6-0.9 eV range [16][17][18] accounts for the very small changes in rate that accompany driving-force (X ) 42, 81) and temperature (X ) 50) 19 variations. Thus the observed rates closely approximate coupling-limited k ET values.

Long-range electron transfer

“…1) provides a benchmark for superexchange-mediated electron tunneling through proteins. The rates of high driving-force ET reactions have been measured for Ͼ30 Ru(diimine)-labeled metalloproteins (7,29,30,38). Driving-force-optimized values are scattered around the Ru-azurin 1.1-Å Ϫ1 exponential distance decay.…”

Flexibility of the metal-binding region in apo-cupredoxins

“…Cycling between the coordination geometries preferred by these metals in each redox state would in principle lead to high reorganization energies (7). Electron transfer iron centers avoid this by using rigid cofactors such as iron-sulfur clusters and heme groups (8)(9)(10). Instead, copper ions in ET centers are only bound to protein residues, as observed in type 1 (blue) copper and Cu A centers; and thus the protein fold around the metal ion is expected to impart rigidity to these otherwise flexible metal centers (10)(11)(12).…”

The metal reductase activity of some multiheme cytochromes c : NMR structural characterization of the reduction of chromium(VI) to chromium(III) by cytochrome c 7

“…One intermediate is identified with hemes I and III largely oxidized and heme IV essentially reduced in accordance with the order of reduction potentials of the unmodified protein. Although no information is available on the intramolecular electron-transfer pathway, it is tempting to propose that heme IV provides the electron to chromium, because they are so close to one another, and hemes I and III reestablish the thermodynamic conditions by electron tunneling to heme IV (41)(42)(43)(44)(45)(46)(47)(48). As far as chromium is concerned, it is reasonable that the three electrons provided by the protein reduce chromate(VI) stepwise (49,50).…”