Sequence Determination of Reduction Potentials by Cysteinyl Hydrogen Bonds and Peptide Dipoles in [4Fe-4S] Ferredoxins

Abstract

A sequence determinant of reduction potentials is reported for bacterial [4Fe-4S]-type ferredoxins. The residue that is four residues C-terminal to the fourth ligand of either cluster is generally an alanine or a cysteine. In five experimental ferredoxin structures, the cysteine has the same structural orientation relative to the nearest cluster, which is stabilized by the SH...S bond. Although such bonds are generally considered weak, indications that Fe-S redox site sulfurs are better hydrogen-bond acceptors than most sulfurs include the numerous amide NH...S bonds noted by Adman and our quantum mechanical calculations. Furthermore, electrostatic potential calculations of 11 experimental ferredoxin structures indicate that the extra cysteine decreases the reduction potential relative to an alanine by approximately 60 mV, in agreement with experimental mutational studies. Moreover, the decrease in potential is due to a shift in the polar backbone stabilized by the SH...S bond rather than to the slightly polar cysteinyl side chain. Thus, these cysteines can "tune" the reduction potential, which could optimize electron flow in an electron transport chain. More generally, hydrogen bonds involving sulfur can be important in protein structure/function, and mutations causing polar backbone shifts can alter electrostatics and thus affect redox properties or even enzymatic activity of a protein.

Limited Effectiveness of Antioxidants in the Protection of Yeast Defective in Antioxidant Proteins

“…[16] We hypothesized that the main basis for the prooxidative action of some antioxidants is the reactivity, mainly the high redox potential of their secondary radicals formed in the reactions with the oxidants. [2] This feature may be of special importance if the Fe -S clusters of proteins constitute the critical target since their one-electron redox potential is low, ranging between 2 645 mV and 0 V. [23,24] The prooxidative action of ascorbate has been broadly discussed. [25 -27] However, it seems that similar property may be even more important for other antioxidants.…”

Density functional theory calculations of redox properties of iron–sulphur protein analogues

“…In our related study using classical MM methods, several sequence determinants of reduction potentials involved hydrogen bonds to the redox site, in which the hydrogen bond donors included a backbone amide in some [1Fe]-rubredoxins [17] and a non-ligating cysteine in some [4Fe -4S]-Fds [18], and an asparagine and a histidine in the [1Fe]-rubrerythrin [102]. Although the effects on the reduction potential were modelled purely as electrostatic interactions, the possibility that the hydrogen bonds perturb the electronic structure of the redox site remained.…”

Redox-dependent Structural Reorganization in Putidaredoxin, a Vertebrate-type [2Fe-2S] Ferredoxin from Pseudomonas putida

“…The electron-rich sulfur containing residues in the vicinity of the active site were demonstrated to be able to tune the redox potential of ferredoxins by changing their electrostatic potential. 44 In reduced Pdx, the flexible side-chain of Met70 follows the movement of the Cys45-Ala46 peptide upon its isomerization and shifts closer to the Ser44-Cys45 fragment (Figure 4). On the other side, Ser73 (Cys73 in the wild-type Pdx) moves closer to Met70.…”