Rubredoxins are a class of low-molecular-weight iron-containing proteins found in sulfur-metabolizing bacteria and archaea. Sometimes rubredoxins are classified as iron-sulfur proteins; however, in contrast to iron-sulfur proteins, rubredoxins do not contain inorganic sulfide. Like cytochromes, ferredoxins and Rieske proteins, rubredoxins are thought to participate in electron transfer in biological systems. Recent work in bacteria and algae have led to the hypothesis that some rubredoxins may instead have a role in delivering iron to metalloproteins.
Structure
The 3-D structures of a number of rubredoxins have been solved. The fold belongs to the α+β class, with 2 α-helices and 2-3 β-strands. Rubredoxin active site contains an iron ion which is coordinated by the sulfurs of four conserved cysteine residues forming an almost regular tetrahedron. This is sometimes denoted as a [1Fe-0S] or an Fe1S0 system, in analogy to the nomenclature for iron-sulfur proteins. While the vast majority of rubredoxins are soluble, there exists a membrane-bound rubredoxin, referred to as rubredoxin A, in oxygenic photoautotrophs.
Rubredoxins perform one-electron transfer processes. The central iron atom changes between the +2 and +3 oxidation states. In both oxidation states, the metal remains high spin, which helps to minimize structural changes. The reduction potential of a rubredoxin is typically in the range +50 mV to -50 mV.
This iron-sulphur protein is an electron carrier, and it is easy to distinguish its metallic centre changes: the oxidized state is reddish (due to a ligand metal charge transfer), while the reduced state is colourless (because the electron transition has an energy of the infrared level, which is imperceptible to the human eye).
Rubredoxin in some biochemical reactions
camphor 1,2-monooxygenase [(+)-camphor, reduced-rubredoxin:oxygen oxidoreductase (1,2-lactonizing)] (+)-bornane-2,5-dione + reduced rubredoxin + O2 = 5-oxo-1,2-campholide + oxidized rubredoxin + H2O alkane 1-monooxygenase (alkane, reduced-rubredoxin:oxygen 1-oxidoreductase) octane + reduced rubredoxin + O2 = 1-octanol + oxidized rubredoxin + H2O superoxide reductase (rubredoxin:superoxide oxidoreductase) reduced rubredoxin + superoxide + 2 H+ = rubredoxin + H2O2 rubredoxin—NAD+ reductase (rubredoxin:NAD+ oxidoreductase) reduced rubredoxin + NAD+ = oxidized rubredoxin + NADH + H+ rubredoxin—NAD(P)+ reductase (rubredoxin:NAD(P)+ oxidoreductase) reduced rubredoxin + NAD(P)+ = oxidized rubredoxin + NAD(P)H + H+
Electron transfer rate
The electron exchange rate is accurately determined by standard kinetics measurements of visible absorption (490 nm) spectra. The electron transfer rate has three parameters: electronic coupling, reorganization energy and free energy of reaction (ΔG°).
Protein mechanism and effects
The electron transfer reaction of rubredoxin is carried out by a reversible Fe3+/Fe2+ redox coupling by the reduction of Fe3+ to Fe2+ and a gating mechanism caused by the conformational changes of Leu41.
Upon the reduction of Fe3+ to Fe2+, the four Fe-S bond lengths increase and the amide-NH H-bonding to the S(Cys) become shortened. The reduced Fe2+ structure of rubredoxin results in a small increase in electrostatic stabilization of the amide-NH H-bonding to the S-Cys, leading to a lower reorganizational energy that allows faster electron transfer.