Difference Between Ferredoxin and Rubredoxin

The key difference between ferredoxin and Rubredoxin is that ferredoxin has a considerably lower redox potential compared to Rubredoxin.

Both ferredoxin and Rubredoxin are iron-containing proteins. However, we can find ferredoxin in bacterial forms and in plants because it is a chloroplast-protein. However, Rubredoxin is a protein that occurs only in bacteria and archaea. These two compounds have a closely similar structure.

1. Overview and Key Difference
2. What is Ferredoxin
3. What is Rubredoxin
4. Side by Side Comparison – Ferredoxin vs Rubredoxin in Tabular Form
5. Summary

What is Ferredoxin?

Ferredoxin is an iron-sulfur containing protein. It is involved in mediating the electron transfer in a variety of metabolic reactions. These are small proteins which are water-soluble, and they exist in chloroplasts. The iron and sulfur atoms in this protein are arranged in iron-sulfur clusters. They can act as biological capacitors by accepting and discharging electrons. Here, the oxidation state of iron atoms changes from +2 to +3. Therefore, they act as electron transferring agents in redox reactions that take place in a biological environment. Comparatively, the redox potential of this protein is low. A ferredoxin protein molecule can contain two, three or four iron atoms per protein molecule. There are three common types of ferredoxins: Fe₂S₂ ferredoxins, Fe₄S₄ ferredoxins and Fe₃S₄ ferredoxins.

The major role of ferredoxin is to allocate high energy electrons in the chloroplast, and these proteins are involved in distributing electrons required for carbon dioxide fixation, nitrile reduction, sulfite reduction, glutamate synthesis, cyclic electron flow, etc.

What is Rubredoxin?

Rubredoxin is an iron-containing protein that can be found in bacteria and archaea. It is a type of low-molecular-weight protein (usually proteins are high-molecular-weight compounds). However, unlike ferredoxin, Rubredoxin protein does not contain inorganic sulfides. The major role of Rubredoxin is that it participates in electron transfer mechanisms in redox reactions that occur in biological systems.

When considering the structure of Rubredoxin, it contains a central iron atom which has nearly a tetrahedral geometry. The four groups that are bound to this iron atom are cysteine residues. Majority of Rubredoxin proteins are water-soluble chemical species. However, there are some insoluble species that exist as membrane-bound proteins. E.g. Rubredoxin-A.

During the electron transfer mechanism, the oxidation state of the central iron atom changes from +2 to +3. We can easily recognize this change in oxidation state because the colour changes from red to colourless. During this changing, the metal ion remains in a high-spin state because it is helpful to minimize the structural changes of the protein. Typically, the reduction potential of Rubredoxin is higher than ferredoxin; it is in the range of +50 mV to -50 mV.

What is the Difference Between Ferredoxin and Rubredoxin?

Ferredoxin and Rubredoxin are protein compounds which contain both iron and sulfur as components. The key difference between ferredoxin and Rubredoxin is that ferredoxin has a considerably lower redox potential compared to Rubredoxin. The redox potential of ferredoxin is about -420 mV, and the redox potential of Rubredoxin ranges from -50 to +50 mV. Furthermore, ferredoxin can contain two, three or four iron atoms per protein molecule, but in Rubredoxin, there is one central iron atom. However, both these molecules have a closely similar tetrahedral geometry around the iron atoms.

Moreover, ferredoxin contains inorganic sulfur as a component in the protein molecule, but there is no inorganic sulfur in Rubredoxin. When considering the occurrence, ferredoxin can occur in both bacterial forms and in plants, but Rubredoxin occurs in bacteria and archaea.

Below tabulation summarizes the difference between ferredoxin and Rubredoxin.

Summary – Ferredoxin vs Rubredoxin

Reference:

1. Chipperfield, J.r. “IRON | Properties and Determination.” Encyclopedia of Food Sciences and Nutrition, 2003, pp. 3367–3373., doi:10.1016/b0-12-227055-x/00650-7.

Image Courtesy:

1. “3P1M.pdb1” By Jmol Development team – Jmol (CC0) via Commons Wikimedia
2. “PDB 1s24 EBI” By Jawahar Swaminathan and MSD staff at the European Bioinformatics Institute – (Public Domain) via Commons Wikimedia