Question

What makes that iron-sulfur (Fe-S) cluster so prevalent in living organisms? Why was the “great oxidation” initially a big problem for living organisms? What adaptations were made as a result?

Answer 1

Iron-Sulfur clusters are very important in living organisms as they can be utilized for a variety of biochemical processes inside living organisms. Ferredoxin is the most common. These are best known for their oxidation-reduction reactions of mitochondrial electron transport. They also play vital roles in photosynthesis and nitrogen fixation. Main role: electron transport proteins in biological redox reactions.

When Fe-S clusters are proteins, the iron-sulfur linkage contains two types of sulfur atoms, 1) Free sulfur atoms which are labile. 2) Cysteinyl sulfur atoms which cannot be replaced with other atoms. Free sulfur atoms can be easily removed as H₂S on acidification. These proteins are extremely important in living systems.

Certain examples of such proteins:

1. Ferredoxin: 2Fe-2S protein: This contains two irons and has one iron in +2 oxidation state and the other ion in +3 oxidation state. It acts as a one-electron transfer protein as the other iron can be oxidized to +3 oxidation state to reach to it's oxidized form.

2. 4Fe-4S: (4Fe-Ferredoxin): Usually undergoes a one-electron transfer.

There are several other types of Ferredoxins (8Fe-8S), (3Fe-4S). These electron transfer properties are very important in living systems.

Before the "Great Oxygenation", the earth was mostly anaerobic. Iron and sulfur proteins clusters provided many enzymes with the ability to transfer electrons. Because free oxygenation is very reactive to elemental iron the Iron in +2 oxidation state was then oxidized to +3 oxidation state. By oxidizing environmental iron, the bioavailability of oxygen was reduced. This created unstable Fe-S clusters which caused them to decompose. As a response to this, the microorganisms relying on electron transport had to adapt. As oxygen concentration increased, iron became limited in aerobic species.

Microorganisms tackled this problem by excreting siderophores, soluble organic molecules that avidly bind iron and can leach it off mineral precipitates.