What accounts for this difference in iron oxidation states between hemoglobin and cytochromes?

Bruce P. answered • 09/25/17

20+ year college biology/genetics teacher; I want you to understand.

Hello, Manohar! When you think of protein machines and what they can/cannot do, it often comes down to how each is precisely fitted for its purpose. The cytochromes are designed to "hold and pass" electrons, so they are built to engage in 'normal' operations--the ability for the iron to have an electron (Fe2+) or lose it (Fe3+) depending on what other molecule is present to 'take' or 'donate' an electron.

Hemoglobin is precisely designed NOT to permit this. Basically, the oxygen is "trying" to steal an electron from the Fe2+, but the oxygen and iron are held so that this reaction cannot take place. Since the oxygen is 'trapped' in this state, it can be carried around 'held in place' by the hemoglobin. So a different way of asking your question would be "Why can cytochrome C go to Fe3+ whereas hemoglobin cannot? A lot of it has to do with the GEOMETRY of the interaction between the O2 and the Fe2+, as well as the atoms holding on to each.

You can get a feel for the precision of this 'dance' from the animation here:

https://thinkbio.guru/3D_Directory/Flotsam_and_Jetsam/Hemo_myo/MyoHemeMorph3D.php

(make sure and use the buttons to walk through different views)

Cheers

Bruce

Hi Manohar,

This is what I found. Apparently the Fe in hemoglobin and myoglobin is always in the ferrous state (Fe 2+), while in cytochoromes switches between Fe2+ and Fe3+. For most cytochromes the heme Fe is bound to two amino acid side chains v/s one in hemoglobin molecules. This configuration prevents the binding of O2, CO2 and CO to cytochromes.

Great question!

Best

Sanjay