# What oxidation state is iron in hemoglobin?

Jun 27, 2016

"Iron"("II"). It must be, in order for the iron in heme to get oxidized upon binding with ${\text{O}}_{2}$.

Heme in hemoglobin starts out like this:

A histidine can bind at the fifth coordination site, while ${\text{O}}_{2}$ can bind to the sixth coordination site.

When ${\text{O}}_{2}$ binds to iron in this case, it temporarily and reversibly binds as the ${\text{O}}_{2}^{-}$ superoxide, so the reduction half-reaction is

${\text{O"_2 + e^(-) -> "O}}_{2}^{-} ,$

and the oxidation half-reaction is

${\text{Fe"^(2+) -> "Fe}}^{3 +} + {e}^{-} ,$

giving the overall reaction as:

${\text{O"_2 + cancel(e^(-)) -> "O}}_{2}^{-}$
${\text{Fe"^(2+) -> "Fe}}^{3 +} + \cancel{{e}^{-}}$
$\text{-----------------------------------------}$
$\textcolor{b l u e}{{\text{O"_2 + "Fe"^(2+) -> "Fe"^(3+) + "O}}_{2}^{-}}$

On the other hand, "Iron"("III"), in methemoglobin, cannot bind oxygen unless the iron center is reduced back to "Iron"("II").