What oxidation state is iron in hemoglobin?

1 Answer
Jun 27, 2016

"Iron"("II")Iron(II). It must be, in order for the iron in heme to get oxidized upon binding with "O"_2O2.


Heme in hemoglobin starts out like this:

A histidine can bind at the fifth coordination site, while "O"_2O2 can bind to the sixth coordination site.

When "O"_2O2 binds to iron in this case, it temporarily and reversibly binds as the "O"_2^(-)O2 superoxide, so the reduction half-reaction is

"O"_2 + e^(-) -> "O"_2^(-),O2+eO2,

and the oxidation half-reaction is

"Fe"^(2+) -> "Fe"^(3+) + e^(-),Fe2+Fe3++e,

giving the overall reaction as:

"O"_2 + cancel(e^(-)) -> "O"_2^(-)
"Fe"^(2+) -> "Fe"^(3+) + cancel(e^(-))
"-----------------------------------------"
color(blue)("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").