What oxidation state is iron in hemoglobin?

1 Answer
Truong-Son N.
Jun 27, 2016

"Iron"("II"). It must be, in order for the iron in heme to get oxidized upon binding with "O"_2.

Heme in hemoglobin starts out like this:

A histidine can bind at the fifth coordination site, while "O"_2 can bind to the sixth coordination site.

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

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

and the oxidation half-reaction is

"Fe"^(2+) -> "Fe"^(3+) + 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").

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