Question

What oxidation state is iron in hemoglobin?

Answer 1

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

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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")`.