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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