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# Bound vs. Unbound Heme as a Cofactor

BIOCHM-EHSKVM

Consider the schematic below:

The affinity of carbon monoxide to unbound heme is much greater compared to oxygen; however, that affinity is reduced with bound heme (as in heme that is bound to myoglobin or hemoglobin).

Which of the following statements describes that observation?

A

The oxygen atoms are $sp^2$ hybridized in $O_2$ and that explains why it binds at a $120^o$ angle to both bound and unbound heme structures.

B

With bound heme, binding of $O_2$ is of greater affinity compared to $CO$ simply because oxygen levels are higher compared to carbon monoxide (assuming non-toxic environment).

C

The carbon and oxygen atoms are $sp$ hybridized in $CO$ and that explains why it binds at a $180^o$ angle to unbound heme structures but due to the histidine residue placements in myoglobin, it can only bind at a $120^o$ angle.

D

The Fe–O bond is stronger compared to the Fe-C bond.