Question

What does the hemoglobin in a healthy adult, not possessing the sickle cell trait, consist of?

Answer 1

Healthy-adult hemoglobin is an `\mathbf(alpha_2beta_2)` heterotetramer, whose central cavity contains a heme molecule.

The `alpha` chains are depicted as helices above, and the `beta` chains are depicted as thick arrows above (perhaps not visible on this X-ray crystallographic depiction).

Heme is a porphyrin ring that has a central binding site that contains a metal-ion cofactor; specifically, the `"Fe"("II")` cation, rather than zinc, like in zinc protoporphyrin (ZPP), which is found in sickle-cell anemic adults (whose hemoglobin experienced an `"E6V"` mutation on its `beta` chain).

Iron has 6 binding sites. Four of these are occupied by heme's four nitrogens. One of these is occupied by a nearby histidine sidechain. The sixth is unoccupied, but a histidine sidechain is also nearby.

Here is a depiction of heme binding `"O"_2`:

where `"His"` is a histidine sidechain:

On the iron, heme can bind such ligands as oxygen and carbon monoxide in between the histidine sidechain's protonated nitrogen (the one on the bottom right of the above depicted sidechain) and the iron.