What portion of hemoglobin molecule binds oxygen?

Answer 1

Hemoglobin contains a heme group with an Iron ion attached to it. The iron is what binds to O2.

Answer 2

There is an Fe2+ (iron) within the heme portion of hemoglobin. It binds exactly one molecule of oxygen.

This means that each hemoglobin can bind 4 oxygen molecules.

Answer 3

The iron in the hemoglobin is the site of the oxygen binding. Each iron can bind one O2 molecule, therefore the hemoglobin molecule is capable of binding a total of four O2 molecules.