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Amino Acid vs. Protein

For the actual grabbing of the oxygen molecule itself, a special partner molecule called 'heme' is used. Heme contains an iron atom (shown in pink) in contact with the oxygen.

spacefill Heme

wireframe Heme

Side note: without the influence of hemoglobin amino acids, the heme iron would be oxidized by the oxygen (it'd rust!), and that would be the end of that. The partnership between heme and myoglobin allows reversible, non-modifying contact between the oxygen and the iron.