FRANCIS LEROY, BIOCOSMOS / SCIENCE PHOTO LIBRARY FRANCIS LEROY, BIOCOSMOS / SCIENCE PHOTO LIBRARY
Diagram showing the atomic structure of the haem group of the haemoglobin molecule, the oxygen- transporting protein in red blood cells that transfers oxygen from the lungs to the body's tissues. A single iron atom (Fe, colour-coded orange) is bound to 4 nitrogen atoms held in the centre of a tetrapyrrole ring that is incorporated into a large protein structure. A fifth bond exists between a group on the protein molecule. Here, iron is in its +2 oxidation state & has a coordination number (bonding capacity) of 6. The structure therefore allows for 1 oxygen molecule to be bound to iron without oxidising it.
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