Page "Hemoglobin" Paragraph 54

The iron atom in the heme group must initially be in the ferrous ( Fe < sup > 2 +</ sup >) oxidation state to support oxygen and other gases ' binding and transport ( it temporarily switches to ferric during the time oxygen is bound, as explained above ).

Initial oxidation to the ferric ( Fe < sup > 3 +</ sup >) state without oxygen converts hemoglobin into " hemiglobin " or methemoglobin ( pronounced " MET-hemoglobin "), which cannot bind oxygen.

Hemoglobin in normal red blood cells is protected by a reduction system to keep this from happening.

Nitric oxide is capable of converting a small fraction of hemoglobin to methemoglobin in red blood cells.

The latter reaction is a remnant activity of the more ancient nitric oxide dioxygenase function of globins.