METHEMOGLOBIN 

Clinical Significance Methemoglobin is a form of hemoglobin wherein its heme iron has been oxidized, changing its iron configuration from the ferrous (Fe2+) to the ferric (Fe3+) state. Unlike normal hemoglobin, methemoglobin does not bind oxygen and as a result cannot deliver oxygen to the tissues. Methemoglobin is an altered state of hemoglobin in which the heme iron is oxidized from the ferrous (Fe2+) to the ferric (Fe3+) state. Oxidation differs from oxygenation in that it changes the redox state of heme iron (causes removal of an electron), whereas oxygenation involves reversible binding of O2 without a redox change.

 

Oxyhemoglobin dissociation curve (See link below)

Figure 1

The ferric hemes of methemoglobin do not bind O2. The ferric heme in the hemoglobin tetramer also causes the remaining normal ferrous hemes within the same tetrameric hemoglobin molecule to have increased O2 affinity [1]. This produces a left shift of the hemoglobin oxygen dissociation curve and in turn further decreases O2 delivery to the tissues (figure 1). As a result of these two changes (inability to bind oxygen and left-shifting of the oxygen-hemoglobin curve), methemoglobin causes decreased oxygen delivery from the same amount of hemoglobin, producing "functional anemia."