Hb comes in different forms (4):
1. Myoglobin: a single monomer of Hb; it binds a single oxygen; is present in slow (red) skeletal muscle; it serves to store O2 for times when PO2 is very low.
2. Fetal hemoglobin (hemoglobin F, HbF): this Hb has a higher affinity for O2
3. Hemoglobin S: is an abnormal variant involving a single amino acid substitution.
This substitution causes oxygenated HbS to crystallize into long fibers that change the shape of the red cell (cause it to sickle).
Non-crystallized HbS has the same oxygen affinity as normal Hb, but when it becomes fibrous, its affinity decreases.
As a consequence the oxygen carrying capacity of individuals with HbS is less than normal.
4. Methehemoglobin: the iron is in the ferric (Fe
+3) rather than the ferrous (Fe+2) form
which prevents O2 binding