Hemoglobin is a tetramer

The HbA tetramer consists of 2 α and 2 β globin chains Each globin chain contains one iron atom in the “haem-pocket” which allows very efficient oxygen (O2) binding. A 15° shifting in the axle of the tetramer enables the transformation of the tetramer from “de-oxy-” to “oxy-Hb” in the lung. At lower oxygen tension, O2 is released to the tissue, while 2,3,DPG binds to the haem-pocket. The Hb-tetramer travels back to the lung as “de-oxy-Hb”, where it is oxygenated again.