Hemoglobin consists of a tetrameric protein with a2 and b2 subunits. Each subunit has a heme group tightly surrounded by structural segments of the protein.

The consists of an iron atom coordinated by four porphyrin N atoms. The iron is bound on one side to an N from a Histidine. During the , one oxygen molecule binds to the heme, pulling the heme into a plane. When the oxygen molecule is lost in the , the heme forms slightly concave at the center. This change in the shape of the heme group is responsible for the change of the entire hemoglobin during the and states.

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