Haemoglobin, molecular model. This protein is composed of two alpha subunits (pink) and two beta subunits (blue). Haemoglobin is the main oxygen transport protein in the blood. This haemoglobin is bound to 2,3-bisphosphoglyceric acid (BPG, yelllow). The binding of BPG which is found in the blood, decreases haemoglobin's affinity for oxygen. Decreasing the affinity makes it easier for haemoglobin to release oxygen and deliver it to tissues that need it.