Foetal haemoglobin, molecular model. This protein is composed of two alpha subunits (red) and two gamma subunits (pink). Haemoglobin is the main oxygen transport protein in the blood. Foetal haemoglobin has a higher affinity for binding oxygen than haemoglobin found in adult blood. One of the changes to the molecule that allows foetal haemoglobin to bind oxygen more strongly is the replacement of the amino acid histidine with serine (red spheres at centre).