haemoglobin

Protein used by all vertebrates and some invertebrates for oxygen transport because the two substances combine reversibly. In vertebrates it occurs in red blood cells (erythrocytes), giving them their colour.

In the lungs or gills where the concentration of oxygen is high, oxygen attaches to haemoglobin to form oxyhaemoglobin. This process effectively increases the amount of oxygen that can be carried in the bloodstream. The oxygen is later released in the body tissues where it is at a low concentration, and the deoxygenated blood returned to the lungs or gills. Haemoglobin will combine also with carbon monoxide to form carboxyhaemoglobin, which has the effect of reducing the amount of oxygen that can be carried in the blood.

According to US research in 1996, haemoglobin also plays a role in the regulation of blood pressure. The haemoglobin molecule contains a small quantity of the amino acid cysteine that binds with nitric oxide ions (NO+) in the blood. Nitric oxide acts to relax the muscles around the blood vessels, aiding their dilation and thus lowering blood pressure.

Haemoglobin is made up of two pairs of protein chains. Outside of a blood cell these pairs of chains would separate and be toxic. In 1993, genetically engineered haemoglobin was made by inserting the synthetic versions of the genes that code for these proteins into the bacterium Escherichia coli and manipulating them so that the pairs of proteins are bonded together and so not toxic.