Structurally, hemoglobin consists of four protein molecules (globins) that are connected together. Each one of these molecules contains 2 alpha-globulin chains and 2 beta-globulin chains. Each globulin chain holds the heme molecule. Wrapped by the heme molecule is an iron atom, which is the most important part of hemoglobin, for it is responsible to transport the oxygen and carbon dioxide in our blood. The iron contained in hemoglobin is also responsible for the red color of blood.
Hemoglobin is also found in nonerythroid cells including the A9 dopaminergic neurons in the substantia nigra, macrophages, alveolar cells, and mesangial cells in the kidney. In these tissues, it has a non-oxygen carrying function as an antioxidant and a regulator of iron metabolism.