A globin is a group of four globulin protein molecules that become bound by the iron in heme molecules to form hemoglobin or myoglobin. The Globins are a related family of proteins, all of which have similar amino acid sequence and folding. Globin is a constituent of the hemoglobin molecule and is composed of two alpha and two beta chains. The two alpha chains differ slightly from the beta chains in the amino acid sequence.
A globin molecule is a coiled polypeptide chain. It becomes associated with an iron-containing group which attracts oxygen; this group is called heme group. Four globin molecules and four heme groups make up hemoglobin.