Haemoglobin is a tetramer composed of four globin molecules; two alpha globins and two beta globins. These are also known as subunits. The alpha globin is composed of 141 amino acids, and the beta globin consists of 146. Both alpha and beta globin proteins share similar secondary and tertiary structures, each with eight helical segments. Each globin chain contains one haem molecule. Haem is a prosthetic group (a non-protein molecule linked to its structure by covalent or other weak bonds).
Each subunit contains regions with a coiled shape; many of the amino acids that make up the polypeptide chain interact to form this particular structure, known as an alpha helix. In an alpha helix, each amino acid each amino acid is "hydrogen-bonded" to the amino acid that is four residues ahead of it in the chain. In haemoglobin, the hydrogen-bonding interaction occurs between the H of an -NH group and the O of a -CO group of the polypeptide backbone chain; the amino-acid side chains extend outward from the backbone of the helix. Approximately 75% of the amino-acid composition of haemoglobin adopts an alpha-helical structure. Another common structural motif is the beta-pleated sheet, in which amino acids line up in straight parallel rows.
Figure 1: This is a molecular model of the alpha-helix structure in a subunit of haemoglobin. The blue strands are a ribbon representation to emphasize the helical structure. The green dotted lines show the hydrogen bonding between the -NH and -CO functional groups.
In the middle of each haem group there is an iron ion, which can associate with one oxygen molecule. Since haemoglobin has four globin groups, each molecule can carry four oxygen molecules.
Figure 2: This is a molecular model of haemoglobin with the subunits displayed in the ribbon representation. A ribbon representation traces the backbone atoms of a protein and is often used to represent its three-dimensional structure. The four haem...