Haemoglobin is one of the two oxygen binding proteins found in vertebrates. It's function is to carry oxygen in the blood from the lungs to other tissues in the body, in order to supply the cells with the oxygen required by them for oxidative phosphorylation of foodstuffs. Haemoglobin is found in the blood within the erythrocytes (red blood cells). These cells essentially act as a sack for carrying haemoglobin, since mature erythrocytes lack any internal organelles (nucleus, mitochondria, etc). The other oxygen binding protein found in vertebrates is myoglobin, which stores the oxygen in the tissues of the body ready for when the tissues require it. The highest concentration of myoglobin are found in skeletal and cardiac muscle which requires large amounts of oxygen because of there need for large amounts of energy during contraction.
Mygolobin is the simpler of the two oxygen binding proteins. It is made up of 153 amino acids in a single polypeptide chain, and it was the first protein to have its three dimensional structure determined by x-ray crystallography. Within a hydrophobic crevice of the globular protein, formed by the folding of the poly peptide chain is the heme prosthetic group (shown in the picture above, the polypeptide part shown in pink, and the heme group shown in green). The heme group has a central iron atom which is essential in the binding of oxygen.
Haemoglobin is essentially composed of four polypeptide peptide units that are very similar to myoglobin.
On this web site the structure, and chemistry of the heme group (responsible for the binding of oxygen) and of the actual polypeptide will be discussed, along with a brief look at other oxygen binding proteins found in nature.
This diagram shows one molecule of myoglobin, haemoglobin is made up of four of these units bound together in a tetrahedron, via intermolecular forces to give the quaternary structure (which will be discussed later).