7.1 Haemoglobin

• Haemoglobins are proteins with a quaternary structure that has evolved to make it efficient at loading oxygen under one set of conditions, but unloading under a different set of conditions.
• Haemoglobin has a primary structure, which is the sequence of amino acids in the four polypeptide chains.
• Haemoglobin has a secondary structure, in which each of its four polypeptide chains is coiled into a helix.
• Haemoglobin has a tertiary structure, in which each coiled polypeptide chain is folded further into a precise shape.
• Haemoglobin has a specific quaternary structure, in which all four polypeptides are linked together to form a spherical molecule. Each polypeptide has a haem group, which contains a ferrous (Fe 2+) ion.
• Each Fe 2+ ion in haemoglobin can combine with a single O2 molecule, making a total of four O2 molecules that can be carried by one haemoglobin.
• Loading/associating is where haemoglobin binds to oxygen.
• Unloading/disassociating is where haemoglobin releases oxygen.
• Haemoglobins with a high oxygen affinity bind easily, but release less easily. Haemoglobins with a low oxygen affinity bind less easily, but release more easily.
• To be efficient at oxygen transport, haemoglobin must readily associate with oxygen at the exchange surface. It must also readily disassociate at those tissues requiring oxygen.
• The affinity of haemoglobin for oxygen changes depending on the conditions.
• Every species produces a haemoglobin with a slightly different amino acid sequence, therefore each species has a haemoglobin with a slightly different quaternary and tertiary structure, and therefore oxygen binding properties.