Haemoglobin (Cambridge (CIE) AS Biology): Revision Note

The molecular structure of haemoglobin

Structure

• Haemoglobin is a globular protein which is an oxygen-carrying pigment found in vast quantities in red blood cells

• It has a quaternary structure as there are four polypeptide chains

  • These chains or subunits are globin proteins (two α–globins and two β–globins) and each subunit has a prosthetic haem group

• The four globin subunits are held together by disulphide bonds and arranged so that:

  • The hydrophobic R groups are facing inwards (helping preserve the three-dimensional spherical shape)

  • The hydrophilic R groups are facing outwards (helping maintain its solubility)

• The arrangements of the R groups is important to the functioning of haemoglobin

• If changes occur to the sequence of amino acids in the subunits this can result in the properties of haemoglobin changing

  • This is what happens to cause sickle cell anaemia

• The prosthetic haem group contains an iron II ion (Fe²⁺) which is able to reversibly combine with an oxygen molecule forming oxyhaemoglobin

  • This results in the haemoglobin appearing bright red

• Each haemoglobin with the four haem groups can therefore carry four oxygen molecules (eight oxygen atoms)

Function

• Haemoglobin is responsible for binding oxygen in the lung and transporting the oxygen to tissue to be used in aerobic respiration

• As oxygen is not very soluble in water and haemoglobin is, oxygen can be carried more efficiently around the body when bound to the haemoglobin

• The presence of the haem group (and Fe²⁺) enables small molecules like oxygen to be bound more easily:

  • As each oxygen molecule binds, it alters the quaternary structure (due to alterations in the tertiary structure) of the protein

  • This causes haemoglobin to have a higher affinity for the subsequent oxygen molecules and they bind more easily

• The existence of the iron II ion (Fe²⁺) in the prosthetic haem group also allows oxygen to reversibly bind

  • This is because none of the amino acids that make up the polypeptide chains in haemoglobin are well suited to binding with oxygen