7.1 Haemogloin

• Haemoglobins are protein molecules with a quaternary structure that has evolved to make it efficient at loading oxygen under one set of conditions but unloading it under a different set of conditions
• • A hemoglobin molecule is made up of 4 polypeptide chains consisting of 2 pairs of alpha and beta polypeptide chains
  • Each chain is attached to a haem group that can combine with oxygen
• The primary structure of a haemoglobin molecule is the sequence of amino acids in the 4 polypeptide chains
• The secondary structure of a haemoglobin molecule is in which each of these polypeptide chains is coiled into a helix
• The tertiary structure of a haemoglobin molecule is in which each polypeptide chain is folded into a precise shape which is an important factor in its ability to carry oxygen
• The quaternary structure of a haemoglobin molecule is in which all 4 polypeptides are linked together to form an almost spherical molecule

  Each polypeptide is associated with a haem group - which contains a ferrous ion (Fe2+)
• In the quaternary structure, each ferrous ion can combine with a single oxygen molecule, making a total of 4 oxygen molecules that can be carried by a single haemoglobin molecule
• Loading is the process by which haemoglobin binds with oxygen, which takes place in the lungs in humans
• Unloading is the process by which haemoglobin releases its oxygen, which takes place in the tissues in humans
• Haemoglobins with a high affinity for oxygen take up oxygen more easily, but release it less easily
• Haemoglobins with a low affinity for oxygen take up oxygen less easily, but release it more easily
• Structure of a haemoglobin molecule:

  1 = haem group

  2 = Iron atom

  3 = Polypeptide chain

  
• The role of haemoglobin is to transport oxygen

  To be efficient at this, haemoglobin must:

  • readily associate with oxygen at the gas exchange surface
  • Readily dissociate from oxygen at those tissues requiring it for respiration
• Haemoglobin binds with oxygen in the lungs and releases it in the tissues because a change in the environment of a protein changes its tertiary structure and therefore affects the way it functions
• • Haemoglobin changes its affinity (chemical attraction) for oxygen under different conditions
  • It achieves this because its shape changes in the presence of certain substances, such as carbon dioxide
• • At gas exchange surfaces,
  • Oxygen concentration is high
  • Carbon dioxide concentration is low
  • The affinity of haemoglobin for oxygen is high
  • So oxygen is associated
• • At respiring tissues,
  • Oxygen concentration is low
  • Carbon dioxide concentration is high
  • The affinity of haemoglobin for oxygen is low
  • So oxygen is dissociated
• Conditions in which the affinity of haemoglobin for oxygen is different: Oxygen concentration and carbon dioxide concentration
• In the presence of carbon dioxide, the new shape of the haemoglobin molecule binds more loosely to oxygen

  As a result, haemoglobin releases its oxygen
• • Different haemoglobins in species have different affinities for oxygen due to the different shape of the molecule
  • Each species produces a haemoglobin with a slightly different amino acid sequence so a different tertiary and quaternary structure and so different oxygen binding properties
• Depending on its structure, haemoglobin molecules range from those that have a high affinity for oxygen to those that have a low affinity for oxygen
• • When the body is at rest, only 1 of 4 oxygen molecules carried by haemoglobin is normally released into the tissues
  • This could be an advantage because it helps to reserve oxygen molecules in order to supply tissues when the organism becomes more active
• • Carbon monoxide binding permanently to haemoglobin instead of oxygen means carbon monoxide will gradually occupy all the sites on haemoglobin instead
  • This means no oxygen will be carried to tissues such as the brain therefore stop them from respiring and functioning, making the person lose consciousness