Collagen Vs. Haemoglobin

Here are the similarities and differences between the structure and function of haemoglobin (as an example of a globular protein) and collagen (as an example of a fibrous protein):

  • Haemoglobin’s quaternary structure is four polypeptide subunits (2 alpha and 2 beta chains) whereas Collagen’s structure is 3 polypeptide chains wound around each other like rope.
  • Haemoglobin has a prosthetic group – each chain contains a haem group (Fe2+) but collagen has no prosthetic group.
  • Haemoglobin is made of a wide range of amino acid constituents in its primary structure whereas approx 35% of collagen’s primary structure is just one type of amino acid – glycine.
  • Much of a Haemoglobin molecule is wound into alpha helix structures but collagen’s molecule mainly consists of left-handed helix structures.
  • Haemoglobin’s function is as a transport molecule and carries oxygen around the body. In contrast collagen’s function is as a structural molecule and to give strength to many cells including artery walls, tendons, bones, and cartilage. This is why it is important that Haemoglobin is soluble (so it can travel around the body in the blood) and Collagen is insoluble (wouldn’t be able to provide support otherwise as would always dissolve).  Haemoglobin’s shape is important so that red blood cells become biconcave disks which can easily travel around the body and bind with oxygen.
  • Haemoglobin is a globular protein meaning its 3D feature is to roll up into balls, yet collagen is a fibrous protein so its 3D feature is to form fibres.
  • Haemoglobin molecules to not bond with each other. However, collagen molecules form covalent bonds between molecules called cross-links which are staggered along the collagen molecules, both increasing strength and forming a fibril. Many fibrils joined together make a collagen fibre.

Haemoglobin:

Collagen:

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The Transport of Carbon Dioxide

  • Respired carbon dioxide has to be removed:
    • 5% Dissolves as a gas in the plasma
    • 10% Combines with Haemoglobin (Carbaminohaemoglobin)
    • 85% Dissolved in the form of hydrogen carbonate ions HCO3
  • Carbonic acid is produced using the enzyme carbonic anhydrase:
    • CO2 + H2O  →  H2CO3
  • Carbonic acid in solution produces hydrogen ions and hydrogen carbonate ions:
    • H2CO3 → HCO3 + H+
  • Chloride shift:
    • The negatively charged hydrogen carbonate ions diffuse out of the red blood cells. To balance the charge, Cl ions move in.
  • Hydrogen ions cause the red blood cell to become acidic. To control this, haemoglobin takes up the hydrogen ions to form haemoglobinic acid. The haemoglobin acts as a buffer.
  • The Bohr Effect:
    • The hydrogen ions compete for the space on the haemoglobin originally taken up by oxygen.
    • The hydrogen ions displace the oxygen making the oxyhaemoglobin dissociate faster.
    • More CO2 → more H+ ions → more freely O2 dissociates from oxyhaemoglobin.