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GCE CIE BIOL - Haemoglobin (Globular Proteins) 

Chapter 2 Biological Molecules

May 5, 2020

2.3.c Describe the molecular structure of haemoglobin as an example of a globular protein, and of collagen as an example of a fibrous protein and relate these structures to their functions (The importance of iron in the haemoglobin molecule should be emphasised. A haemoglobin molecule is composed of two alpha (α) chains and two beta (β) chains, although when describing the chains the terms α-globin and β-globin maybe used. There should be a distinction between collagen molecules and collagen fibres). 

GLOBULAR PROTEINS

  • Globular proteins curl up into a ball shape so that their hydrophobic regions are inside the sphere and the hydrophilic regions point outwards. 
  • The hydrophobic R groups inside are important for the three-dimensional shape of the molecule. 
  • The hydrophilic R groups on the surface of the molecule are important in maintaining its solubility. 
  • Globular proteins are soluble in water
  • Examples include: myoglobin, haemoglobin and enzymes.

The above image shows the structure of a globular protein. 

HAEMOGLOBIN

  • Haemoglobin is the oxygen-carrying pigment found in red blood cells.
  • Has a quaternary structure with four polypeptide chains, two α-globin and two β-globin chains.
  • Each of the four polypeptide chains contains a haem group (i.e., four haem groups)
  • Haem contains iron atoms. 
  • One oxygen molecule (O2) can bind to one iron atom. 
  • Each haemoglobin has four haem groups, therefore, can carry four oxygen molecules at one time. 
  • Haem determines colour of haemoglobin - if iron combines with oxygen, it is called oxyhaemoglobin, and is bright red. 
  • If iron does not combine with oxygen, colour is purplish.

The above image shows haemoglobin structure when loaded with oxygen (oxyhaemoglobin), and when not loaded with oxygen.

  • Sickle cell anaemia is a genetic disorder in which the amino acid sequence coding the haemoglobin protein changes from glutamic acid (polar) to valine (non-polar). As such, the red blood cells become insoluble. This can cause problems such as blockages in blood vessels.

The above image shows a comparison between normal blood cells, and sickle cell anaemia cells.

Finally done! See you soon! 🤙

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