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GCE CIE BIOL - Haemoglobin (Globular Proteins) 

Chapter 2 Biological Molecules

· biology,GCE CIE,Haemoglobin,globular protein,sickle cell anaemia

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.
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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.
Haemoglobin structure when loaded with oxygen (oxyhaemoglobin), and when not loaded with oxygen.

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.
Comparison between a normal blood cells, and a sickle cell anaemia cells.

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

Finally done! See you soon! 🤙

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Written by Aayushi | GCE CIE Biology Specialist @ TUTTEE

References:

  • Cornell, B. Fibrous vs Globular Proteins. Retrieved from https://ib.bioninja.com.au/standard-level/topic-2-molecular-biology/24-proteins/fibrous-vs-globular-protein.html 
  • Gillam, P. Haemoglobin. Retrieved from https://pmgbiology.com/tag/haemoglobin/
  • Jones, M., & Parkin, M. (2018). Cambridge International AS and A Level Biology. Cambridge: Cambridge University Press.
  • What is sickle cell anaemia? (2017, May 17). Retrieved from https://www.yourgenome.org/facts/what-is-sickle-cell-anaemia
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