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.
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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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