3.4.8 Amino Acids - Proteins secondary structure
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Four levels of protein structure
The structures formed by polypeptides is complex and can be considered to have four levels, named primary, secondary, tertiary and quaternary structures.
The primary structure is the order of amino acids that make up the protein. This is often shown as an abbreviated list, using three letter, or even one letter abbreviations for each constituent amino acid. For example glycine may be represented as 'gly' or simply 'U'.
This chain of amino acids then twists itself into a secondary structure that is held in position by mainly hydrogen bonding, with possibly ionic interactions, disulfide bridges and van der Waals forces.
There are three fundamental secondary structures adopted by a protein:
- 1 alpha-helix
- 2 beta-pleated sheets
- 3 strings and turns
In alpha helix secondary structure the polypeptide chain spirals into a right handed helix (viewed from above), which is held in place by hydrogen bonds. There are approximately 3.6 amino acids per turn of the helix.
In beta pleated sheet secondary structure the amino acid sequence runs parallel
to other amino acid sequences giving the appearance of a sheet which has pleats.
These can be subdivided into structures in which the chains of amino acids run
in the same direction (parallel) or in an opposite sense (antiparallel). Once
again the chains are held together by hydrogen bonds
Tertiary and quaternary protein structure
The helices and sheets formed by the secondary level of structure then can fold and interact with one another to wrap the polypeptide strand into its final shape. The quaternary structure of a protein is when two or more tertiary structures interact and group together to make the final molecule.
All four levels of protein structure are exemplified in the haemoglobin molecule