3.4.8 Amino Acids - Proteins

Specification

Students should:
  • understand that proteins are sequences of amino acids joined by peptide links
  • understand that hydrolysis of the peptide link produces the constituent amino acids
  • know that mixtures of amino acids can be separated by chromatography

Proteins

Proteins are naturally occurring large molecules formed by linking amino acids together.

As each amino acid contains at least one acid group and one basic amine group, they can condense together at either end. There are 21 different naturally occurring amino acids and this makes a staggering number of possible protein molecular structures.

Polypeptide (protein) formation


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The peptide linkage

As stated above amino acids can join together by a reaction known as condensation (addition-elimination) when the carboxylic acid group of one amino acid reacts with the amine group of another.

Condensation


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Hydrolysis of polypeptides

The peptide linkage is easily broken by warming with dilute acid or base. This is known as hydrolysis; the result of which is that the original amine and carboxylic acid groups are regenerated:

Hydrolysis


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Analysis of proteins

The protein is first hydrolysed into its constituent amino acids. These can then be separated by a technique known as electrophoresis. This depends on the fact that at any given pH the overall charge on the amino acid is a function of the amount of negative charge from dissociation of the carboxylic acid group, and the amount of positive charge from protonation of the amine group.

The mixture of amino acids is placed at the centre of a buffered gel and an electric field is applied acrosss the gel

Each individual amino acid is either pulled towards the positive or negative poles of the applied potential depending on its overall charge at that buffered pH.

Electrophoresis

Mixtures of aminoacids can also be separated by chromatography, either thin layer or column.


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