The primary structure of a protein also sometimes referred to as a polypeptide, is decided by sequences of amino acids. The amino acids are sequenced in a linear polypeptide chain which folds as it is being produced. it is this three - dimensional folded structure that decides the proteins biological activity.

The amino acids in the chain are linked together by a condensation reaction. This reaction occurs between an amino and carboxylic acid functional group. There is always a free amino group at the left hand end of the chain and a free carboxylic acid group at the right, these are known respectively as the N - terminus and the C - terminus. These two free functional groups react together and remove one molecule of water. The resulting link is called a peptide bond.

There are 20 different amino acids and each contain an R group. The higher - order structure of a protein is decided by the interactions that form between the R groups when the chain is folded. There are three weak interactions that occur; hydrophobic interactions, hydrogen bonds and ionic interactions.

  • A hydrophobic interaction will occur between two water hating groups.

Hydrogen bonds and ionic interactions occur because the majority of hydrophillic groups are on the surface of the molecule and so are surrounded by water. When the chain folds these hydrophillic groups will be on the interior of the molecule and will not be surround by water and so cannot react. Therefore they form either a hydrogen bond or a ionic interaction depending on the circumstances.

  • A hydrogen bond is formed when an electronegative atom/s bond weakly with a hydrogen atom.

  • An ionic intraction occurs due to the attraction of oppositely charged groups.

It is due to these bonds and interactions that the higher - order structure of the polypeptide is determined and therefore the biological activity.

If the wrong amino acid is added into a growing polypeptide chain, and goes undetected by the body it will affect the higher - order structure of the final chain. Take haemoglobin as an example, if the protein which codes for haemglobin has a DNA base mutation within the chain the higher - order structure will be different from a chain without a DNA base mutation. This difference occurs because of a change in the folding of the molecule. As previously said the folding occurs between the different R groups of the amino acids and the interactions they form. If there is a mutation in the chain a different amino acid is added than should have been, a different R group will be present and so a different interaction will form changing where the fold is, this will alter the final higher - order structure of the protein.

If the higher - order structure of a protein is changed it will affect how that protein functions. It might mean an enzyme cannot connect with an active site on a protein as either the enzyme or active site could be the wrong shape, meaning that some processes won't occur, will occur to much or be performed incorrectly. A mutation in the DNA could affect one of the 23 pairs of chromosomes as in the disease sickle - cell anaemia. Sickle - cell anaemia involves a DNA base change in the protein that codes for haemoglobin which is found on chromosome 11. This means that a sufferer can pass the disease on to offspring. A sufferer of sickle - cell anaemia will have problems with their haemoglobin.

Haemoglobin is found in red blood cells which are normally bi - con - cave in shape. In sickle - cell the haemoglobin turns into a gel like substance altering the shape of some red blood cells. These cells can then get stuck in the capillaries, blocking some of the of the oxygen getting around the body, causing severe pain which can lead to permanent damage if the blockage is not fixed quickly.

The primary structure of the protein consists of amino acids in a specific order. The amino acids contain R groups that interact with each other creating a three - dimensional structure. This higher - order structure decides the biological activity of that protein. if the wrong amino acid joins the polypeptide chain the higher - order structure will change, effecting the functioning of the protein.