Explain the structure of a Protein and how this can relate to its function.

A protein is a polymer made up of 20 possible amino acid monomers. The amino acid monomers are made up of three components:

 

1)An acid group

2)A variable group

3)An amine group.

 

The amine group of one amino acid will form a peptide bond with the adjacent amino acid’s acid group through a condensation reaction. The condensation reaction removes one hydrogen from the amine group and one hydroxide from the acid group.  A series of condensation polymerisation reactions occur resulting in a chain of amino acids forming, known as a polypeptide. This sequence of amino acids is known as the primary structure and dictates the future role of the protein. A change in the primary structure of amino acids therefore affects the protein’s function.

 

The secondary structure of a protein is created through the formation of hydrogen bonds between the positive and negative regions of the amino acid chain. The amine groups of the amino acids are relatively positive due to the presence of nitrogen and the acid group is relatively negative due to the presence of oxygen. As a consequence of these bonds forming, the polypeptide chain twists to form an alpha helix.

 

The tertiary structure of a protein provides the protein with a unique and specific shape through the creation of disulphide bonds. These bonds are formed due to some amino acids containing sulphur. The creation of a specific and unique shape is important in functional proteins such as enzymes. Enzymes act as biological catalysts in reactions by increasing the rate of reaction whilst not getting involved in the reaction itself. In order to complete this function, enzymes contain a specific active site which shape is dictated by the tertiary structure.  The active site is where the enzyme reacts with its specific substrate. If the disulphides bonds break in the tertiary structure, due to excess heat for example, the enzyme’s active site may denature and change shape disabling the enzyme from catalysing the reaction.

 

Large proteins are often formed of complex molecules in a quaternary structure where two or more polypeptide chains may join together. As a result of the diverse nature of roles proteins carry out, how the polypeptide chains are joined relates specifically to its function. For example, fibrous proteins, which carry out a structural role, have a different quaternary structure to globular proteins such as enzymes that have a metabolic function. An example of a fibrous protein is collagen, which is formed by the joining of three fibrous polypeptide chains through cross linkages between the chains’ amino acids. At the point where one collagen molecule ends and the other begins is spread throughout the fibre, the strength and stability of the complete molecule is increased. This is important as collagen form tendons which join muscle to bone. Alternatively a prosthetic group (non protein) may become attached to the protein. An example of this is a Haem group (containing a ferrous ion) in haemoglobin. This group allows for the loading and unloading of oxygen into the haemoglobin molecule. This feature enables haemoglobin, formed of four polypeptide chains linked in a spherical molecule each containing a haem group, to carry out its function of carrying oxygen around the body.