Within living organisms proteins carry out a number of different functions. The structure of a protein is made up of amino acids, the general structure of which is:

The difference between amino acids can be found in the R-group.

Proteins can be composed of different combinations of 20 amino acids. When two amino acids join they release the -H and -OH groups in a process called condensation. A peptide-bond is then formed between nitrogen and an alpha-carbon.

There are four structures to a protein:

• The primary structure is the sequence of amino acids which are joined together by covalent peptide bonds. It’s this sequence which dictates the shape the protein will take.
• The secondary structure refers to the hydrogen bonds between the amino acids. There are two types of secondary structure: alpha helices and beta sheets. The frequency of either is dependent on the primary structure.
• The tertiary structure is how the polypeptides twist and fold to make the protein’s globular shape. The polypeptides are composed of amino acids which are joined together by peptide bonds.
• The quaternary structure is how the polypeptide chains (or tertiary structures) are joined. This structure is only present when the protein is composed of two or more polypeptide chains. For example, collagen is made up of three polypeptide chains while haemoglobin is comprised of four.

The final protein can be either globular or fibrous. A globular protein is made from folded chains and is soluble. A fibrous protein is composed of long, twisted chains and is insoluble.