How are proteins structured?

There are 4 main structural levels of proteins which allow them to be the most diverse molecules in the cell. First is the primary structure which is the sequence of amino acids found in the protein. Amino acids are composed of an amino group, a carboxyl group and the variable R group. The R group varies from amino acid to amino acid, and has different chemical properties depending on the amino acid. For instance, they may be negatively or positively charged, hydrophobic or form disulphide bonds. There are 20 different amino acids which can combine to make an enormous array of proteins with different functions. These amino acids are connected to one another by what is known as a peptide bond and it is the amino acids and the bonds that hold them together that characterise the primary structure. The secondary structure is made when the amino groups (-NH) and the carboxyl group (-CO) hydrogen bond to one another. This can cause the initial chain of amino acids to fold into an alpha helix or a beta pleated sheet. These are just two of the secondary structures that form as a way to optimise the number of hydrogen bonds in the protein so it is the most stable. Next we have the tertiary structure. Remember that proteins have variable R groups that may have different properties? Well these R groups can interact with one another too. Some interactions include ionic interactions which occur between positively charged and negatively charged R groups. Hydrophobic R groups tend to group together in the centre of the protein away from the water filled cytoplasm of the cell. Cysteine is an amino that has a sulphur group which can react with other cysteine R groups to form a disulphide bond. Unlike the other interactions mentioned a disulphide bond is a covalent bond and so is very strong. The other interactions are only temporary non-covalent interactions. Although they can be quite weak individually, there may be thousands of these interactions occurring in one protein allowing it to keep its structure. Finally, some proteins are made of more than one polypeptide chain. For example haemoglobin needs four chains to carry oxygen. It is also important for these chains to be kept together and they are done so by the same interactions and bonds that we see in the tertiary structure. The only difference is that the tertiary structure is about interactions within the same protein strand, whereas the quaternary structure is formed by interactions between different strands.