What is the structure of proteins?

Most proteins have 3 or 4 levels of structure which include the primary, secondary and tertiary structure, with many also having a Quaternary structure. Firstly, the primary structure is the unique sequence of amino acids joined together by peptide bonds which creates a polypeptide chain. Joining two amino acids together is a condensation reaction and the bond formed is between the carbon atom on one amino acid to the nitrogen on the other. Water is formed in the process. After synthesis, the polypeptide chain is folded into different shapes in the secondary structure which are held together by hydrogen bonds between the double bonded electronegative oxygen and the (delta) positive hydrogen on the nitrogen. The two main types of secondary structure include the alpha helix where the polypeptide curls round and is held together by the hydrogen bonds running parallel to the helix. The second structure is the beta pleated sheet where the polypeptide chain ‘zig zags’ back and forward which forms a sheet of anti-parallel strands which is again held together by hydrogen bonds. The tertiary structure is the final 3D shape of the protein which is formed by further folding of the polypeptide chain. This structure is specific to the unique sequences of amino acids in the primary structure. 3 different types of bonds hold this structure together including: Hydrogen bonds, ionic bonds and disulphide bridges. Finally, many proteins also form a Quaternary structure which is where several polypeptide chain interact/ are arranged together. A common example is haemoglobin, but other quaternary proteins include collagen and antibodies. These structures are stages of protein folding, once completed the 3D shape of a protein can be classified as fibrous or globular. Fibrous proteins are long and thin and composed of many polypeptide chains. They usually have structural roles such as keratin in your hair, or collagen in the bones. However, most proteins are globular meaning they are compacted, ‘ball shaped’ and more rounded. This group classification includes many types of protein including enzymes, membrane proteins, storage proteins and receptors.