Describe the structure of proteins

Proteins are polymers of amino acids, formed by long chains of amino acids joined together by peptide bonds. These bonds are formed by condensation reactions which allow two single amino acid molecules to join together forming a dipeptide molecule and water. There are four levels of protein structure. The primary structure is the order of amino acids in the peptide. Secondary structure is the folding of the polypeptide chain due to hydrogen bonding into an alpha helix structure, resembling a coiled spring, or a beta pleated sheet. Tertiary structure is the 3-D folding due to ionic and disulphide bonds between sulphur molecules in distant parts of the chain. These bonds may also be hydrogen bonds which occur between distant parts of the chain. The majority of proteins, such as enzymes, only exhibit primary, secondary and tertiary structure. Some proteins, such as haemoglobin, have quarternary structure which is when the protein has two or more polypeptide chains joined together.