Describe the first three levels of protein structure and the interactions which stabilise them.

Primary structure - sequence of amino acids in a polypeptide chain, amino acids joined by peptide bonds. Secondary structure - folding of the polypeptide chain into regular structures such as alpha-helices and beta-pleated sheets, stabilised by hydrogen bonds between the polypeptide chain backbone. Tertiary structure - further folding into a specific 3D shape which determines protein function, stabilised by hydrogen, ionic and disulphide bonds between the R groups of the amino acid residues.