Outline the folding of a protein to form its 3D structure.

Folding of proteins occurs in 4 distinct stages: primary, secondary, tertiary and quaternary. Primary structure is the formation of the amino acid (peptide) sequence, here the amino acids are joined together by peptide bonds. Secondary structure is the formation of the backbone of the protein, the peptide sequence can either fold into an alpha helix structure or a beta pleated sheet structure. They’re held in these structures by hydrogen bonds. Tertiary structure is the final folding of the single peptide sequence. There are 4 different bonds which contribute to tertiary structure: hydrogen bonding, disulphide bridges, ionic bonds, and hydrophilic and hydrophobic interactions. The interactions that occur in a single peptide sequence depend on the properties of the amino acids that are a part of it, e.g. if cysteine is common in the peptide sequence then there will be a lot of disulphide bridges. Finally, quaternary structure is the folding of more than one peptide sequences together, bonds involved in this folding are the same as the ones involved in tertiary structure, except they exist between many peptides. During quaternary structure non proteins subunits can also be combined with the peptides to form the protein, e.g. the haem group involved in haemoglobin.