Proteins are made up of polymers of amino acids. The amino acids are joined together by peptide bonds in a condensation reaction. This series of peptide bonds is also known as the polypeptide backbone, off which are side chains made up of amino acids. This type of reaction is catalysed by the ribosome in the cytoplasm and releases a water molecule. There are four levels of protein structure. The primary structure is the linear sequence of amino acids joined by covalent peptide bonds. . The main secondary structures of proteins are the alpha helix and the beta pleated sheet. The secondary protein sturcture is stabilised by hydrogen bonds which are 1/10 the strength of covalent bonds. The protein then continues to fold forming a three-dimensional structure. This is known as the tertiary structure. Disulphide bridges are formed in the tertiary structure, these are formed between amino acids which contain a thiol group (SH). The H+ is lost very easily in an oxidation reaction with another SH group and the two join with a disulphide bond. Proteins that have to work outside of the cell use disulphide bonds to increase their stability. If two or more tertiary structures form a single structure then it is a quaternary structure. An example of a quaternary structure are haemoglobin molecules, that are made up of four globin molecules. These are also known as red blood cells and are found in blood .
Proteins can come in all different shapes and sizes  due to the fact that there is any possible sequence of amino acids and that a protein can be made of an alpha helix, a beta pleated sheet or both. The amino acids which tend to be conserved in proteins are those which make up the active site, as this is the part of the protein which has most functional significace.
- ↑ http://www.chemguide.co.uk/organicprops/aminoacids/proteinstruct.html
- ↑ Alberts et al., (2008) Molecular Biology of the Cell, 5th Edition, Garland Science, Chapter 3, Page 136
- ↑ Alberts et al.(2008) Molecular Biology of the Cell, 5th Edition, Garland Science Chapter 3 Page 144