Protein Structure

Amino acids are the monomeric form of proteins, meaning, amino acids are the foundation bricks of proteins.  In total there are only 20 different amino acids. Amino acids are also known as Zwitterions as they are dipolar in a neutral pH solution. The amino acid sequence is known as the primary structure of a protein. The primary structure of a protein is dependent on the genes that are transcribed and translated to produce a polypeptide. Typically one gene equates to the production of one polypeptide. An example of this would be insulin, which is one polypeptide long. Each amino acid is joined to the adjacent amino acid by a peptide bond. The peptide bonds are covalent bonds. A short sequence of amino acids can form an oligopeptide or peptide. 

A polypeptide can fold, bend and twist. This leads to the formation of the secondary structure of a    protein. It must be noted here, that this folding, bending and twisting is not random. Once folded, there will have been the formation of hydrogen, ionic and disulfide bonding. Along with this there may be some weak Van der Waals interactions maintaining a loosely constructed structure. There are two common types of structure- the  Alpha helix and the Beta pleated sheet. 

Secondary Structure- sourced http://www.drzolghadri.com/?attachment_id=139

The tertiary structure of a protein is formed by further bending and folding. Side chain interactions are decisive of the three-dimensional structure. The quarternary structure of a protein is produced through the interaction and binding of many sub-units (polypeptides). These work together to form one larger functional unit. An example of a quarternary structure (protein) would be haemoglobin. 

Structures- sourced: http://alevelnotes.com/Protein-Structure/61

Further reading: Lodish, H.[et al.] (2008), Molecular cell biology , 6^th edn. W.H Freeman, New York.