What are the different protein structures?

There are 4 levels you use to describe protein structure. 

Primary Structure - This is the simplest structure of a protein. It is essentially the sequence of amino acids that form a protein.  

Secondary Structure - In longer protein chains, parts are organised into two structures known as alpha-helices, or beta-pleated sheets. These structures are held together by hydrogen bonds. Alpha-helices are cylindrical whereas beta-pleated sheets tend to be flat or planar. 

Tertiary Structure - This is the 3D structure the protein chain folds itself into. A protein chain is organised into it's tertiary structure through various interactions. These include - ionic interactions (between -COOH and -NH2 groups), hydrogen bonds, van der Waals forces/hydrophobic interactions or disulphide bridges (between two cysetine residues). 

Quarternary structure - this is the arrangement of one or more protein molecules (polypeptide chains) into a multi-protein complex. An example includes haemoglobin, which is made of 4 polypeptide chains.