The structure of proteins can be described at four different levels.

• Primary Structure - the sequence of amino acids in the peptide or protein chain, listed from N-terminus to C-terminus
• Secondary Structure - the folding of short regions of the primary chain into ordered three-dimensional structures, including alpha-helices and beta-pleated sheets
• Tertiary Structure - the further folding of a single chain of a protein held together by interactions between the sidechains
• Quaternary Structure - the arrangement of all chains (subunits) in a protein comprised of more than one protein chain