Four levels of protein organization
From amino acid sequence to 3D shape
Protein structure is described on four levels. The primary structure is the sequence of amino acids. Hydrogen bonds between the CO and NH groups of the backbone fold short stretches into secondary structures, mainly the alpha-helix and the beta-sheet. The tertiary structure is the complete three-dimensional fold of one chain, stabilized by interactions between side chains, including disulfide bonds. Proteins made of several chains, such as hemoglobin with its four subunits, also have a quaternary structure.