Flexi Says: The amino acid sequence is the primary structure of a protein. As explained in Figure below, a protein may have up to four levels of structure, from primary to quaternary. The complex structure of a protein allows it to carry out its biological functions. Secondary structure refers to local sub-structures generated from the primary structure, usually involving alpha helix and beta pleated sheet structures. These secondary structures form through hydrogen bonding between amino acids. Tertiary structure refers to the three-dimensional structure of a single polypeptide. The alpha-helices and beta-sheets are folded into a compact globule structure. Stability is maintained through hydrogen bonds, disulfide bonds and other interactions. Quaternary structure is a larger assembly of several polypeptide chains, now referred to as subunits of the protein. The quaternary structure is stabilized by the same interactions as the tertiary structure. Complexes of two or more polypeptides are called multimers. Specifically, a dimer contains two subunits, a trimer contains three subunits, and a tetramer contains four subunits.
