The shape of a protein is determined by the four levels of protein structure: primary, secondary, tertiary, and quaternary. Other chaperon proteins are present in the cell and ensure that a protein folds into its proper shape.
Proteins are composed of polypeptides in which a sequence of amino acids connected through peptide bonds. This sequence is referred to as a proteins primary structure. The primary structure is most often reported through one letter or three letter abbreviations for the amino acids, listed beginning from the N-terminal side. Secondary structure is a regularly folded segment of a protein formed through hydrogen bonds between the backbone atoms of the amino acids. The two main types of secondary structure are alpha helices and beta sheets. An alpha helix contains 3.6 amino acids per helical turn. The amino acid proline does not fit within a helix due to its rigid structure. Beta sheets contain chains of peptide bonds that line up along side of one another. They can be either parallel, in which both strands align directionally, or anitparallel, in which each sheet is oppositely directed.
Tertiary structure is the three-dimensional structure of the entire peptide chain. In protein diagrams, alpha helices are generally shown as coils and beta sheets are shown as a flat ribbon. Tertiary structure is maintained by the interactions of an amino acids side chains as determined by their characteristics. Quaternary structure is present in proteins that contain more than one polypeptide chain, referred to as a domain. It is the spatial arrangement of the domains.