Proteins are chains of amino acids that fold into three-dimensional shapes. The shape of the protein is very important for its function and the three-dimensional structure is specified by a sequence of amino acids. Protein structure has 4 levels of organization known as primary, secondary, tertiary and quaternary. Proteins are initially produced as a primary sequence composed of a linear sequence of amino acids joined by peptide bonds that continue to fold into secondary, tertiary, and finally quaternary structures. Twenty different amino acids are incorporated into proteins, the sequence of amino acids of a protein is called the primary structure (Loughlin, 2017). Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay A primary structure is the simplest level of protein structure: it is a sequence of amino acids in a polypeptide chain. Each chain has its own set of amino acids assembled in a particular order with a typical basic chemical structure, as shown below in Figure 1. A central carbon atom (carbon a) bonded to a hydrogen atom, an amino group of base comprising one nitrogen atom and two hydrogen atoms (-NH2), a carboxyl group (-COOH) and a specific side chain or R group made up of different atoms. The R group gives each amino acid its identity; it can be polar, non-polar or even unchanged (Loughlin, 2017). The amino acids of a polypeptide are linked together via covalent bonds known as a peptide bond, each bond forming a condensation reaction. During protein synthesis the carboxyl group of the amino acid at the end of the growing polypeptide chain reacts with the amino group of an incoming amino acid, releasing a water molecule. The resulting bond between amino acids is a peptide bond. Due to the structure of amino acids, a polypeptide chain has two ends that are chemically distinct from each other. At one end of the polypeptide chain it has a free amino group called the amino terminus (N-terminus) and the other end which has a free carboxyl group known as the carboxyl terminus (C-terminus). Interactions between amino acids cause a protein to fold; from an amino acid sequence of a polypeptide to a three-dimensional structure of a mature functioning protein (Loughlin, 2017). The two most important protein secondary structures, the alpha helix (a helices) and the beta sheet (ß sheet), were predicted. by Linus Pauling (1951) cited in Loughlin (2017, p. 9) . With the use of X-ray diffraction (Loughlin, 2017, p.12, Box 1.3) Pauling was able to determine the shape of proteins, discovering the spiral structure of proteins; the polypeptide skeleton. He recognized that the folding of peptide chains, among other things such as steric hindrance, should maintain bond angles and the planar structure of the peptide bond, as well as prevent the atoms from getting close enough to repel each other. Both types of secondary structures, a helix and a ß-sheet, are held in shape by hydrogen bonds, which form between carbonyl (C=O) and amine (NH) shown below, pulling the polypeptide chain into a helical structure allowing the lateral chain to protrude and interact freely. Most characteristics of proteins are consistent with their secondary structures, they can be fibrous which are important in the formation of biological or globular structures, spherical in shape with recognizable regions of helices and ß-sheet structures connected to known non-uniform shapes as random reels (Loughlin, 2017). As described in Loughlin (2017), the structure, 2017).