Structures of Proteins
Proteins are polymeric chains that are built from monomers called amino acids. All structural and functional properties of proteins derive from the chemical properties of the polypeptide chain. There are four levels of protein structural organization: primary, secondary, tertiary, and quaternary. Primary structure is defined as the linear sequence of amino acids in a polypeptide chain. The secondary structure refers to certain regular geometric figures of the chain. Tertiary structure results from long-range contacts within the chain. The quaternary structure is the organization of protein subunits, or two or more independent polypeptide chains. The primary structure of a protein is simply the sequence of amino acids. The structure or amino acid sequence is unique for each protein. The sequence in which the amino acids are attached to one another ultimately is dependent on the genetic code from DNA. This primary structure dictates the function of the protein indirectly through additional levels of structure. Of course, the formation or the putting together of a protein does not stop at the di-peptide stage. Instead, these two amino acid residues are bonded to another amino acid, which in turn is bonded to another, which in turn is bonded to still more. In the process, quite extensive arrangements of amino acid residues are built up to form a protein. Ultimately, this is the primary structure of a protein. According to IUPAC, “the secondary structure of a segment of polypeptide chain is the local spatial arrangement of its main-chain atoms without regard to the conformation of its side chains or to its relationship with other segments.” Basically, the secondary structure of a protein is the initial arrangement of the primary structure. Sometimes part of the amino acid sequence folds into a coil called a helix. Sometimes part of the amino acid sequence folds into a sheet of parallel strands. The complete protein may have some parts where the secondary structure