Proteins are natural polymer molecules consisting of amino acid units. The number of amino acids in proteins may range from two to several thousand.
Protein it is a polymer which differ from carbohydrate and lipids due to the presence of nitrogen on its structure i.e. the element of carbon, oxygen, hydrogen are associated with nitrogen to form protein, some protein contain phosphorous, iron, zinc and copper. The following are the main four structures of protein;-
(a) Primary Structure: The primary structure of a protein is its amino acid sequence. This amino acid sequence is determined by the base sequence of the gene which codes for the protein.
(b) Secondary Structure: Secondary structures have α-helices and β-pleated sheets. These form as a result of hydrogen bonds between the peptide groups of the main chain. Therefore, proteins that contain secondary structures will have regions that are cylindrical (α-helices) and/or regions that are planar (β-pleated sheets).
(c) Tertiary Structure: The tertiary structure of a protein is its three-dimensional conformation which occurs as a result of the protein folding. This folding is stabilised by hydrogen bonds, hydrophobic interactions, ionic bonds and disulphide bridges. These intramolecular bonds form between the R groups of different amino acids.
(d) Quaternary Structure: A quaternary structure is formed when two or more polypeptide chains associate to form a single protein. An example is haemoglobin which consists of four polypeptide chains. In some cases, some proteins can have a non-polypeptide structure called a prosthetic group. These proteins are called conjugated proteins. The haem group in haemoglobin is a prosthetic group.