Proteins are polymers of amino acids, linked by peptide bonds known as polypeptides. Amino acids can be essential or non-essential. There are 20 types of amino acids that come together to form proteins and so they are also known as heteropolymers. In a heteropolymer, there are different types of monomers that are repeated “n” number of times.
Chemists consider the molecular formula (C6H6) and not the structural formula. On the other hand, organic chemists consider a two-dimensional view of molecular structure. Physicists consider a three-dimensional view of molecular structure. However, biochemists consider the protein structure at four levels – primary, secondary, tertiary and quaternary structures. The structure of these proteins is crucial in determining their functions.
In the primary structure, left end of a protein is represented by the first N-terminal amino acid and the right end is represented by the last C-terminal amino acid, which refers to the positional information of amino acids in a protein.
In the secondary structure, amino acid links are longer and due to the heaviness of the molecule, it tends to bend. It is either an alpha helix or beta sheets.
The tertiary structure represents the three-dimensional structure of the entire polypeptide chain. The function of a protein depends on its tertiary structure. If this is disrupted, the protein is said to be denatured and it loses its activity.
The fourth level of protein structure is the quaternary structure. This structure is formed by several protein molecules or polypeptides also known as protein subunits. These subunits are arranged as a linear string of spheres one upon each other in the form of a cube or plate, where two identical alpha type subunits and two identical beta type subunits collectively form human haemoglobin.
Some of the most common proteins we come across are collagen, RUBISCO, trypsin, insulin, antibodies, receptors and GLUT-4.