Levels of Protein Structure

Introduction: Proteins are fundamental components of all living cells. They exist in the form of enzymes, hormones, antibodies etc. Proteins are a group of complex organic macromolecules that contain carbon, hydrogen, nitrogen and usually sulfur and are made up of one or more chains of amino acids linked with each other by peptide bond with a very unique three dimensionally folded structure. 

Levels of Protein Structure: Due to its complexity, protein structure has to be studied at four levels.

 (i) Primary structure – is essentially a polypeptide formed by attachment of amino acids one after the other in the form of a linear chain by means of a peptide bond. Here due to involvement of the two functional groups namely; carboxylic group -COOH, and amino group -NH2, every polypeptide chain shows orientation as C-terminus (referring to a free carboxylic group) and N-terminus (referring to a free amino group). 

 (ii) Secondary structure – is attained by twisting of the polypeptide chain in a very specific manner. The twists occur due to the chemical nature of each amino acid that constitute the polypeptide chain. Two types of secondary structure are seen a) alpha-helix is a spiral arrangement stabilized by extensive hydrogen bonding. Each turn of alpha-helix contains 3.6 amino acids, travels a distance of 0.54 nm, with a space of 0.15 nm between two consecutive amino acids. b) beta-sheet is formed by two or more fully extended polypeptide chains linked sideways by hydrogen bonds. Depending upon the orientation of each polypeptide chains with respect to N-terminus and C-terminus, the beta-sheet may be parallel or antiparallel. 

(iii) Tertiary structure – It involves the three dimensional arrangement of alpha-helix and / or beta-sheet in such a way that the hydrophilic parts face the outer side and hydrophobic parts face the inner side of the protein molecule. This is achieved by formation of hydrogen bonds along with disulphide linkages in the interior of the molecule. 

 (iv) Quaternary structure – Some proteins are formed by a single polypeptide chain. But in many cases there is more than one copy of the polypeptide (oligomers) or different polypeptides of different structures (complex assembly). In such cases these individual polypeptides are connected to each other by non-covalent bonds such as hydrogen bond or ionic bond.

 As such it is the tertiary and quaternary structure that is responsible for the functional efficiency of the protein molecule making it extremely important. 

Reference - Biochemistry by Voet and Voet.