Types of Structure of Proteins : Primary, Secondary and Tertiary MCQ - Practice Questions with Answers

Structure of Proteins - Primary Structure

A protein can have up to four levels of structures but all proteins have all four levels. 

a) Primary Structure of Protein: 

• The specific sequence of amino acids in the polypeptide chain is called the primary structure.
• The linear polymer of amino acids, called the polypeptide contains polarity.
• By convention, the beginning of any polypeptide chain is at the α-amino group and the end is at the α-carboxyl group.
• Each amino acid is called a residue.
• The unique sequence for every polypeptide is determined by the genes encoding that protein.

Structure of Proteins - Secondary Structure

• Once the primary structure of the protein is formed, it begins to twist into regular patterns that make its secondary structure. 
• These twists are formed as a result of the regular pattern of H-bonds.
• There are two types of secondary structures: α-helix and β-pleated sheets.

α-helix: 

• It is a rod-like structure.
• Inner portion contains the backbone of the polypeptide and outer portion contains the side chains.
• The helical shape occurs because of the formation of H-bonds between the oxygen atom of the -CO group of one amino acid and the nitrogen atom of the -NH group of the fourth amino acid.
• Thus, the H-bonds are formed every fourth amino acid.

β-pleated sheets:

• In this structure, two linear polymers of amino acids run in opposite direction.
• In this arrangement, the -NH and -C=O groups of an amino acid in one strand form H-bonds with -C=O and -NH groups of the opposing amino acids on the other strand.

Tip: The α-helix and β-pleated sheet structures are found in most globular and fibrous proteins and they play an important structural role.

Structure of Proteins - Tertiary & Quaternary Structure

• The tertiary structure of a protein is its unique three-dimensional structure. 
• It occurs due to the following interactions:

                          -Hydrophobic Interactions
                          -Van Der Waals Interactions
                          -Disulfide Bridges
                          -H-bonds and Ionic Bonds

• Some proteins are formed from several polypeptides, also known as subunits, and the interaction of these subunits forms the quaternary structure.
• For example, Hemoglobin protein has two α and two β polypeptides together with four heme groups.
• Weak interactions between the subunits help to stabilize the overall structure.