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Before I explain what maintains the secondary structure of a protein, I would like to explain about the secondary structure of a protein. I hope you know about the primary structure of the protein (the polypeptide chain).
The secondary structure of a protein is a regular recurring three-dimensional structure found in the protein’s native structure. There are mainly two types of secondary structures and these are α-helix and β-pleated sheets. There are four different weak (non-covalent) interactions that are responsible for the 3D structure of a protein and these are; H-bonding (Hydrogen bonding), Ionic interaction, Van-der-Waals force of attraction, and hydrophobic interaction. Now, I will jump to the topic; what maintains the secondary structure of a protein?
The secondary structure of a protein is purely maintained by Hydrogen bonding. Yes, it is the H- bonding that maintains the secondary structure of a protein. To explain in a detailed way, let’s take an example.
Example 1: α-helix
The α-helix is one of the most common secondary structures of the proteins that are abundant in globular proteins. It is a right-handed coiled strand that looks like a DNA double-stranded structure. As shown above, in the α-helix, a single turn (that is called as a pitch) contains 3.6 amino acid residues.
The carbonyl oxygen (C=O) of the first amino acid residue of a polypeptide chain forms H-bond with an amide nitrogen(C-NH-) of the fourth amino acid residues. The formation of the Hydrogen bonding follows the same pattern in α-helix. In α-helix, R groups (side chain) of each of the amino acid residues of the polypeptide chain are projected outward of the α-helix.
Example 2: β-pleated sheet
You might be curious about how the H-bonding helps polypeptide chain attaining a β-pleated sheet. the β-pleated sheet is another type of secondary structure that is predominant in fibrous proteins such as hair and nail. β-pleated sheets can be two types; parallel and anti-parallel.
In β-pleated sheets, the Hydrogen bonding occurs in between the two strands (inter-strand Hydrogen bonding) in which amide nitrogen of an amino acid residue of one strand forms H-bond with the carbonyl oxygen of an amino acid residue of the another strand. Here, two types of H-bonding may be possible; intramolecular H-bonding (of there are two different polypeptide chains forming β-pleated sheet) or intermolecular H-bonding (if there is only one polypeptide chain forming β-pleated sheet). In the β-pleated sheet, R groups are projected either upward or downward the plane. I hope you are clear about the question what maintains the secondary structure of a protein?