Secondary Structure of Proteins

Secondary Structure

The local folding of the polypeptide in some regions gives rise to the secondary structure of the protein. The α-helix and β-pleated sheet structures are the most common secondary structures (see image below). Both structures are the α-helix structure—the helix which hydrogen bonds hold in shape. The hydrogen bonds form between the oxygen atom in the carbonyl group in one amino acid and another amino acid that is four amino acids farther along the chain.


The α-helix and β-pleated sheet are secondary structures of proteins. They form because of hydrogen bonding between carbonyl and amino groups in the peptide backbone. Certain amino acids have a propensity to form an α-helix, while others have a propensity to form a β-pleated sheet. Image Attribution: OpenStax Biology

Every helical turn in an alpha helix has 3.6 amino acid residues. The R groups (the variant groups) of the polypeptide protrude out from the α-helix chain. In the β-pleated sheet, the “pleats” form by hydrogen bonding between atoms on the backbone of the polypeptide chain.

The R groups are attached to the carbons and extend above and below the folds of the pleat. The pleated segments align parallel or antiparallel to each other. In addition, hydrogen bonds form between the partially positive nitrogen atom in the amino group and the partially negative oxygen atom in the carbonyl group of the peptide backbone. You can find the α-helix and β-pleated sheet structures in most globular and fibrous proteins. As a matter of fact, they play an important structural role.

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