Medicine & USMLE

Secondary Protein Structure

Protein Structure
  1. Primary Protein Structure
  2. Secondary Protein Structure
  3. Tertiary Structure
  4. Quaternary Protein Structure


The secondary structure of proteins refers to the 3-dimensional structure of local segments in proteins. This local structure is primarily dictated by hydrogen bonding between the backbones of amino acids. These bonds result in two types of common motifs: alpha-helices and beta-sheets. 

Key Points

  • Secondary Structure
    • 3-dimensional structure of local polypeptide segments
    • Determined by hydrogen bonds between backbones of amino acids
      • between carbonyl and amino groups
    • Two common motifs:
      • Alpha-helix: hydrogen bonding within the same polypeptide, usually between peptides 3-4 residues away
        • Proline can disrupt alpha helices (R group folds back on itself to form a ring, forming a kink in the chain. This prevents the amino group from being accessible for hydrogen bonding)
      • Beta-sheet: hydrogen bonding between neighboring polypeptide chains
        • Parallel strands run in same direction, resulting in bent hydrogen bonds (weaker) 
        • Anti-parallel strands run in opposite directions, resulting in linear hydrogen bonds (stronger)