In relation to a protein, what does the term Secondary Structure describe?

Question

BlackTom AI · Accepted Answer

Secondary structure refers to the local, regularly repeating shapes adopted by the polypeptide backbone—most commonly alpha (α) helices and beta (β) sheets—stabilized primarily by hydrogen bonds between backbone amide N–H and carbonyl C=O groups. Relevant concepts include the peptide bond planarity, backbone dihedral angles (phi, psi), and the geometry of backbone hydrogen bonding that allows repeating motifs like the right-handed α-helix and antiparallel/parallel β-sheets.

Step-by-step reasoning:
1. Identify levels of protein structure:
   - Primary: linear sequence of amino acids (the order of residues).
   - Secondary: local backbone conformations stabilized by backbone hydrogen bonds (α-helices, β-sheets, turns).
   - Tertiary: the overall 3D fold of a single polypeptide, driven largely by side-chain (R-group) interactions: hydrophobic packing, ionic bonds, disulfide bridges, van der Waals, and hydrogen bonds involving side chains.
   - Quaternary: arrangement and number of multiple polypeptide subunits in a multi-subunit protein.
2. Examine each answer option in light of these definitions:
   - Option A ("The order of amino acids in a peptide chain") describes primary structure, not secondary.
   - Option B ("Structures formed due to hydrogen bonding between atoms in the peptide back bone") matches the textbook definition of secondary structure: backbone N–H···O=C hydrogen bonds forming α-helices and β-sheets.
   - Option C ("Folding of the peptide due to interactions between amino acid side chains") describes tertiary structure because it emphasizes side-chain interactions.
   - Option D ("The number an organisation of peptitide subunits in a protein") (sic) describes quaternary structure.

Why the correct answer is correct:
Backbone hydrogen bonds are the hallmark of secondary structure; these interactions repeat along the chain and produce the characteristic helical or sheet motifs independent of the exact amino-acid sequence (though sequence affects propensity). Side chains mostly influence higher-order folding (tertiary) or break/permit certain secondary motifs (e.g., proline disrupts α-helices).

Therefore, the correct choice is: Structures formed due to hydrogen bonding between atoms in the peptide back bone. This matches the provided answer.

In relation to a protein, what does the term Secondary Structure describe?Single choice

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