Which of the following best defines a protein’s secondary structure?

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Question recap: Which of the following best defines a protein’s secondary structure? The four choices present various interpretations of protein structure at different hierarchical levels.
Option a: 'The complete three-dimensional conformation of a single polypeptide chain, including side-chain interactions and disulfide bonds.' This describes the overall tertiary structure (and aspects like quaternary if multiple chains are involved), not specifically the secondary structure. It conflates local folding with the full 3D fold, so it’s not a definition of secondary structure.
Option b: 'The linear order of amino acids within a polypeptide, determined by the nucleotide sequence of the gene.' This is the primary structure of a protein, which is the sequence of amino acids. It does not address the local folding patterns that characterize secondary structure, so this option misdefines secondary structure.
Option c: 'The local folding patterns of a polypeptide backbone, such as alpha helices and beta sheets, stabilised primarily by hydrogen bonding.' This precisely captures secondary structure: it focuses on recurring, local motifs formed by backbone interactions, with hydrogen bonds driving the formation of helices and sheets. This aligns with standard definitions in molecular biology and biochemistry.
Option d: 'The spatial organisation of multiple polypeptide subunits held together by covalent and non-covalent interactions.' This describes quaternary structure, which concerns how multiple polypeptide chains assemble together, not the local patterns within a single chain. Therefore it does not define secondary structure.
In summary, option c uniquely identifies the essence of secondary structure as the localized, hydrogen-bond-stabilized folding patterns like alpha helices and beta sheets, while the other options correspond to other levels of protein structure (primary, tertiary, and quaternary) and thus do not define secondary structure.

Which of the following best defines a protein’s secondary structure?

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