Q.Name the four hierarchical levels of protein structure, in order.

[!TLDR]

The four levels of protein structure, in order, are primary, secondary, tertiary and quaternary.

[!ANSWER]

Primary structure, secondary structure, tertiary structure, and quaternary structure.

Protein structure is organised at four hierarchical levels, each built on the one before it: primary structure (the linear sequence of amino acids joined by peptide bonds), secondary structure (local folding patterns such as the alpha-helix and beta-pleated sheet, stabilised by backbone hydrogen bonds), tertiary structure (the overall three-dimensional folding of a single polypeptide chain, stabilised by additional interactions such as disulphide bridges), and quaternary structure (the association of two or more separate polypeptide subunits into one functional protein, as in haemoglobin).

[!ANSWER]

Primary structure, secondary structure, tertiary structure, and quaternary structure.

Recall the four levels in their standard hierarchical order, from the flat amino-acid sequence up to multi-subunit assembly.

Listing the four levels out of order, or omitting quaternary structure since it does not apply to every single-chain protein.