What is the primary structure of a protein?

• A. Sequence of nucleotides
• B. Linear sequence of amino acids
• C. Folding into a 3D structure
• D. Association of multiple polypeptides

Answer: (B)

The primary structure of a protein is defined as the linear sequence of amino acids that make up the polypeptide chain. This sequence is critical because it determines the way the protein will fold into its secondary and tertiary structures, which are essential for its functionality. The order of amino acids in the chain dictates the interactions between the different parts of the molecule, influencing how it assumes its final three-dimensional shape.

In contrast, the other options refer to different aspects of protein structure. The sequence of nucleotides pertains to DNA or RNA, not proteins. The folding into a three-dimensional structure is related to the secondary and tertiary structure of proteins, which are determined by the primary structure but are not themselves the primary structure. The association of multiple polypeptides describes the quaternary structure of proteins, which involves the assembly of several polypeptide chains into a larger functional complex. Thus, the key aspect that defines the primary structure is indeed the linear sequence of amino acids.