What defines the primary structure of a protein?

The primary structure of a protein is defined by the unique sequence of amino acids that make up the polypeptide chain. This sequence is determined by the genetic code and dictates how the protein will fold and ultimately function. Each amino acid in the chain contributes to the protein's properties and shape, influencing subsequent levels of structure, such as secondary, tertiary, and quaternary structures.

The folding of protein domains relates to the secondary and tertiary structures of proteins, which encompass various types of folding patterns and interactions between regions of the polypeptide. The presence of alpha helices and beta sheets specifically refers to the arrangements typical of secondary structures, created by hydrogen bonding patterns among the backbone of the polypeptide chain. Lastly, the quaternary structure pertains to the assembly of multiple polypeptide chains into a functional protein complex, which comes into play after the protein's primary structure has formed.

Thus, the defining characteristic of the primary structure is solely the sequence of amino acids, making that the correct and foundational element in understanding protein biochemistry.