What defines the primary structure of proteins?

The primary structure of proteins is defined by the specific sequence of amino acids in a polypeptide. This linear arrangement of amino acids is crucial because it dictates how the protein will fold and function. Each amino acid in the sequence contributes unique chemical properties that influence interactions between them, ultimately leading to the protein’s three-dimensional shape and biological activity.

Consequently, the primary structure is fundamental to protein biology because any alteration in the amino acid sequence can lead to significant changes in the protein's properties and functions, potentially resulting in diseases. The other aspects listed, such as the folding and three-dimensional shape, the number of polypeptide chains, and features like alpha helices and beta sheets, pertain to higher-level structures of the protein (secondary, tertiary, and quaternary structures) and are influenced by the primary structure but do not define it.