What defines the primary structure of a protein?

The primary structure of a protein is defined by the specific sequence of amino acids in a chain. This sequence is critical because it determines how the protein will fold and function. Each amino acid has unique properties that contribute to the protein’s characteristics, and the order in which these amino acids are arranged dictates the subsequent interactions that lead to higher levels of protein structure, such as secondary, tertiary, and quaternary structures. This foundational level of organization is essential for the protein's biological activity, as even a single change in the amino acid sequence can alter a protein's function and potentially lead to disease. The primary structure is thus the starting point for all additional structural levels and is fundamental to understanding protein biology.