What defines the primary structure of proteins?

The primary structure of proteins is defined by the sequence of amino acids that compose the polypeptide chain. This sequence is crucial because it determines the overall structure and function of the protein. Each amino acid is linked by peptide bonds, creating a specific order that is dictated by the genetic code. The unique arrangement of these amino acids influences how the protein will fold into its secondary, tertiary, and quaternary structures, ultimately determining its role within the cell and the organism.

In contrast, the folding of the polypeptide chain refers to the secondary and tertiary structures that are influenced by the primary structure but are not part of it. The arrangement of alpha helices and other structural motifs represents higher levels of protein structure that emerge from the interactions between amino acids but do not define the primary structure itself. Lastly, the formation of disulfide bonds, which are covalent links between certain amino acids, can stabilize a protein's three-dimensional structure but again does not pertain to the primary sequence of amino acids.