What primarily determines the shape, size, and function of functional proteins?

The shape, size, and function of functional proteins are primarily determined by the sequence of amino acids. This sequence dictates how the protein will fold into its three-dimensional structure, which is crucial for its specific functions. The interactions between the amino acids—including hydrogen bonds, ionic bonds, van der Waals interactions, and disulfide bridges—dictate how the protein will twist and turn to achieve its final conformation.

While the number and arrangement of polypeptide chains contribute to protein structure (e.g., quaternary structure), it is ultimately the sequence of the amino acids within those polypeptide chains that governs how they will fold and organize. Each protein's unique amino acid sequence is what grants it distinct properties and functionalities essential for biological processes. Thus, understanding the primary sequence is key to understanding the protein's overall characteristics.