Here's why:
* Primary structure refers to the linear sequence of amino acids in a polypeptide chain. The amino acids are linked together by peptide bonds, which are covalent bonds.
* Secondary structure (alpha-helices and beta-sheets) is stabilized by hydrogen bonds between the backbone atoms of the polypeptide chain.
* Tertiary structure (the 3D shape of a single polypeptide chain) is stabilized by various interactions, including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges (which are covalent bonds between cysteine residues).
* Quaternary structure (the arrangement of multiple polypeptide chains in a protein complex) is also stabilized by the same interactions as tertiary structure.
So, while covalent bonds play a role in tertiary structure (disulfide bridges), the primary structure is the only level directly defined and held together by covalent bonds (peptide bonds).
