Here's why:
* Primary Structure: This level refers to the linear sequence of amino acids in a protein chain. It's held together by peptide bonds, which are covalent bonds and much stronger than hydrogen bonds. Disrupting hydrogen bonds doesn't break the peptide bonds, so the primary structure remains intact.
* Secondary Structure: This level involves local folding patterns like alpha-helices and beta-sheets, which are stabilized by hydrogen bonds between backbone atoms. Disrupting these hydrogen bonds would significantly alter the secondary structure.
* Tertiary Structure: This refers to the overall 3D shape of a single protein chain. It's stabilized by a variety of interactions including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges. Disrupting hydrogen bonds would significantly affect the tertiary structure.
* Quaternary Structure: This applies to proteins composed of multiple polypeptide chains (subunits). It's also held together by interactions like hydrogen bonds. Disrupting these hydrogen bonds would affect the quaternary structure.
In summary: While hydrogen bonds play a crucial role in stabilizing the higher levels of protein structure, they don't affect the primary structure, which is determined by the strong covalent peptide bonds.
