Which is more likely to be changed by heating a protein—its primary structure or its tertiary structure?
Heating a protein will not change its primary structure, but may change its secondary, tertiary or quaternary structure.
There are 4 levels of structure in proteins:
1) Primary Structure : the linear structure of amino acids in the polypeptide chain
2) Secondary Structure : hydrogen bonds between peptide group chains form the polypeptide chain into an alpha helix or beta-pleated sheet
3) Tertiary Structure : interaction between side chains causes the 3-D structure of alpha helices and beta-pleated sheets to become folded into a more compact globular structure
4) Quaternary Structure : multiple polypeptide chains coming together to form a functional unit.
Heat will not alter the order of amino acids in the chain (the primary structure), because it will not effectively break the peptide bonds between them.
However, heat can force the bonds between the many polypeptide chains forming the quaternary structure to break. By doing so, the side chains that are joined together in the tertiary structure are also broken apart from one another. It can even cause the alpha-helices and beta-pleated sheets themselves to untwist or unfold, losing its secondary structure.