AFinal answer:
The primary structure of a protein, which is the sequence of amino acids, is the least likely to be affected by changes in pH due to the strength of its covalent bonds, unlike the higher levels of protein structure.
Step-by-step explanation:
The level of protein structure least likely to be affected by a shift in pH below its optimal range is the primary structure. The primary structure refers to the sequence of amino acids in the protein, which is held together by covalent peptide bonds. These bonds are not easily disrupted by changes in pH compared to the hydrogen bonds and ionic interactions that hold the secondary structure (alpha helices and beta pleated sheets) and the tertiary structure (the overall 3D structure due to side chain interactions). The quaternary structure, which occurs when two or more tertiary subunits interact, can also be disrupted in lower pH as it relies on the same types of non-covalent interactions as the secondary and tertiary structures.