#Question id: 909

Most proteins denature at both high and low pH. At high pH, the ionization of internal tyrosines is thought to be the main destabilizing influence, whereas at low pH, the protonation of buried histidines. A titration curve for the unfolding of the enzyme ribonuclease is shown in Figure. Given the relation between the titration curves for protein unfolding and histidine protonation, which of the following conclusion can be drawn correctly?

A. The sharp transition in graph indicates a highly cooperative process; when the protein starts to unfold, it completes the process rapidly

B. The sharp transition in graph indicates a highly noncooperative process

C. Several buried histidines can ionize when the chain starts to unfold, so that when one goes, they all go together due to cooperative behaviour of protein folding

D. Buried histidine (pK of 4 in this example) becomes accessible to solvent, its pK will shift toward its normal value of 6, significantly steepening its titration curve.