The image below plots the denaturation of ribonuclease A as a function of guanidine hydrochloride (GdnHCl) concentration, monitored by circular dichroism. The shape of the curve in the figure supports which statement?
A) The addition of GdnHCl has little effect on protein secondary structure.
B) Increasing temperature causes the protein to become unfolded.
C) Unfolding of this protein is a cooperative process.
D) Ribonuclease A is a heat-stable protein.
E) The peptide bonds of the protein are broken around 3.4 M GdnHCl.

Question

The image below plots the denaturation of ribonuclease A as a function of guanidine hydrochloride (GdnHCl) concentration, monitored by circular dichroism. The shape of the curve in the figure supports which statement? 
A) The addition of GdnHCl has little effect on protein secondary structure. 
B) Increasing temperature causes the protein to become unfolded. 
C) Unfolding of this protein is a cooperative process. 
D) Ribonuclease A is a heat-stable protein. 
E) The peptide bonds of the protein are broken around 3.4 M GdnHCl.

Quizplus · Accepted Answer

The sigmoidal shape of the curve indicates that the denaturation of ribonuclease A is a cooperative process. As the concentration of GdnHCl increases, more and more protein molecules unfold, leading to an abrupt transition from the folded to the unfolded state. Thus, the curve supports the choice that "unfolding of this protein is a cooperative process." The other choices are not supported by the curve. Choice A is not likely, as the addition of GdnHCl disrupts protein structure. Choice B is not supported by the curve, as there is no temperature data shown. Choice D is incorrect, as the curve shows the unfolding of ribonuclease A with increasing GdnHCl concentration, and not heat stability. Choice E is not supported by the curve, as there is no specific point on the curve where peptide bond breakage occurs.

The Image Below Plots the Denaturation of Ribonuclease a as a Function