Question

In the 1950s, Christian Anfinsen demonstrated the renaturation of the protein ribonuclease (RNase) in vitro. After reduction (to reduce the disulfide links) and the addition of urea (to denature the protein), the protein was in an unfolded state. After removing the urea and the reducing agent, the protein refolded, with greater than 90% activity. If the urea were removed after oxidation occurred, the protein had less than 5% activity. Why would the protein not refold correctly if the urea were removed after the reducing agent was removed? (In other words, what would happen if the urea were removed after oxidation?) Choose the best answer.

a. Urea would participate in weak bonding interactions with RNase, preventing oxidation of Cys.

b. The protein would not fully unfold (denature).

c. Disulfide bonds are not positioned correctly unless weak bonding interactions are present.

d. Contaminants in the RNase preparation would form covalent bonds with the protein, preventing reactivation.

Answer 1

The common chemical denaturant of proteins is urea. The high concentration of urea causes unfolding of protein and thus results in loss of function of protein. The urea interacts with the protein and prevents folding of protein.

During oxidation, the disulphide bonds that are required for proper functioning and stabilization of protein are formed, while in presence of urea, the disulfide bonds are not positioned correctly. The protein oxidation results in covalent modification of protein that results in change of physical and chemical properties of protein.

The change of physical and chemical properties of protein after oxidation and in presence of urea cannot be modified even after removal of urea. Thus, protein does not fold properly.

Hence, the correct option among the following is c- Disulfide bonds are not positioned correctly unless weak bonding interactions are present.