Exposing a protein to high concentrations of urea, which denatures the protein, then after removing the urea, the protein refolds into its native conformation is evidence that: A) Proteins fold randomly. B) Proteins will fold into multiple conformations under normal cellular conditions. C) Proteins will spontaneously fold into one native conformation under normal cellular conditions. D) Urea is a protease.
The Correct Answer and Explanation is:
Correct Answer: C) Proteins will spontaneously fold into one native conformation under normal cellular conditions.
Explanation:
The experiment described—exposing a protein to high concentrations of urea to denature it, followed by removal of the urea and observation of the protein refolding into its original structure—provides critical insight into the nature of protein folding. This classic experiment was first performed by Christian Anfinsen in the 1950s using the enzyme ribonuclease A. His findings formed the basis of what is now known as the Anfinsen’s Dogma, which states that the primary amino acid sequence of a protein contains all the necessary information for the protein to fold into its correct three-dimensional structure.
Urea disrupts the non-covalent interactions—such as hydrogen bonds, hydrophobic interactions, and ionic bonds—that maintain the protein’s tertiary and secondary structures, causing it to unfold or denature. Importantly, the covalent structure, particularly the primary sequence of amino acids, remains intact during this process. Once urea is removed, the denatured protein is often able to spontaneously refold into its native, functional conformation without the need for any external guidance or template. This behavior strongly supports the conclusion that proteins, under normal physiological conditions, will fold into a specific, energetically favorable native conformation.
This experiment refutes the idea that protein folding is random (as in option A), as random folding would take an impractical amount of time due to the vast number of possible conformations (a concept known as Levinthal’s paradox). It also negates the idea in option B that proteins adopt multiple stable conformations under normal conditions. Lastly, option D is incorrect because urea is not a protease; it is a chemical denaturant that disrupts non-covalent interactions, not a protein-degrading enzyme.
Thus, the correct conclusion is that proteins spontaneously fold into a single, stable, native conformation dictated by their amino acid sequence.
