Question

5. In class, we stated that protein denaturation is reversible. When we cook an egg, which contains a large amount of protein, we clearly denature all of its protein. Why is it not possible to renature this protein by cooling our omelet?
A. Because covalent bonds in the protein have been broken.
B. Because the protein has fallen out of solution.
C. Because there are no chaperone molecules to help fold the protein.
D. It is possible to renature the protein by cooling the omelet - we just can’t see it happen.

Answer 1

B. Because the protein has fallen (or precipitated) out of the solution

Answer 2

Hmm, I disagree. Protein typically does not precipitate (though through some wet lab wizardry involving solvents and spinning the sample in a centrifuge scientists can make protein precipitate, after a fashion, in the lab) in solution.

The crude structural elements of proteins are held together with disulfide bonds between different cystine residues. All the finer structure - which amounts to most of the structure - is provided by weaker non-covalent bonds. Boiling an egg disrupts the non-covalent bonds at very low temperatures and it also disrupts the disulfide bonds at much higher temperatures.

As the proteins - now a collection of unfolded peptides - in the egg white cool, they form all sorts of inappropriate bonds with other residues in their own chain and with other peptides all together. The result is one big tangled mat of peptides. The absence of chaperones, necessary for proper protein folding and now totally unfolded too) also contributes to the impossibility of refolding the peptides into properly structured proteins.