When a cysteine residue in a protein is replaced with serine, the protein folds normally and exhibits 95 percent of its normal biological activity.However, when this same cysteine residue is replaced with glycine, the protein does not fold normally and exhibits only 35 percent of its normal biological activity.Which hypothesis is both reasonable and consistent with these observations?

Question

Quizplus · Accepted Answer

The fact that replacing the cysteine residue with serine maintains the protein's activity at 95 percent suggests that the side chain of cysteine is not necessary for the protein's stability or function. However, when glycine is substituted, which is a small and flexible residue, the protein is destabilized and loses its biological activity. This suggests that the lack of a side chain to interact with other residues in the interior of the protein destabilizes the protein. A reasonable explanation is that the cysteine side chain may form hydrogen bonds with other residues in the interior of the protein to help stabilize its tertiary structure.

When a Cysteine Residue in a Protein Is Replaced with Serine