When a Cysteine Residue in a Protein Is Replaced with Serine

Multiple Choice

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?

A) A hydrogen bond between the cysteine side chain and another residue in the interior of the protein helps stabilize the protein's tertiary structure.
B) The cysteine side chain is exposed to the external environment, where it forms salt bridges with ions dissolved in the water surrounding the protein.
C) The cysteine residue forms a disulfide bridge with another cysteine in the polypeptide chain to stabilize the protein's tertiary structure.
D) A salt bridge between the cysteine side chain and an acidic residue in the interior of the protein helps stabilize the protein's tertiary structure.
E) The cysteine side chain is located in a tight bend in the polypeptide chain, where only a small side chain can be accommodated.