Disulfide Bridges Stabilize Folded Proteins

 
Cysteine is one of the common amino acids found in all proteins. Under certain conditions two cysteine molecules can be covalently linked through their sulfur atoms creating a disulfide bridge. The figure shows how this can happen with free amino acids but disulfide bridges can also form between cysteine residues in within a single polypeptide chain. The joined molecules are called cystine.

Internal disupfide bridges are very common in proteins that are excreted from the cell or in proteins that are located on the outside surface of the cell membrane. Such proteins are often exposed to harsh conditions that are very unlike the conditions inside the cell where the protein folded into its native conformation.

Pancreatic ribonuclease A (the cow version is shown on the right) is secreted into the intestine where it aids in the digestion of RNA by cleaving it into small pieces. The acidic conditions of the small intestine would cause many proteins to unfold.

RNase A is stabilized by formation of four disulfide bridges between cysteine residues that are only brought together once the protein folds into its proper conformation. For example, the cysteine residue at position 26 is part of an α-helix and it's a long way from the cysteine at position 84, which is part of a β-strand region of secondary structure.

Once the protein folds in the cytoplasm the Cys-26 and Cys-84 side chains are almost in contact and the disulfide bridge is easily created by an oxidation reaction that involves glutathione. This disulfide bridge is not easily reduced back to free side chains so it serves to lock in the correct folded conformation. Since there are four such bridges in riboneculease A, the folded structure is very resistant to denaturation in the harsh conditions outside the cell.

Recall that the correctly folded form of a protein is the lowest energy conformation. But this minimum free energy state only applies to the conditions inside the cell where folding takes place. If the protein has to function in a different environment, it might unfold and become inactive. Over time, there was selection for increased stability by substituting cysteine residues that could form disulfide bridges.