Final answer:
Cysteine residues in PDI shuffle disulfide bonds in proteins by forming temporary bonds with cysteine residues in improperly folded proteins, assisting in correct protein folding.
Step-by-step explanation:
Cysteine residues in PDI (Protein Disulfide Isomerase) shuffle disulfide bonds by forming temporary disulfide bonds with cysteine residues in improperly folded new protein. This essential molecular process involves oxidation where a sulfhydryl (-SH) group from one cysteine reacts with another, forming a disulfide bridge (-S-S-). This action helps to stabilize the tertiary structure of proteins and correct misfolded proteins by rearranging improper disulfide bonds, contributing to the proper folding necessary for the protein's functionality.
Examples of proteins with disulfide linkages include insulin, which contains both intra- and inter-chain disulfide bonds. Intra-chain bonds occur within a single polypeptide chain, whereas inter-chain bonds link different polypeptide chains, such as the A and B chains in insulin. Through the reversible oxidative reactions facilitated by PDI, proteins can attain their native, functional conformations, which are robust and resistant to a variety of environmental conditions.