What is the primary function of Protein disulfide isomerase (PDI)?

The primary function of Protein disulfide isomerase (PDI) is to catalyze oxidation reactions to produce cystine. PDI plays a crucial role in the formation and rearrangement of disulfide bonds within and between polypeptide chains during protein folding. These disulfide bonds, which are covalent linkages formed between the sulfur atoms of cysteine residues, are important for stabilizing the three-dimensional structure of proteins.

When proteins are being synthesized in the endoplasmic reticulum, they often require the correct formation of disulfide bonds to achieve their proper functional conformation. PDI promotes these oxidative reactions that lead to the formation of cystine (the oxidized form of two cysteine molecules connected by a disulfide bond), thereby facilitating the correct folding and stability of proteins.

The other options do not accurately capture the primary role of PDI. For instance, producing hydrogen bonds between proteins refers to non-covalent interactions that contribute to protein structure but is not a function of PDI. Assisting in protein folding by reducing disulfide bonds does not reflect the correct mechanism of action, as PDI is involved in the oxidation process that forms those bonds rather than reducing them. Hydrolyzing peptide bonds pertains