Final answer:
Disulfide bonds in insulin and proinsulin are formed in the endoplasmic reticulum of cells, as proteins fold and cysteine residues within the chain undergo oxidation. These bonds are important for the tertiary structure and bioactivity of insulin, including when synthetic human insulin is created by reforming disulfide bridges to connect the A and B chains.
Step-by-step explanation:
The disulfide bonds in both insulin and proinsulin molecules most likely are formed in the endoplasmic reticulum of cells. The endoplasmic reticulum is the site where proteins are folded and where disulfide bonds are formed due to the oxidation of sulfhydryl (-SH) groups on cysteine residues within the protein sequence. This occurs during the tertiary structure formation of the protein.
Insulin is synthesized as a preproinsulin, which is processed to proinsulin and then to active insulin. The cleavage of the 'C' chain from proinsulin by proteolytic enzymes results in active insulin. Disulfide bonds, crucial for the stability and structure of insulin, are formed between cysteine residues on the A and B chains of insulin and are essential for the bioactivity of the hormone.
In the process of creating synthetic human insulin, or humulin, disulfide cross bridges are reformed to connect the A and B chains after the chains are purified and mixed. This is essential for maintaining the correct tertiary structure which is necessary for the function of the insulin molecule.