What is the role of sulfur in insulin structure?

Sulfur plays a critical role in the structure and stability of insulin through the formation of disulfide bridges, which are covalent bonds between sulfur atoms in cysteine residues. These disulfide bridges contribute to the three-dimensional conformation of the insulin molecule, which is essential for its biological activity. Accurate folding and structural integrity are necessary for insulin to effectively bind to its receptors and perform its role in regulating blood glucose levels.

While the other choices involve important biological functions—such as enzyme activation, hydrogen bond formation, and glucose transport—they do not specifically pertain to the structural role of sulfur in insulin. The formation of disulfide bridges is crucial for maintaining the stability and functionality of insulin, making this the correct answer.