Which type of bonds links the subunits of Protein X covalently?

• A. Thiol groups of methionine residues.
• B. Thiol groups of cysteine residues.
• C. Hydroxyl groups of serine residues.
• D. Hydroxyl groups of threonine residues.

Answer: (B)

The correct answer is that the thiol groups of cysteine residues link the subunits of Protein X covalently. Cysteine contains a thiol (-SH) group that can undergo oxidation to form disulfide bonds (S-S) with another cysteine residue. These disulfide bridges are important for stabilizing the three-dimensional structure of proteins, particularly in extracellular environments where the oxidative conditions can promote such bonds.

Disulfide bonds play a crucial role in maintaining the functional conformation of proteins and can be found in various proteins, especially those that are secreted and thus exposed to the extracellular environment. This covalent linkage provides added stability to the protein structure compared to non-covalent interactions, which may be weaker and more susceptible to disruption.

While other amino acids mentioned, like serine and threonine, have hydroxyl groups that can participate in various types of chemical reactions, they do not form covalent bonds analogous to disulfide bridges in the same way that cysteine does. Methionine also contains a sulfur atom in its side chain, but it typically does not form disulfide bonds and its reactivity differs from that of cysteine.