When heavy metals such as [tex]\( Pb^{2+} \)[/tex] (lead) or [tex]\( Hg^{2+} \)[/tex] (mercury) come into contact with proteins, they have a tendency to denature proteins. Denaturation is the process by which a protein loses its three-dimensional structure and, consequently, its function. This occurs because heavy metals disrupt the normal bonding interactions that maintain the protein's structure.
To understand which specific part of the protein these metals react with, let us analyze the given options:
1. The SH group of cysteine residues:
- The SH (thiol) group in cysteine side chains is highly reactive and has a high affinity for heavy metals. The sulfur atom in the SH group can easily donate electrons and form strong bonds with metals like [tex]\( Pb^{2+} \)[/tex] and [tex]\( Hg^{2+} \)[/tex]. This interaction is particularly strong because sulfur is a soft ligand and has a strong tendency to bind to soft metal cations like [tex]\( Pb^{2+} \)[/tex] and [tex]\( Hg^{2+} \)[/tex].
2. The OH group of tyrosine residues:
- The OH (hydroxyl) group in tyrosine side chains is less reactive compared to the thiol group. While it can participate in hydrogen bonding and some other interactions, it does not have as high an affinity for heavy metals as the SH group does.
3. The proline residues:
- Proline is unique among amino acids due to its cyclic structure, which causes rigid kinks in the polypeptide chain. However, it does not possess a specific reactive group that would have a particularly high affinity for heavy metals.
4. The NH [tex]\(_2\)[/tex] group of glutamine:
- The NH [tex]\(_2\)[/tex] (amine) group in glutamine can engage in hydrogen bonding and can act as a nucleophile in some reactions, but like the OH group of tyrosine, it does not have as high an affinity for heavy metals as the SH group does.
Considering the above points, it is clear that the SH group of cysteine residues is the primary site for the reaction with heavy metals such as [tex]\( Pb^{2+} \)[/tex] or [tex]\( Hg^{2+} \)[/tex]. This is due to the strong binding affinity of the thiol (sulfhydryl) group for these heavy metals, leading to the denaturation of the protein structure.
Therefore, heavy metals such as [tex]\( Pb^{2+} \)[/tex] or [tex]\( Hg^{2+} \)[/tex] denature proteins by reacting primarily with the SH group of cysteine residues.
