Final answer:
Cysteine residues are capable of forming disulfide bonds that stabilize protein structure by linking together, creating a covalent linkage known as cystine. While methionine contains sulfur, it does not usually form such bonds. Disulfide bonds are exemplified in insulin's structure, contributing greatly to its tertiary structure and functionality.
Step-by-step explanation:
The amino acids cysteine (Cys) and methionine (Met) contain sulfur atoms in their structures and are known for forming disulfide linkages in proteins. Cysteine, in particular, has a side chain with a thiol (-SH) group that can react with another cysteine residue's thiol group to form a disulfide bond (S-S), when in the presence of oxygen. This reaction is oxidatively driven as illustrated: R-SH + HS-R → R-S-S-R.
The disulfide bond formed between two cysteines is also sometimes referred to as cystine when bound together in a dimer. These bonds play a critical role in stabilizing the tertiary structure of proteins by providing covalent bonds within or between protein chains. An example of proteins with such bonds includes insulin, which consists of A and B chains that are linked by two interchain disulfide bonds and contains one intrachain disulfide bond within the A chain.
While methionine also contains sulfur, it does not typically engage in disulfide bonding. Thus, in the context of protein cross-linking, cysteine is the residue known to form disulfide bonds, crucial in protein structural integrity and function.