Disulfide bonds are formed between which type of groups in cysteine residues?

Disulfide bonds are covalent linkages that form between the thiol groups (-SH) of cysteine residues. Cysteine contains a sulfhydryl group which is particularly reactive under oxidizing conditions. When two cysteine residues come into proximity, the thiol groups from each can undergo an oxidation reaction, resulting in the formation of a disulfide bond (–S–S–). This bond plays a crucial role in stabilizing the tertiary and quaternary structures of proteins by providing additional structural integrity.

The other groups mentioned in the options do not participate in disulfide bond formation. Amino groups (-NH2) are involved in peptide bond formation between amino acids. Carboxyl groups (-COOH) also play a key role in forming peptide bonds, linking amino acids together in protein synthesis. Hydroxyl groups (-OH) are present in some amino acids and can be involved in other types of bonding or interactions but do not form disulfide bonds. Thus, the correct connection for disulfide bond formation specifically involves thiol groups from cysteine residues.