Thiol group in amino acids:
Thiol group (-SH) is a functional group that contains a sulfur atom bonded to a hydrogen atom. It is present in certain amino acids and plays a crucial role in protein structure and function.

Cysteine amino acid:
- Cysteine is one of the 20 standard amino acids commonly found in proteins.
- It contains a thiol group in its side chain, which is unique among amino acids.
- The thiol group of cysteine can form disulfide bonds with another cysteine residue, leading to the formation of cystine.

Formation of cystine:
- When two cysteine molecules come close to each other, the thiol groups can oxidize to form a disulfide bond (-S-S-).
- This covalent bond between two cysteine residues is known as cystine.
- Cystine plays a role in stabilizing protein structures and is important for the formation of disulfide bridges in proteins.

Role of cysteine in proteins:
- Cysteine residues with thiol groups are involved in maintaining the tertiary and quaternary structure of proteins.
- Disulfide bonds formed by cysteine residues contribute to the stability and function of many proteins.
- Cysteine also participates in redox reactions and antioxidant defense mechanisms within cells.
Therefore, the thiol group is present in cysteine, an amino acid that plays a significant role in protein structure and function through the formation of disulfide bonds.