Which amino acid is known for containing a thiol group?

Cysteine is the amino acid that contains a thiol group (-SH) in its structure. This thiol group is significant because it can participate in forming disulfide bonds under oxidative conditions, which are crucial for stabilizing protein structures. These disulfide bonds often play a critical role in the tertiary and quaternary structures of proteins, contributing to their overall stability and function.

In contrast, serine contains a hydroxyl group (-OH), which is involved in various biochemical reactions but does not have the properties associated with a thiol group. Threonine also has a hydroxyl side chain similar to serine, and valine is a branched-chain amino acid that lacks any sulfur-containing functional groups. Thus, it's clear that cysteine's unique thiol group is what makes it distinct among the amino acids listed.