Final answer:
Cysteine is the amino acid with a sulfhydryl group that forms disulfide bridges, crucial for protein stability and structure.
Step-by-step explanation:
The amino acid that contains a sulfhydryl group and is involved in disulfide bridge formation is cysteine. This sulfhydryl (SH) group is highly reactive and when two cysteine molecules come into proximity within a protein, they can oxidize to form a disulfide bond (S-S bond), creating a dimer called cystine. This covalent bond is crucial for the protein's tertiary structure stability and is often found in proteins that need to maintain a certain shape or configuration, such as insulin.