Which amino acid contains a sulfhydryl group in its side chain?

Cysteine is the amino acid that contains a sulfhydryl group (-SH) in its side chain. This functional group is significant because it allows cysteine to participate in the formation of disulfide bonds, which are important in stabilizing the three-dimensional structure of proteins. The presence of the sulfhydryl group also plays a crucial role in various biochemical processes, including enzyme catalysis and redox reactions.

In contrast, glycine has a simple hydrogen as its side chain, making it the smallest amino acid with no functional groups. Alanine features a methyl group as its side chain, while serine has a hydroxymethyl group (-CH2OH). None of these options contain the sulfhydryl group found in cysteine, which is essential for many biological functions and protein interactions.