Side chain of which amino acid contains sulfhydryl group ?

Ans. is 'b' i.e., Cysteine There are two sulfur containing amino acids :- i) Cysteine ii) Methionine The side chain of cysteine contains sulfhydryl group that has a pKa of approximately 8.4 for dissociation of its hydrogen sulfhydryl groups of two cystein molecules can form covalent disulfide bond to form cystine. Methionine although it contains sulfur group, does not containt sulfhydryl group and cannot form disulfide bond.