This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Which of the following amino acid contains free sulphydryl group 
Select and explain. 
A. Cysteine, B. Methionine, C. Serine, D. Glycine
A: Cysteine
Answer: A: Cysteine 
Explanation: Ans. is 'a' i.e.,Cysteine There are two sulfur containing amino acids :?CysteineMethionineThe 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.