This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Proteins and Amino Acids, Subject name: Biochemistry
Sulphur of cysteine are not used/utilised in the body for the following process/product: 
Select and explain. 
A. Help in the conversion of cyanide to thiocyanate, B. Thiosulphate formation, C. Introduction of sulphur in methionine, D. Disulphide bond formation between two adjacent peptide
C: Introduction of sulphur in methionine
Answer: C: Introduction of sulphur in methionine 
Explanation: Ans. C. Introduction of sulphur in methionine.Methionine is an essential amino acid, so it cannot be synthesized from Cysteine.But Sulphur of cysteine is donated by sulphur of methionine.This is called transulfuration reaction.PLP is the coenzyme of transulfuration.The reaction is catalysed by Cystathionine beta Synthase and Cystathionase enzyme.