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 aminoacids is a component of Thioredoxin reductase? 
Select and explain. 
A. Selenocysteine, B. Cysteine, C. Methionine, D. Homocysteine
A: Selenocysteine
Answer: A: Selenocysteine 
Explanation: Ans. is 'a' i.e., Selenocysteine Selenocysteine is present at the active site of some enzymes that catalyze redox reactions, e.g. thioredoxin reductase, glutathione peroxidase, and the deiodinase (conves thyroxin to triiothyronine).