In adults, cysteine can be synthesized from methionine via which metabolic pathway?

• A. Trans-sulfuration pathway
• B. Urea cycle
• C. Glycolysis
• D. Beta-oxidation

Answer: (A)

The key idea is that adults can make cysteine from methionine through the trans-sulfuration pathway. In this route, methionine is converted through a series of steps that lead to homocysteine, which then combines with serine to form cystathionine. Cystathionine is subsequently cleaved to yield cysteine (with enzymes that require vitamin B6 as a cofactor). This pathway links methionine metabolism to cysteine production, providing a route for nonessential cysteine synthesis in adults. The other options aren’t involved in making cysteine from methionine—the urea cycle disposes of ammonia, while glycolysis and beta-oxidation are pathways for carbohydrate and fatty acid metabolism, not cysteine biosynthesis.