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

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

Answer: (C)

Cysteine can be synthesized from methionine in adults through the trans-sulfuration pathway, a branch of sulfur amino acid metabolism. In this route, methionine is first involved in the methionine cycle to form homocysteine. From homocysteine, the trans-sulfuration pathway channels it with serine to produce cystathionine via the enzyme cystathionine beta-synthase, with pyridoxal phosphate (vitamin B6) as a cofactor. Cystathionine is then cleaved by cystathionine gamma-lyase to yield cysteine, along with other byproducts. This makes cysteine semi-essential because, when this pathway functions, the body can generate cysteine endogenously from methionine. This process is distinct from glycolysis, the Krebs (TCA) cycle, and the urea cycle.