Practice question

Pyrrolysine biosynthesis begins with two molecules of L-lysine, confirmed by isotopic labeling tracking carbon skeletons and nitrogen atoms into product. Pathway employs radical SAM chemistry to rearrange carbon chain. One lysine converted via PylB to methyl-ornithine retaining backbone, second lysine provides side-chain amine forming isopeptide linkage catalyzed by PylC ligase. Final cyclization and oxidation by PylD yields pyrroline heterocycle fused to lysine. This route ensures direct link to abundant lysine pool, avoiding complex precursors, explaining why lysine auxotrophs cannot produce pyrrolysine. Organisms synthesizing pyrrolysine therefore regulate lysine biosynthesis and pyrrolysine enzymes coordinately for metabolic economy.