Practice question

PylC functions as ATP-dependent ligase catalyzing second step of pyrrolysine pathway, condensation of (3R)-3-methyl-D-ornithine with second L-lysine molecule via amide bond formation between ornithine carboxylate and lysine epsilon-amino group, producing N6-((2R,3R)-3-methylornithyl)-lysine. Mechanism employs ATP to activate carboxylate as acyl-adenylate intermediate then nucleophilic attack by lysine amine, similar to glutathione synthesis. PylC structure resembles D-alanine-D-alanine ligase fold. Deletion of pylC accumulates methylornithine and ablates pyrrolysine formation preventing amber suppression and methylamine methyltransferase synthesis in Methanosarcina. This condensation generates pseudopeptide intermediate essential for subsequent oxidation cyclization into pyrrolysine heterocycle.