L-Lysine is the homologue of L-ornithine, and it too functions as an alkaloid precursor, using pathways analogous to those noted for ornithine.The extra methylene group in lysine means this amino acid participates in forming six-membered piperidine rings, just as ornithine provided fivemembered pyrrolidine rings. As with ornithine, the carboxyl group is lost, the ε-amino nitrogen rather than the α-amino nitrogen is retained, and lysine thus supplies a C₅N building block (Figure 21).