PylB from M. barkeri

[77] Quitterer F., List A., Eisenreich W., Bacher A., Groll M. 
Crystal Structure of Methylornithine Synthase (PylB): Insights into the Pyrrolysine Biosynthesis
Angew. Chem. Int. Ed., 2012, 51, 1339-42, PDF

The biosynthetic pathway of the recently discovered 22nd amino acid, pyrrolysine, starts with an isomerization of lysine to methylornithine, catalyzed by PylB. The X-ray crystal structure of PylB is determined and shows it has a TIM barrel fold. The sealed central cavity contains a [4Fe-4S] cluster, S-adenosylmethionine (SAM), and methylornithine, whose 2R,3R configuration could be confirmed. The data suggest a fragmentation–recombination mechanism via a glycyl radical intermediate.