This domain is found at the N-terminal end of Pyrrolysine biosynthesis protein PylD from Methanosarcina acetivorans, a dehydrogenase that catalyses the final step in the biosynthesis of the amino acid pyrrolysine [1,2].

1: Structure and reaction mechanism of pyrrolysine synthase (PylD). Quitterer F, Beck P, Bacher A, Groll M; Angew Chem Int Ed Engl. 2013;52:7033-7037. PMID:23720358

2: The formation of pyrroline and tetrahydropyridine rings in amino acids catalyzed by pyrrolysine synthase (PylD). Quitterer F, Beck P, Bacher A, Groll M; Angew Chem Int Ed Engl. 2014;53:8150-8153. PMID:24916332