D

Choline is a quaternary amine nutrient involved in membrane phospholipid and acetylcholine synthesis, but it is not incorporated into proteins as an α-amino acid. Glycine, hydroxy-proline (a post-translationally modified form of proline found in collagen), and glutamic acid are all bona-fide amino acids [Vlădescu, 1975, PMID 812113; Sheard & Zeisel, 1989, PMID 2520248].