Figure 32.5 De novo purine biosynthesis. (1) Glycine is coupled to the amino group of phosphoribosylamine. (2) N¹⁰-Formyltetrahydrofolate (THF) transfers a formyl group to the amino group of the glycine residue. (3) The inner amide group is phosphorylated and then converted into an amidine by the addition of ammonia derived from glutamine. (4) An intramolecular coupling reaction forms a five-membered imidazole ring. (5) Bicarbonate adds first to the exocyclic amino group and then to a carbon atom of the imidazole ring. (6) The imidazole carboxylate is phosphorylated, and the phosphoryl group is displaced by the amino group of aspartate. (7) Fumarate leaves, followed by (8) the addition of a second formyl group from N¹⁰-formyl tetrahydrofolate. (9) Cyclization completes the synthesis of inosinate, a purine nucleotide. Abbreviation: P stands for phosphoryl group.