 The source of the carbon and nitrogen atoms of the purine ring, 5 and 4 respectively, come from multiple sources.The amino acid glycine contributes all its carbon (2) and nitrogen (1) atoms, with additional nitrogen atoms from glutamine (2) and aspartic acid (1), and additional carbon atoms from formyl groups (2), which are transferred from the coenzyme tetrahydrofolate as 10-formyltetrahydrofolate, and a carbon atom from bicarbonate (1).Formyl groups build carbon-2 and carbon-8 in the purine ring system, which are the ones acting as bridges between two nitrogen atoms.