The purine ring is constructed on phosphoribose.The 5-membered imidazole ring is first synthesized. Successive reactions result in the formation of IMP , which is a complete purine nucleotide. IMP occurs at a fork in the pathway of purine biosynthesis. Separate pathways lead to the formation of AMP and GMP . The first and committed step in purine is catalyzed by glutamine PRPP amidotransferase. This reaction is inhibited by AMP and GMP (and several purine nucleotide analogues)and is activated by PRPP .