 Pyrophosphate from PRPP is an excellent leaving group, so little chemical assistance is needed to promote catalysis.Rather, the primary function of the enzyme appears to be bringing the reactants together appropriately and preventing the wrong reaction, such as hydrolysis.Besides having their respective catalytic abilities, the two domains also coordinate with one another to ensure that all the ammonia produced from glutamine is transferred to PRPP and no other nucleophile than ammonia attacks PRPP.