▁rib ose ▁ph osph ate ▁py roph osph ok in ase , ▁which ▁is ▁activ ated ▁by ▁in organ ic ▁ph osph ate ▁and ▁in activ ated ▁by ▁pur ine ▁rib on uc le ot ides . ▁It ▁is ▁not ▁the ▁committed ▁step ▁to ▁pur ine ▁synth esis ▁because ▁PR PP ▁is ▁also ▁used ▁in ▁p yr im id ine ▁synth esis ▁and ▁salv age ▁path ways . ▁ ▁The ▁first ▁committed ▁step ▁is ▁the ▁reaction ▁of ▁PR PP , ▁gl ut am ine ▁and ▁water ▁to ▁ 5 '- ph osph or ib os yl am ine ▁( P RA ), ▁gl ut am ate , ▁and ▁py roph osph ate ▁- ▁catal yz ed ▁by ▁amid oph osph or ib os yl trans fer ase , ▁which ▁is ▁activ ated ▁by ▁PR PP ▁and ▁in hib ited ▁by ▁A MP , ▁G MP ▁and ▁I MP . ▁ ▁PR PP ▁+ ▁L - G l ut am ine ▁+ ▁H 2 O ▁→ ▁P RA ▁+ ▁L - G l ut am ate ▁+ ▁P Pi ▁ ▁In ▁the ▁second ▁step ▁react ▁P RA , ▁g ly c ine ▁and ▁ATP ▁to ▁create ▁G AR , ▁AD P , ▁and ▁py roph osph ate ▁- ▁catal yz ed ▁by ▁ph osph or ib os yl am ine — gly c ine ▁lig ase ▁( G AR ▁synth et ase ). ▁Due ▁to ▁the ▁chemical ▁l ability ▁of ▁P RA , ▁which ▁has ▁a ▁half - life ▁of ▁ 3 8 ▁seconds