ut am ate ▁to ▁form ▁AD P ▁and ▁an ▁a cy l - ph osph ate ▁intermediate , ▁ γ - gl ut am yl ▁ph osph ate , ▁which ▁react s ▁with ▁am mon ia , ▁forming ▁gl ut am ine ▁and ▁in organ ic ▁ph osph ate . ▁AD P ▁and ▁Pi ▁do ▁not ▁dis soci ate ▁until ▁am mon ia ▁bind s ▁and ▁gl ut am ine ▁is ▁released . ▁ ▁ATP ▁bind s ▁first ▁to ▁the ▁top ▁of ▁the ▁active ▁site ▁near ▁a ▁c ation ▁binding ▁site , ▁while ▁gl ut am ate ▁bind s ▁near ▁the ▁second ▁c ation ▁binding ▁site ▁at ▁the ▁bottom ▁of ▁the ▁active ▁site . ▁The ▁presence ▁of ▁AD P ▁causes ▁a ▁conform ational ▁shift ▁in ▁G S ▁that ▁stabil izes ▁the ▁ γ - gl ut am yl ▁ph osph ate ▁mo iety . ▁ ▁Am mon ium ▁bind s ▁strongly ▁to ▁G S ▁only ▁if ▁the ▁a cy l - ph osph ate ▁intermediate ▁is ▁present . ▁Am mon ium , ▁rather ▁than ▁am mon ia , ▁bind s ▁to ▁G S ▁because ▁the ▁binding ▁site ▁is ▁polar ▁and ▁exposed ▁to ▁sol vent . ▁ ▁In ▁the ▁second ▁step , ▁de pro ton ation ▁of ▁am mon ium ▁allows ▁am mon ia ▁to ▁attack ▁the ▁intermediate ▁from ▁its ▁nearby ▁site ▁to ▁form ▁gl ut am ine . ▁Ph osph ate ▁leaves ▁through ▁the ▁top ▁of ▁the ▁active ▁site , ▁while ▁gl ut am ine ▁leaves ▁through ▁the ▁bottom ▁( between ▁two ▁rings