. ▁ ▁The ▁middle ▁of ▁the ▁b if unnel ▁contains ▁two ▁sites ▁in ▁which ▁div al ent ▁c ations ▁bind ▁( M n + 2 ▁or ▁M g + 2 ). ▁One ▁c ation ▁binding ▁site ▁is ▁involved ▁in ▁ph osph ory l ▁transfer ▁of ▁ATP ▁to ▁gl ut am ate , ▁while ▁the ▁second ▁stabil izes ▁active ▁G S ▁and ▁helps ▁with ▁the ▁binding ▁of ▁gl ut am ate . ▁▁▁ ▁H ydro gen ▁bond ing ▁and ▁hydro ph ob ic ▁interactions ▁hold ▁the ▁two ▁rings ▁of ▁G S ▁together . ▁Each ▁sub unit ▁poss esses ▁a ▁C - ter minus ▁and ▁an ▁N - ter minus ▁in ▁its ▁sequence . ▁The ▁C - ter minus ▁( hel ical ▁th ong ) ▁stabil izes ▁the ▁G S ▁structure ▁by ▁inserting ▁into ▁the ▁hydro ph ob ic ▁region ▁of ▁the ▁sub unit ▁across ▁in ▁the ▁other ▁ring . ▁The ▁N - ter minus ▁is ▁exposed ▁to ▁the ▁sol vent . ▁In ▁addition , ▁the ▁central ▁channel ▁is ▁formed ▁via ▁six ▁four - str and ed ▁ β - sheets ▁composed ▁of ▁anti - parallel ▁loops ▁from ▁the ▁twelve ▁sub un its . ▁ ▁Mechan ism ▁▁ ▁G S ▁catal y zes ▁the ▁ATP - dependent ▁cond ens ation ▁of ▁gl ut am ate ▁with ▁am mon ia ▁to ▁yield ▁gl ut am ine . ▁The ▁hydro ly s is ▁of ▁ATP ▁drives ▁ ▁the ▁first ▁step ▁of ▁a ▁two - part , ▁concert ed ▁mechanism . ▁ATP ▁ph osph ory l ates ▁gl