box yl ▁o xygen ▁atom ▁of ▁its ▁ben z oph en one ▁mo iety , ▁fitting ▁between ▁the ▁large ▁and ▁small ▁catal yt ic ▁lo bes ▁of ▁PK A ▁and ▁extending ▁from ▁the ▁inner ▁edge ▁to ▁the ▁outer ▁mouth ▁of ▁the ▁active - site ▁ATP - binding ▁c left ▁of ▁the ▁en zym e . ▁The ▁c left ' s ▁g ly c ine - rich ▁loop ▁and ▁small ▁lo be ▁allow ▁for ▁tight , ▁induced - fit ▁binding . ▁ ▁Each ▁sub site ▁of ▁the ▁ATP - binding ▁pocket ▁in ▁PK A ▁is ▁occupied ▁by ▁a ▁subst itu ent ▁of ▁the ▁bal an ol ▁mole c ule ▁as ▁observed ▁in ▁studies ▁of ▁recom bin ant ▁mouse ▁catal yt ic ▁sub unit ▁of ▁PK A . ▁The ▁ad en ine ▁sub site ▁of ▁PK A ▁has ▁hydro ph ob ic ▁components ▁and ▁has ▁the ▁potential ▁to ▁don ate ▁and ▁accept ▁electrons ▁to ▁make ▁hydro gen ▁b onds ▁with ▁one ▁or ▁two ▁plan ar ▁cy clic ▁rings . ▁Specifically , ▁the ▁back bone ▁nit ro gen ▁of ▁Val 1 2 3 ▁and ▁the ▁car b ony l ▁o xygen ▁atom ▁of ▁G lu 1 2 1 ▁can ▁form ▁hydro gen ▁b onds ▁with ▁bal an ol ' s ▁single ▁hydro x yl ▁group ▁of ▁the ▁ 4 - h ydro x yl ▁ben zo yl ▁mo iety . ▁In ▁the ▁ATP ▁mole c ule , ▁the ▁Val 1 2 3 ▁am ide ▁hydro gen ▁atom ▁accepts ▁electrons ▁from ▁ATP ' s ▁pur ine ▁ring