▁o xygen ▁of ▁the ▁N + 1 ▁base ▁to ▁act ▁as ▁a ▁leaving ▁group ▁. ▁ ▁In ▁comparison , ▁R N ase ▁A , ▁a ▁protein ▁that ▁catal y zes ▁the ▁same ▁reaction , ▁uses ▁a ▁coordin ating ▁hist id ine ▁and ▁l ys ine ▁to ▁act ▁as ▁a ▁base ▁to ▁attack ▁the ▁ph osph ate ▁back bone . ▁ ▁Like ▁many ▁protein ▁en zym es ▁metal ▁binding ▁is ▁also ▁critical ▁to ▁the ▁function ▁of ▁many ▁rib oz ym es . ▁O ften ▁these ▁interactions ▁use ▁both ▁the ▁ph osph ate ▁back bone ▁and ▁the ▁base ▁of ▁the ▁nucle ot ide , ▁causing ▁dr astic ▁conform ational ▁changes . ▁ ▁There ▁are ▁two ▁mechanism ▁classes ▁for ▁the ▁cle av age ▁of ▁ph osph odi ester ▁back bone ▁in ▁the ▁presence ▁of ▁metal . ▁In ▁the ▁first ▁mechanism , ▁the ▁internal ▁ 2 ’ - ▁O H ▁group ▁attacks ▁ph osph orus ▁center ▁in ▁a ▁SN 2 ▁mechanism . ▁Metal ▁ ions ▁promote ▁this ▁reaction ▁by ▁first ▁coordin ating ▁the ▁ph osph ate ▁o xygen ▁and ▁later ▁st abling ▁the ▁o xy an ion . ▁The ▁second ▁mechanism ▁also ▁follows ▁a ▁SN 2 ▁disp lacement , ▁but ▁the ▁nucle oph ile ▁comes ▁from ▁water ▁or ▁ex ogen ous ▁hydro x yl ▁groups ▁rather ▁than ▁R NA ▁itself . ▁The ▁smallest ▁rib oz yme ▁is ▁U U U , ▁which ▁can ▁promote ▁the ▁cle av age ▁between ▁G ▁and ▁A ▁of ▁the ▁G AA A ▁t et ran uc le ot ide ▁via ▁the ▁first