formation , ▁term ed ▁the ▁‘ pre - catal yt ic ’ ▁state , ▁the ▁rib oz yme ▁shows ▁two ▁Sr 2 + ▁binding ▁sites . ▁G 4 3 ▁and ▁G 4 2 ▁interact ▁with ▁one ▁Sr 2 + ▁whereas ▁the ▁second ▁Sr 2 + ▁interact s ▁with ▁the ▁A 4 5 , ▁C 2 3 ▁and ▁G 2 4 . ▁This ▁second ▁Sr 2 + ▁binding ▁site ▁also ▁potentially ▁interact s ▁with ▁the ▁ 2 ’ - OH ▁of ▁the ▁C 2 3 ▁via ▁a ▁water ▁mole c ule . ▁This ▁second ▁binding ▁site ▁explains ▁how ▁P b 2 + ▁could ▁facil itate ▁catal ysis ▁by ▁abstract ing ▁the ▁ 2 - OH ▁pro ton ▁and ▁prepare ▁it ▁for ▁an ▁in - line ▁nucle oph ill ic ▁attack ▁on ▁the ▁sc iss ile ▁ph osph ate . ▁This ▁is ▁also ▁supported ▁by ▁the ▁fact ▁the ▁reaction ▁of ▁the ▁lead zym e ▁is ▁p H ▁dependent . ▁Thus , ▁P b 2 + ▁could ▁be ▁acting ▁as ▁a ▁Lewis ▁acid ▁and ▁activ ating ▁the ▁ 2 - OH ▁of ▁C 2 3 . ▁The ▁cry stal ▁structure ▁is ▁consistent ▁with ▁a ▁two - met al ▁ion ▁mechanism ▁that ▁has ▁been ▁proposed ▁for ▁lead zym e ▁catal ysis . ▁ ▁Le ad ▁to x icity ▁through ▁lead zym e ▁ ▁To xic ▁met als ▁like ▁lead ▁are ▁environmental ▁and ▁health ▁ha z ards ▁and ▁can ▁enter ▁bi ological ▁systems ▁upon ▁expos ure . ▁Le ad ▁is ▁a ▁persistent ▁metal ▁and ▁can ▁accum ulate ▁in