▁The ▁AP ▁site ▁is ▁then ▁further ▁stabil ized ▁through ▁hydro gen ▁bond ing ▁of ▁the ▁ph osph ate ▁group ▁ 5 ´ ▁to ▁the ▁AP ▁site ▁with ▁As n 1 7 4 , ▁As n 2 1 2 , ▁His 3 0 9 , ▁and ▁the ▁M g 2 + ▁ion ▁while ▁its ▁or phan ▁base ▁partner ▁is ▁stabil ized ▁through ▁hydro gen ▁bond ing ▁with ▁Met 2 7 0 . ▁The ▁ph osph ate ▁group ▁ 3 ' ▁to ▁the ▁AP ▁site ▁is ▁stabil ized ▁through ▁hydro gen ▁bond ing ▁to ▁Arg 1 7 7 . ▁Meanwhile , ▁an ▁Asp 2 1 0 ▁in ▁the ▁active ▁site , ▁which ▁is ▁made ▁more ▁re active ▁due ▁to ▁the ▁increase ▁in ▁its ▁p K a ▁( or ▁the ▁negative ▁log ▁of ▁acid ▁dis soci ation ▁constant ) ▁caused ▁through ▁its ▁stabil ization ▁through ▁its ▁hydro gen ▁bond ing ▁between ▁As n 6 8 ▁and ▁As n 2 1 2 , ▁activ ates ▁the ▁nucle oph ile ▁that ▁attacks ▁and ▁cle aves ▁the ▁ph osph odi ester ▁back bone ▁and ▁probably ▁results ▁in ▁the ▁observed ▁maximal ▁A PE 1 ▁activity ▁at ▁a ▁p H ▁of ▁ 7 . 5 . ▁ ▁Mechan ism ▁The ▁A PE 1 ▁en zym e ▁creates ▁a ▁nick ▁in ▁the ▁ph osph odi ester ▁back bone ▁at ▁an ▁ab as ic ▁( bas eless ) ▁site ▁through ▁a ▁simple ▁a cy l ▁substitution ▁mechanism . ▁First , ▁the ▁Asp 2 1 0 ▁resid ue ▁in ▁the ▁active ▁site ▁de pro