, ▁which ▁is ▁not ▁present ▁in ▁A PE 1 , ▁but ▁can ▁also ▁be ▁found ▁in ▁hom olog s ▁of ▁human ▁A PE 2 ▁such ▁as ▁AP N 2 ▁prote ins ▁of ▁S . ▁c ere vis iae ▁and ▁S . ▁pom be . ▁ ▁Str ucture ▁of ▁A PE 1 ▁▁ ▁A PE 1 ▁contains ▁several ▁am ino ▁acid ▁resid ues ▁that ▁enable ▁it ▁to ▁react ▁select ively ▁with ▁AP ▁sites . ▁Three ▁A PE 1 ▁resid ues ▁( Arg 7 3 , ▁A la 7 4 , ▁and ▁L ys 7 8 ) ▁contact ▁three ▁consecutive ▁DNA ▁ph osph ates ▁on ▁the ▁str and ▁opposite ▁the ▁one ▁containing ▁the ▁AP ▁site ▁while ▁T yr 1 2 8 ▁and ▁G ly 1 2 7 ▁span ▁and ▁w iden ▁the ▁minor ▁gro ove , ▁anch oring ▁the ▁DNA ▁for ▁the ▁extreme ▁k inking ▁caused ▁by ▁the ▁interaction ▁between ▁positive ▁resid ues ▁found ▁in ▁four ▁loops ▁and ▁one ▁α - hel ix ▁and ▁the ▁negative ▁ph osph ate ▁groups ▁found ▁in ▁the ▁ph osph odi ester ▁back bone ▁of ▁DNA . ▁ ▁This ▁extreme ▁k inking ▁forces ▁the ▁bas eless ▁portion ▁of ▁DNA ▁into ▁A PE 1 ' s ▁active ▁site . ▁This ▁active ▁site ▁is ▁border ed ▁by ▁P he 2 6 6 , ▁Tr p 2 8 0 , ▁and ▁Le u 2 8 2 , ▁which ▁pack ▁tight ly ▁with ▁the ▁hydro ph ob ic ▁side ▁of ▁the ▁AP ▁site , ▁disc rimin ating ▁against ▁sites ▁that ▁do ▁have ▁bases .