- OH . ▁ ▁Hum ans ▁have ▁two ▁AP ▁end on uc leases , ▁A PE 1 ▁and ▁A PE 2 . ▁A PE 1 ▁exhib its ▁robust ▁AP - end on uc lease ▁activity , ▁which ▁accounts ▁for ▁> 9 5 % ▁of ▁the ▁total ▁cell ular ▁activity , ▁and ▁A PE 1 ▁is ▁considered ▁to ▁be ▁the ▁major ▁AP ▁end on uc lease ▁in ▁human ▁cells . ▁ ▁Human ▁AP ▁end on uc lease ▁( AP E 1 ), ▁like ▁most ▁AP ▁end on uc leases , ▁is ▁of ▁class ▁II ▁and ▁requires ▁an ▁M g 2 + ▁in ▁its ▁active ▁site ▁in ▁order ▁to ▁carry ▁out ▁its ▁role ▁in ▁base ▁exc ision ▁repair . ▁The ▁ye ast ▁hom olog ▁of ▁this ▁en zym e ▁is ▁AP N 1 . ▁ ▁Human ▁AP ▁End on uc lease ▁ 2 ▁( AP E 2 ), ▁like ▁most ▁AP ▁end on uc leases , ▁is ▁also ▁of ▁class ▁II . ▁ ▁The ▁ex on uc lease ▁activity ▁of ▁A PE 2 ▁is ▁strongly ▁dependent ▁upon ▁metal ▁ ions . ▁However , ▁A PE 2 ▁was ▁more ▁than ▁ 5 - fold ▁more ▁active ▁in ▁the ▁presence ▁of ▁mang an ese ▁than ▁of ▁mag nes ium ▁ ions . ▁ ▁The ▁conser ved ▁domains ▁involved ▁in ▁catal yt ic ▁activity ▁are ▁located ▁at ▁the ▁N - term inal ▁part ▁of ▁both ▁A PE 1 ▁and ▁A PE 2 . ▁In ▁addition , ▁the ▁A PE 2 ▁protein ▁has ▁a ▁C - term inal ▁extension