▁disease ; ▁whereas , ▁mut ations ▁in ▁the ▁gene ▁encoding ▁the ▁alpha ▁sub unit ▁( H EX A , ▁this ▁gene ) ▁decrease ▁the ▁hydro ly s is ▁of ▁G M 2 ▁gang li os ides , ▁which ▁is ▁the ▁main ▁cause ▁of ▁T ay – S ach s ▁disease . ▁ ▁Function ▁▁ ▁Even ▁though ▁the ▁alpha ▁and ▁beta ▁sub un its ▁of ▁hex os amin id ase ▁A ▁can ▁both ▁cle ave ▁Gal NA c ▁resid ues , ▁only ▁the ▁alpha ▁sub unit ▁is ▁able ▁to ▁hydro ly ze ▁G M 2 ▁gang li os ides . ▁The ▁alpha ▁sub unit ▁contains ▁a ▁key ▁resid ue , ▁Arg - 4 2 4 , ▁which ▁is ▁essential ▁for ▁binding ▁the ▁N - ac et yl - neur aman ic ▁resid ue ▁of ▁G M 2 ▁gang li os ides . ▁▁ ▁The ▁alpha ▁sub unit ▁can ▁hydro ly ze ▁G M 2 ▁gang li os ides ▁because ▁it ▁contains ▁a ▁loop ▁structure ▁consisting ▁of ▁the ▁am ino ▁ac ids : ▁G ly - 2 8 0 , ▁Ser - 2 8 1 , ▁G lu - 2 8 2 , ▁and ▁Pro - 2 8 3 . ▁ ▁The ▁loop ▁is ▁absent ▁in ▁the ▁beta ▁sub unit , ▁but ▁it ▁serves ▁as ▁an ▁ideal ▁structure ▁for ▁the ▁binding ▁of ▁the ▁G M 2 ▁activ ator ▁protein ▁( GM 2 AP ) ▁in ▁the ▁alpha ▁sub unit . ▁ ▁A ▁combination ▁of ▁Arg - 4 2 4 ▁and ▁the ▁am ino ▁ac ids