/ m ol ). ▁ ▁A ▁ 4 - ring ▁substr ate ▁react ed ▁ 2 7 , 0 0 0 ▁times ▁faster ▁than ▁a ▁single ▁ring ▁substr ate ▁indicating ▁the ▁importance ▁of ▁dist al ▁active ▁site ▁binding ▁mot ifs . ▁ ▁This ▁activity ▁ratio ▁pers ists ▁throughout ▁mut agen esis ▁of ▁resid ues ▁important ▁to ▁o xy an ion ▁hole ▁stabil ization , ▁imp lying ▁that ▁dist al ▁binding ▁is ▁what ▁accounts ▁for ▁the ▁large ▁a for ement ioned ▁react ivity ▁difference . ▁ ▁N umer ous ▁physical ▁changes ▁occur ▁upon ▁ster oid ▁binding ▁within ▁the ▁K SI ▁active ▁site . ▁In ▁the ▁free ▁en zym e ▁an ▁ordered ▁water ▁mole c ule ▁is ▁position ed ▁within ▁hydro gen - b ond ing ▁distance ▁of ▁T yr - 1 6 ▁( the ▁P I ▁equivalent ▁of ▁T I ▁K SI ▁T yr - 1 4 ) ▁and ▁Asp - 1 0 3 ▁( the ▁P I ▁equivalent ▁of ▁T I ▁K SI ▁Asp - 9 9 ). ▁This ▁and ▁additional ▁dis ordered ▁water ▁mole cules ▁present ▁within ▁the ▁un lig and ed ▁active ▁site ▁are ▁disp la ced ▁upon ▁ster oid ▁binding ▁and ▁are ▁substantial ly ▁ex cluded ▁by ▁the ▁dense ▁const ell ation ▁of ▁hydro ph ob ic ▁resid ues ▁that ▁pack ▁around ▁the ▁bound , ▁hydro ph ob ic ▁ster oid ▁ske leton . ▁ ▁As ▁stated ▁above , ▁the ▁degree ▁to ▁which ▁various ▁factors ▁contribute ▁to ▁K SI ▁catal ysis ▁is ▁still ▁deb ated . ▁ ▁Function ▁ ▁K