▁mut agen esis ▁and ▁ex ogen ous ▁base ▁rescue . ▁ ▁Asp - 3 8 ▁was ▁mut ated ▁to ▁G ly , ▁null ifying ▁catal yt ic ▁activity , ▁and ▁ex ogen ous ▁rescue ▁was ▁attempted ▁with ▁car box yl ates ▁of ▁varying ▁size ▁and ▁mol ar ity . ▁ ▁By ▁calculating ▁the ▁concentration ▁of ▁base ▁needed ▁for ▁full ▁rescue , ▁the ▁H ersch lag ▁group ▁determined ▁the ▁effective ▁mol ar ity ▁of ▁Asp - 3 8 ▁in ▁K SI ▁( 6 4 0 0 ▁M ). ▁ ▁Thus , ▁Asp - 3 8 ▁is ▁critical ▁for ▁K SI ▁catal ysis . ▁ ▁Sig ala ▁et ▁al . ▁found ▁that ▁sol vent ▁ex clusion ▁and ▁replacement ▁by ▁the ▁remote ▁hydro ph ob ic ▁ster oid ▁rings ▁neg lig ibly ▁alter ▁the ▁electro static ▁environment ▁within ▁the ▁K SI ▁o xy an ion ▁hole . ▁ ▁In ▁addition , ▁lig and ▁binding ▁does ▁not ▁gross ly ▁alter ▁the ▁conform ations ▁of ▁back bone ▁and ▁side ▁chain ▁groups ▁observed ▁in ▁X - ray ▁structures ▁of ▁P I ▁K SI . ▁However , ▁N MR ▁and ▁U V ▁studies ▁suggest ▁that ▁ster oid ▁binding ▁restrict s ▁the ▁mot ions ▁of ▁several ▁active - site ▁groups , ▁including ▁T yr - 1 6 . ▁ ▁Rec ently , ▁the ▁H ersch lag ▁group ▁proposed ▁that ▁remote ▁binding ▁of ▁hydro ph ob ic ▁regions ▁of ▁the ▁substr ate ▁to ▁dist al ▁port ions ▁of ▁the ▁active ▁site ▁contribute ▁to ▁K SI ▁catal ysis ▁( > 5 ▁k cal