▁bond ▁at ▁ Δ 5 , ▁but ▁cannot ▁grow ▁on ▁e pi andro ster one , ▁which ▁la cks ▁a ▁double ▁bond ▁at ▁ Δ 5 , ▁indicating ▁that ▁C . ▁test oster oni ▁K SI ▁is ▁responsible ▁for ▁transfer ▁of ▁the ▁double ▁bond ▁from ▁ Δ 5 ▁to ▁ Δ 4 ▁and ▁transfer ▁of ▁the ▁double ▁bond ▁by ▁hydro gen ation ▁at ▁ Δ 5 ▁and ▁following ▁de h ydro gen ation ▁at ▁ Δ 4 ▁is ▁not ▁possible . ▁ ▁Model ▁en zym e ▁▁ ▁K SI ▁has ▁been ▁used ▁as ▁a ▁model ▁system ▁to ▁test ▁different ▁theories ▁to ▁explain ▁how ▁en zym es ▁achieve ▁their ▁catal yt ic ▁efficiency . ▁Low - bar rier ▁hydro gen ▁b onds ▁and ▁unusual ▁p K a ▁values ▁for ▁the ▁catal yt ic ▁resid ues ▁have ▁been ▁proposed ▁as ▁the ▁basis ▁for ▁the ▁fast ▁action ▁of ▁K SI . ▁Ger lt ▁and ▁G ass man ▁proposed ▁the ▁formation ▁of ▁unus ually ▁short , ▁strong ▁hydro gen ▁b onds ▁between ▁K SI ▁ox an ion ▁hole ▁and ▁the ▁reaction ▁intermediate ▁as ▁a ▁means ▁of ▁catal yt ic ▁rate ▁enh ancement . ▁In ▁their ▁model , ▁high - energy ▁states ▁along ▁the ▁reaction ▁coordinate ▁are ▁specifically ▁stabil ized ▁by ▁the ▁formation ▁of ▁these ▁b onds . ▁Since ▁then , ▁the ▁catal yt ic ▁role ▁of ▁short , ▁strong ▁hydro gen ▁b onds ▁has ▁been ▁deb ated . ▁Another ▁proposal ▁explaining ▁en zym e ▁catal ysis ▁tested ▁through ▁K SI ▁is ▁the ▁geomet rical ▁complement ar ity