▁argument ▁to ▁suggest ▁the ▁intermediate ▁exists ▁as ▁the ▁en olate . ▁ ▁The ▁general ▁base ▁Asp - 3 8 ▁abstract s ▁a ▁pro ton ▁from ▁position ▁ 4 ▁( alpha ▁to ▁the ▁car b ony l , ▁next ▁to ▁the ▁double ▁bond ) ▁of ▁the ▁ster oid ▁ring ▁to ▁form ▁an ▁en olate ▁( the ▁rate - lim iting ▁step ) ▁that ▁is ▁stabil ized ▁by ▁the ▁hydro gen ▁bond ▁don ating ▁T yr - 1 4 ▁and ▁Asp - 9 9 . ▁T yr - 1 4 ▁and ▁Asp - 9 9 ▁are ▁position ed ▁deep ▁within ▁the ▁hydro ph ob ic ▁active ▁site ▁and ▁form ▁a ▁so - called ▁ox an ion ▁hole . ▁Pro ton ated ▁Asp - 3 8 ▁then ▁trans fers ▁its ▁pro ton ▁to ▁position ▁ 6 ▁of ▁the ▁ster oid ▁ring ▁to ▁complete ▁the ▁reaction . ▁ ▁Although ▁the ▁mechan istic ▁steps ▁of ▁the ▁reaction ▁are ▁not ▁disput ed , ▁the ▁contributions ▁of ▁various ▁factors ▁to ▁catal ysis ▁such ▁as ▁electro stat ics , ▁hydro gen ▁bond ing ▁of ▁the ▁o xy an ion ▁hole , ▁and ▁dist al ▁binding ▁effects ▁are ▁discussed ▁below ▁and ▁still ▁deb ated . ▁ ▁The ▁War sh el ▁group ▁applied ▁statistical ▁mechanical ▁computational ▁methods ▁and ▁empir ical ▁val ence ▁bond ▁theory ▁to ▁previous ▁experimental ▁data . ▁ ▁It ▁was ▁determined ▁that ▁electro static ▁pre organ ization - including ▁ion ic ▁resid ues ▁and ▁fixed ▁di pol es ▁within ▁the ▁active ▁site - cont ributes ▁most ▁to ▁K SI ▁catal ysis