Catalytic role of enzymes: short strong H-bond-induced partial proton shuttles and charge redistributions.

A two-step reaction mechanism (catalyzed alternatively by acid and base) with partial proton shuttles and charge redistributions promoted by short strong H bonds (SSHBs) (playing a dual role as an amphi-acid/base catalyst) is proposed to explain the enormous rate enhancement observed in enzymatic reactions involving carbanion intermediates. The SSHBs in the two-step reactions are found to be responsible for enhancing enzyme-substrate interactions in favor of the transition state structure over that of reactant. The detailed quantum theoretical studies of ketosteroid isomerase provide evidence of assisting roles of SSHB in enzymatic activity. The understanding of the two-step reaction mechanism would be a useful aid in designing novel functional enzymes and abzymes.