Several small ribozymes carry out self-cleavage at a specific phosphodiester bond to yield 2′,3′-cyclic phosphate and 5′-hydroxyl termini. Prior mechanistic and structural studies on the HDV ribozymes led to the proposal that the pKa of C75 is shifted toward neutrality, making it an effective general acid. Recent mechanistic studies on the hairpin ribozyme have led to models in which protonation of G8 is required for phosphodiester cleavage, either for general acid catalysis or for electrostatic stabilization. Inspection of recent crystal structures of the hairpin ribozyme, including a complex with a vanadate transition state mimic, suggests an alternative model involving general acid-base catalysis with G8 serving as the general base and A38 as the general acid. This model is consistent with the literature on the hairpin ribozyme, including pH-rate profiles of wild-type and mutant ribozymes and solvent isotope effects. General mechanistic considerations for RNA catalysis suggest that the penalty for having general acids and bases with pKas removed from neutrality is not as severe as expected. These considerations suggest that general acid-base catalysis may be a common mechanistic strategy of RNA enzymes.
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