Explanation by the double-metal-ion mechanism of catalysis for the differential metal ion effects on the cleavage rates of 5′-oxy and 5′-thio substrates by a hammerhead ribozyme

In a previous examination using natural all-RNA substrates that contained either a 5'-oxy or 5'-thio leaving group at the cleavage site, we demonstrated that (i) the attack by the 2'-oxygen at C17 on the phosphorus atom is the rate-limiting step only for the substrate that contains a 5'-thio group (R11S) and (ii) the departure of the 5' leaving group is the rate-limiting step for the natural all-RNA substrate (R11O) in both nonenzymatic and hammerhead ribozyme-catalyzed reactions; the energy diagrams for these reactions were provided in our previous publication, In this report we found that the rate of cleavage of R11O by a hammerhead ribozyme was enhanced 14-fold when Mg2+ ions were replaced by Mn2+ ions, whereas the rate of cleavage of R11S was enhanced only 2.2-fold when Mg2+ ions were replaced by Mn2+ ions. This result appears to be exactly the opposite of that predicted from the direct coordination of the metal ion with the leaving 5'-oxygen, because a switch in metal ion specificity was not observed with the 5'-thio substrate. However, our quantitative analyses based on the previously provided energy diagram indicate that this result is in accord with the double-metal-ion mechanism of catalysis.