Mechanism of metal-promoted catalysis of nucleic acid hydrolysis by Escherichia coli ribonuclease H - Use of inert chromium complexes to evaluate hydrogen bonding and electrostatic stabilization of the transition state

Catalytic activation of Escherichia coli ribonuclease H by a series of inert chromium complexes [Cr(NH3)(6-x)(H2O)(x)](3+) (x=0-6) that bear water and ammine ligands in well-defined geometries in the inner coordination shell has been examined. Such complexes are observed to function by transition state stabilization. The importance of hydrogen bonding and electrostatics to catalytic activation of this reaction were quantitatively evaluated. The availability of [Cr(NH3)(6x)(H2O)(x)](3+) complexes of varying coordination geometry also affords a probe of the preferred structural arrangement for hydrogen-bonding interactions. Under the solution conditions employed, a facial array of bound water molecules is required to promote catalysis, as expected from comparison with the ligation of the enzyme-bound Mg2+-cofactor. These results exclude a structural role for the essential metal cofactor. Hydrogen bonding appears to be the dominant stabilizing interaction. In the absence of bound water ligands (for example, in the specific cases of Cr(NH3)(6)(3+) and Co(NH3)(6)(3+)), hydrogen bond stabilization is precluded: however, catalysis is observed as a result of the increased positive charge on the complex. Apparently the trivalent charge offsets the poorer hydrogen bonding abilities of the ammine ligands.