Metal-ion stoichiometry of the HIV-1 RT ribonuclease H domain: evidence for two mutually exclusive sites leads to new mechanistic insights on metal-mediated hydrolysis in nucleic acid biochemistry
Ja. Cowan et al., Metal-ion stoichiometry of the HIV-1 RT ribonuclease H domain: evidence for two mutually exclusive sites leads to new mechanistic insights on metal-mediated hydrolysis in nucleic acid biochemistry, J BIOL I CH, 5(1), 2000, pp. 67-74
Crystallographic studies of the Mn2+-doped RNase H domain of human immunode
ficiency virus type 1 reverse transcriptase (HIV-1 RT) [1] have revealed tw
o bound Mn2+ separated by approximately 4 Angstrom and surrounded by a clus
ter of four conserved carboxylates. Escherichia coli RNase H is structurall
y similar to the RNase H domain of HIV-1 RT, but requires one divalent meta
l cation for its activity [2, 3], implying either that the HIV-1 RT RNase H
domain contrasts in its ability to bind two divalent metal ions, or that t
he crystallographic data reflect specific use of Mn2+ and/or the doping tec
hnique employed. Metal binding stoichiometry has been determined for Mn2+ a
nd the biologically more relevant Mg2+ cation by solution calorimetric stud
ies of native and recombinant p66/p51 HIV-1 RT. Three Mn2+ ions bind to HIV
-1 RT ape-enzyme: one at the DNA polymerase and two at the RNase H catalyti
c center, the latter being consistent with crystallographic results. Howeve
r, only one Mg2+ ion is bound in the RNase H catalytic center. Several mech
anistic implications arise from these results, including the possibility of
mutually exclusive Mg2+ binding sites that might be occupied according to
the specific reaction being catalyzed by the multifunctional RNase H domain
. The occurrence of distinct binding stoichiometries for Mg2+ and Mn2+ to m
ultifunctional enzymes has previously been reported [4].