HIV-1 reverse transcriptase (RT) contains a C-terminal ribonuclease H (RH) domain on its p66 subunit that can be expressed
as a stable, although inactive protein. Recent studies of several RH enzymes demonstrate that substrate binding plays a major
role in the creation of the active site. In the absence of substrate, the C-terminal helix E of the RT RNase H domain is dynamic,
characterized by severe exchange broadening of its backbone amide resonances, so that the solution characterization of this
region of the protein has been limited. Nuclear magnetic resonance studies of 13C-labeled RH as a function of experimental conditions reveal that the δ1 methyl resonance of Ile556, located in a short, random
coil segment following helix E, experiences a large 13C shift corresponding to a conformational change of Ile556 that results from packing of helix E against the central β-sheet.
This shift provides a useful basis for monitoring the effects of various ligands on active site formation. Additionally, we
report that the RNase H complexes formed with one or both divalent ions can be individually observed and characterized using
diamagnetic Zn2+ as a substitute for Mg2+. Ordering of helix E results specifically from the interaction with the lower affinity binding to the A divalent ion site.
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[Show abstract][Hide abstract] ABSTRACT: Most phosphate processing enzymes require Mg2+ as a cofactor to catalyze nucleotide cleavage and transfer reactions. Ca2+ ions inhibit many of these enzymatic activities, despite Ca2+ and Mg2+ having comparable binding affinities and overall biological abundances. Here we study the molecular details of the calcium inhibition mechanism for phosphodiester cleavage, an essential reaction in the metabolism of nucleic acids and nucleotides, by comparing Ca2+ and Mg2+ catalyzed reactions. We study the functional roles of the specific metal ion sites A and B in enabling the catalytic cleavage of an RNA/DNA hybrid substrate by B. halodurans Ribonuclease (RNase) H1 using hybrid quantum-mechanics/molecular mechanics (QM/MM) free energy calculations. We find that Ca2+ substitution of either of the two active-site Mg2+ ions increases the height of the reaction barrier and thereby abolishes the catalytic activity. Remarkably, Ca2+ at the A site is inactive also in Mg2+-optimized active-site structures along the reaction path, whereas Mg2+ substitution recovers activity in Ca2+-optimized structures. Geometric changes resulting from Ca2+ substitution at metal ion site A may thus be a secondary factor in the loss of catalytic activity. By contrast, at metal ion site B geometry plays a more important role, with only a partial recovery of activity after Mg2+ substitution in Ca2+-optimized structures. Ca2+-substitution also leads to a change in mechanism, with proton transfer from the water nucleophile requiring a closer approach to the scissile phosphate, which in turn increases the barrier. As a result, Ca2+ is less efficient in activating the water. As a likely cause for the different reactivities of Mg2+ and Ca2+ ions in site A, we identify differences in charge transfer and the associated decrease in the pKa of the oxygen nucleophile attacking the phosphate group.
Preview · Article · Feb 2014 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: HIV-1 reverse transcriptase (RT), a critical enzyme of the HIV life cycle and an important drug target, undergoes complex and largely uncharacterized conformational rearrangements that underlie its asymmetric folding, dimerization and subunit-selective ribonuclease H domain (RH) proteolysis. In the present article we have used a combination of NMR spectroscopy, small angle X-ray scattering and X-ray crystallography to characterize the p51 and p66 monomers and the conformational maturation of the p66/p66' homodimer. The p66 monomer exists as a loosely structured molecule in which the fingers/palm/connection, thumb and RH substructures are connected by flexible (disordered) linking segments. The initially observed homodimer is asymmetric and includes two fully folded RH domains, while exhibiting other conformational features similar to that of the RT heterodimer. The RH' domain of the p66' subunit undergoes selective unfolding with time constant ∼6.5 h, consistent with destabilization due to residue transfer to the polymerase' domain on the p66' subunit. A simultaneous increase in the intensity of resonances near the random coil positions is characterized by a similar time constant. Consistent with the residue transfer hypothesis, a construct of the isolated RH domain lacking the two N-terminal residues is shown to exhibit reduced stability. These results demonstrate that RH' unfolding is coupled to homodimer formation.
Full-text · Article · Feb 2014 · Nucleic Acids Research
[Show abstract][Hide abstract] ABSTRACT: HIV-1 reverse transcriptase utilizes a metamorphic polymerase domain that is able to adopt two alternate structures that fulfill catalytic and structural roles, thereby minimizing its coding requirements. This ambiguity introduces folding challenges that are met by a complex maturation process. We have investigated this conformational maturation using NMR studies of methyl-labeled RT for the slower processes in combination with molecular dynamics simulations for rapid processes. Starting from an inactive conformation, the p66 precursor undergoes a unimolecular isomerization to a structure similar to its active form, exposing a large hydrophobic surface that facilitates initial homodimer formation. The resulting p66/p66' homodimer exists as a conformational heterodimer, after which a series of conformational adjustments on different time scales can be observed. Formation of the inter-subunit RH:thumb' interface occurs at an early stage, while maturation of the connection' and unfolding of the RH' domains are linked and occur on a much slower time scale.