Alchemical Free-Energy Calculations of Watson-Crick and Hoogsteen Base Pairing Interconversion in DNA

Correct Nucleotide Selection Is Confined at the Binding Site of Polymerase Enzymes

DNA polymerases (Pols) add incoming nucleotides (deoxyribonucleoside triphosphate (dNTPs)) to growing DNA strands, a crucial step for DNA synthesis. The insertion of correct (vs incorrect) nucleotides relates to Pols' fidelity, which defines Pols' ability to faithfully replicate DNA strands in a template-dependent manner. We and others have demonstrated that reactant alignment and correct base pairing at the Pols catalytic site are crucial structural features to fidelity. Here, we first used equilibrium molecular simulations to demonstrate that the local dynamics at the protein-DNA interface in the proximity of the catalytic site is different when correct vs incorrect dNTPs are bound to polymerase β (Pol β). Formation and dynamic stability of specific interatomic interactions around the incoming nucleotide influence the overall binding site architecture. This explains why certain Pols' mutants can affect the local catalytic environment and influence the selection of correct vs incorrect nucleotides. In particular, this is here demonstrated by analyzing the interaction network formed by the residue R283, whose mutant R283A has an experimentally measured lower capacity of differentiating correct (G:dCTP) vs incorrect (G:dATP) base pairing in Pol β. We also used alchemical free-energy calculations to quantify the G:dCTP →G:dATP transformation in Pol β wild-type and mutant R283A. These results correlate well with the experimental trend, thus corroborating our mechanistic insights. Sequence and structural comparisons with other Pols from the same family suggest that these findings may also be valid in similar enzymes.

Unveiling the Intermolecular Interactions between Drug 5-Fluorouracil and Watson-Crick/Hoogsteen Base Pairs: A Computational Analysis

The adsorption of 5-fluorouracil (5FU) on Watson-Crick (WC) base pairs and Hoogsteen (HT) base pairs has been studied using the dispersion-corrected density functional theory (DFT). The adsorption, binding energy, and thermochemistry for the drug 5FU on the WC and HT base pairs were determined. The most stable geometries were near planar geometry, and 5FU has a higher preference for WC than HT base pairs. The adsorption energies of 5FU on nucleobase pairs are consistently higher than pristine nucleobase pairs, indicating that nucleobase pair cleavage is less likely during the adsorption of the 5FU drug. The enthalpy change for the formation of 5FU-DNA base pairs is higher than that for the formation of 5FU-nucleobases and is enthalpy-driven. The E gap of AT base pairs is higher, suggesting that their chemical reactivity toward further reaction would be less than that of GC base pairs. The electron density difference (EDD) analysis shows a significant decrease in electron density in aromatic regions on the purine bases (adenine/guanine) compared to the pyrimidine bases. The MESP diagram of the stable 5FU-nucleobase pair complexes shows a directional interaction, with the positive regions in a molecule interacting with the negative region of other molecules. The atoms in molecule analysis show that the ρ(r) values of C=O···H-N are higher than those of N···H/N-H···O. The N···H intermolecular bonds between the base pair/drug and nucleobases are weak, closed shell interactions and are electrostatic in nature. The noncovalent interaction analysis shows that several new spikes are engendered along with an increase in their strength, which indicates that the H-bonding interactions are stronger and play a dominant role in stabilizing the complexes. Energy decomposition analysis shows that the drug-nucleobase pair complex has a marginal increase in the electrostatic contributions compared to nucleobase pair complexes.

Elucidating the Molecular Determinants of the Binding Modes of a Third-Generation HIV-1 Integrase Strand Transfer Inhibitor: The Importance of Side Chain and Solvent Reorganization

The first- and second-generation clinically used HIV-1 integrase (IN) strand transfer inhibitors (INSTIs) are key components of antiretroviral therapy (ART), which work by blocking the integration step in the HIV-1 replication cycle that is catalyzed by a nucleoprotein assembly called an intasome. However, resistance to even the latest clinically used INSTIs is beginning to emerge. Developmental third-generation INSTIs, based on naphthyridine scaffolds, are promising candidates to combat drug-resistant viral variants. Among these novel INSTIs, compound 4f exhibits two distinct conformations when binding with intasomes from HIV-1 and the closely related prototype foamy virus (PFV) despite the high structural similarity of their INSTI binding pockets. The molecular mechanism and the key active site residues responsible for these differing binding modes in closely related intasomes remain elusive. To unravel the molecular determinants governing the two distinct binding modes, we applied a novel molecular dynamics-based free energy method that utilizes alchemical pathways to overcome the sampling challenges associated with transitioning between the two bound conformations of ligand 4f within the crowded environments of the INSTI binding pockets in these intasomes. The calculated conformational free energies successfully recapitulate the experimentally observed binding mode preferences in the two viral intasomes. Analysis of the simulated structures suggests that the observed binding mode preferences are caused by amino acid residue differences in both the front and the central catalytic sub-pocket of the INSTI binding site in HIV-1 and PFV. Additional free energy calculations on mutants of HIV-1 and PFV revealed that while both sub-pockets contribute to binding mode selection, the central sub-pocket plays a more important role. These results highlight the importance of both side chain and solvent reorganization, as well as the conformational entropy in determining the ligand binding mode, and will help inform the development of more effective INSTIs for combatting drug-resistant viral variants.

Deciphering the Role of Anions of Ionic Liquids in Modulating the Structure and Stability of ct-DNA in Aqueous Solutions

Stabilizing biomolecules under ambient conditions can be extremely beneficial for various biological applications. In this context, the utilization of ionic liquids (ILs) in enhancing the stability and preservation of nucleic acids in aqueous solutions is found to be promising. While the role of the cationic moiety of ILs in the said event has been thoroughly explored, the importance of the anionic moiety in ILs, if any, is rather poorly understood. Herein, we examine the function of anions of ILs in nucleic acid stabilization by examining the stability and structure of calf thymus-DNA (ct-DNA) in the presence of various ILs composed of a common 1-ethyl-3-methylimidazolium cations (Emim+) and different anions, which includes Cl-, Br-, NO 3 - , Ac - , HS O 4 - and B F 4 - by employing various spectroscopic techniques as well as Molecular Dynamics (MD) simulation studies. Analysis of our data suggests that the chemical nature of anions including polarity, basicity, and hydrophilicity become an important factor in the overall DNA-IL interaction event. At lower concentrations, the interplay of intermolecular interaction between the IL anions with their respective cations and the solvent molecules becomes a very crucial factor in inducing their stabilizing effect on ct-DNA. However, at higher concentrations of ILs, the ct-DNA stabilization is additionally governed by specific-ion effect. MD simulation studies have also provided valuable insights into molecular-level understanding of the DNA-IL interaction event. Overall, the present study clearly demonstrated that along with the cationic moiety of ILs, the anions of ILs can play a significant role in deciding the stability of duplex DNA in aqueous solution. The findings of this study are expected to enhance our knowledge on understanding of IL-DNA interactions in a better manner and will be helpful in designing optimized IL systems for nucleic acid based applications.

Elucidating the molecular determinants for binding modes of a third-generation HIV-1 integrase strand transfer inhibitor: Importance of side chain and solvent reorganization

The first and second-generation clinically used HIV-1 integrase (IN) strand transfer inhibitors (INSTIs) are key components of antiretroviral therapy (ART), which work by blocking the integration step in the HIV-1 replication cycle that is catalyzed by a nucleoprotein assembly called an intasome. However, resistance to even the latest clinically used INSTIs is beginning to emerge. Developmental third-generation INSTIs, based on naphthyridine scaffold, are promising candidates to combat drug-resistant viral variants. Among these novel INSTIs, compound 4f exhibits two distinct conformations when binding to intasomes from HIV-1 and the closely related prototype foamy virus (PFV), despite the high structural similarity of their INSTI binding pockets. The molecular mechanism and the key active site residues responsible for these differing binding modes in closely related intasomes remain elusive. To unravel the molecular determinants governing the two distinct binding modes, we employ a novel molecular dynamics-based free energy approach that utilizes alchemical pathways to overcome the sampling challenges associated with transitioning between two ligand conformations within crowded environments along physical pathways. The calculated conformational free energies successfully recapitulate the experimentally observed binding mode preferences in the two viral intasomes. Analysis of the simulated structures suggests that the observed binding mode preferences are caused by amino acid residue differences in both the front and the central catalytic sub-pocket of the INSTI binding site in HIV-1 and PFV. Additional free energy calculations on mutants of HIV-1 and PFV revealed that while both sub-pockets contribute to the binding mode selection, the central sub-pocket plays a more important role. These results highlight the importance of both side chain and solvent reorganization, as well as the conformational entropy in determining the ligand binding mode and will help inform the development of more effective INSTIs for combatting drug-resistant viral variants.