Unraveling the binding characteristics of small ligands to telomeric DNA by pressure modulation

Stabilization of G-Quadruplex Structures of the SARS-CoV-2 Genome by TMPyP4, BRACO19, and PhenDC3

The G-quadruplex is one of the non-canonical structures formed by nucleic acids, which can be formed by guanine-rich sequences. They became the focus of much research when they were found in several oncogene promoter regions and also in the telomeres. Later on, they were discovered in viruses as well. Various ligands have been developed in order to stabilize DNA G-quadruplexes, which were believed to have an anti-cancer or antiviral effect. We investigated three of these ligands, and whether they can also affect the stability of the G-quadruplex-forming sequences of the RNA genome of SARS-CoV-2. All three investigated oligonucleotides showed the G-quadruplex form. We characterized their stability and measured their thermodynamic parameters using the Förster resonance energy transfer method. The addition of the ligands caused an increase in the unfolding temperature, but this effect was smaller compared to that found earlier in the case of G-quadruplexes of the hepatitis B virus, which has a DNA genome.

Effects of Crowding and Cosolutes on Biomolecular Function at Extreme Environmental Conditions

The extent of the effect of cellular crowding and cosolutes on the functioning of proteins and cells is manifold and includes the stabilization of the biomolecular systems, the excluded volume effect, and the modulation of molecular dynamics. Simultaneously, it is becoming increasingly clear how important it is to take the environment into account if we are to shed light on biological function under various external conditions. Many biosystems thrive under extreme conditions, including the deep sea and subseafloor crust, and can take advantage of some of the effects of crowding. These relationships have been studied in recent years using various biophysical techniques, including neutron and X-ray scattering, calorimetry, FTIR, UV-vis and fluorescence spectroscopies. Combining knowledge of the structure and conformational dynamics of biomolecules under extreme conditions, such as temperature, high hydrostatic pressure, and high salinity, we highlight the importance of considering all results in the context of the environment. Here we discuss crowding and cosolute effects on proteins, nucleic acids, membranes, and live cells and explain how it is possible to experimentally separate crowding-induced effects from other influences. Such findings will contribute to a better understanding of the homeoviscous adaptation of organisms and the limits of life in general.

Enhancement of the thermal stability of G-quadruplex structures by urea

The solute urea has been used extensively as a denaturant in protein folding studies; double-stranded nucleic acid structures are also destabilized by urea, but comparatively less than proteins. In previous research, the solute has been shown to strongly destabilize folded G-quadruplex DNA structures. This contribution demonstrates the stabilizing effect of urea on the G-quadruplex formed by the oligodeoxyribonucleotide (ODN), G3T (d[5'-GGGTGGGTGGGTGGG-3']), and related sequences in the presence of sodium or potassium cations. Stabilization is observed up to 7 M urea, which was the highest concentration we investigated. The folded structure of G3T has three G-tetrads and three loops that consist of single thymine residues. ODNs related to G3T, in which the thymine residues in the loop are substituted by adenosine residues, also exhibit enhanced stability in the presence of molar concentrations of urea. The circular dichroism (CD) spectra of these ODNs in the presence of urea are consistent with that of a G-quadruplex. As the urea concentration increases, the spectral intensities of the peaks and troughs change, while their positions change very little. The heat-induced transition from the folded to unfolded state, T_m, was measured by monitoring the change in the UV absorption as a function of temperature. G-quadruplex structures with loops containing single bases exhibited large increases in T_m with increasing urea concentrations. These data imply that the loop region play a significant role in the thermal stability of tetra-helical DNA structures in the presence of the solute urea.

Harnessing Pressure-Axis Experiments to Explore Volume Fluctuations, Conformational Substates, and Solvation of Biomolecular Systems

Intrinsic thermodynamic fluctuations within biomolecules are crucial for their function, and flexibility is one of the strategies that evolution has developed to adapt to extreme environments. In this regard, pressure perturbation is an important tool for mechanistically exploring the causes and effects of volume fluctuations in biomolecules and biomolecular assemblies, their role in biomolecular interactions and reactions, and how they are affected by the solvent properties. High hydrostatic pressure is also a key parameter in the context of deep-sea and subsurface biology and the study of the origin and physical limits of life. We discuss the role of pressure-axis experiments in revealing intrinsic structural fluctuations as well as high-energy conformational substates of proteins and other biomolecular systems that are important for their function and provide some illustrative examples. We show that the structural and dynamic information obtained from such pressure-axis studies improves our understanding of biomolecular function, disease, biological evolution, and adaptation.

Solutions: how adaptive changes in cellular fluids enable marine life to cope with abiotic stressors

The seas confront organisms with a suite of abiotic stressors that pose challenges for physiological activity. Variations in temperature, hydrostatic pressure, and salinity have potential to disrupt structures, and functions of all molecular systems on which life depends. During evolution, sequences of nucleic acids and proteins are adaptively modified to "fit" these macromolecules for function under the particular abiotic conditions of the habitat. Complementing these macromolecular adaptations are alterations in compositions of solutions that bathe macromolecules and affect stabilities of their higher order structures. A primary result of these "micromolecular" adaptations is preservation of optimal balances between conformational rigidity and flexibility of macromolecules. Micromolecular adaptations involve several families of organic osmolytes, with varying effects on macromolecular stability. A given type of osmolyte generally has similar effects on DNA, RNA, proteins and membranes; thus, adaptive regulation of cellular osmolyte pools has a global effect on macromolecules. These effects are mediated largely through influences of osmolytes and macromolecules on water structure and activity. Acclimatory micromolecular responses are often critical in enabling organisms to cope with environmental changes during their lifetimes, for example, during vertical migration in the water column. A species' breadth of environmental tolerance may depend on how effectively it can vary the osmolyte composition of its cellular fluids in the face of stress. Micromolecular adaptations remain an under-appreciated aspect of evolution and acclimatization. Further study can lead to a better understanding of determinants of environmental tolerance ranges and to biotechnological advances in designing improved stabilizers for biological materials.

Binding Properties of RNA Quadruplex of SARS-CoV-2 to Berberine Compared to Telomeric DNA Quadruplex

Previous studies suggest that berberine, an isoquinoline alkaloid, has antiviral potential and is a possible therapeutic candidate against SARS-CoV-2. The molecular underpinnings of its action are still unknown. Potential targets include quadruplexes (G4Q) in the viral genome as they play a key role in modulating the biological activity of viruses. While several DNA-G4Q structures and their binding properties have been elucidated, RNA-G4Qs such as RG-1 of the N-gene of SARS-CoV-2 are less explored. Using biophysical techniques, the berberine binding thermodynamics and the associated conformational and hydration changes of RG-1 could be characterized and compared with human telomeric DNA-G4Q 22AG. Berberine can interact with both quadruplexes. Substantial changes were observed in the interaction of berberine with 22AG and RG-1, which adopt different topologies that can also change upon ligand binding. The strength of interaction and the thermodynamic signatures were found to dependent not only on the initial conformation of the quadruplex, but also on the type of salt present in solution. Since berberine has shown promise as a G-quadruplex stabilizer that can modulate viral gene expression, this study may also contribute to the development of optimized ligands that can discriminate between binding to DNA and RNA G-quadruplexes.

Characterization of a G-quadruplex from hepatitis B virus and its stabilization by binding TMPyP4, BRACO19 and PhenDC3

Specific guanine rich nucleic acid sequences can form non-canonical structures, like the four stranded G-quadruplex (GQ). We studied the GQ-forming sequence (named HepB) found in the genome of the hepatitis B virus. Fluorescence-, infrared- and CD-spectroscopy were used. HepB shows a hybrid form in presence of K⁺, but Na⁺, Li⁺, and Rb⁺ induce parallel structure. Higher concentrations of metal ions increase the unfolding temperature, which was explained by a short thermodynamic calculation. Temperature stability of the GQ structure was determined for all these ions. Na⁺ has stronger stabilizing effect on HepB than K⁺, which is highly unusual. The transition temperatures were 56.6, 53.8, 58.5 and 54.4 °C for Na⁺, K⁺, Li⁺, and Rb⁺ respectively. Binding constants for Na⁺ and K⁺ were 10.2 mM and 7.1 mM respectively. Study of three ligands designed in cancer research for GQ targeting (TMPyP4, BRACO19 and PhenDC3) showed unequivocally their binding to HepB. Binding was proven by the increased stability of the bound form. The stabilization was higher than 20 °C for TMPyP4 and PhenDC3, while it was considerably lower for BRACO19. These results might have medical importance in the fight against the hepatitis B virus.

Pressure Perturbation Studies of Noncanonical Viral Nucleic Acid Structures

G-quadruplexes are noncanonical structures formed by guanine-rich sequences of the genome. They are found in crucial loci of the human genome, they take part in the regulation of important processes like cell proliferation and cell death. Much less is known about the subjects of this work, the viral G-quadruplexes. We have chosen three potentially G-quadruplex-forming sequences of hepatitis B. We measured the stability and the thermodynamic parameters of these quadruplexes. We also investigated the potential stabilization of these G-quadruplexes by binding a special ligand that was originally developed for cancer therapy. Fluorescence and infrared spectroscopic measurements were performed over wide temperature and pressure ranges. Our experiments indicate the small unfolding volume change of all three oligos. We found a difference between the unfolding of the 2-quartet and the 3-quartet G-quadruplexes. All three G-quadruplexes were stabilized by TMPyP4, which is a cationic porphyrin developed for stabilizing the human telomere.

Ions in the Deep Subsurface of Earth, Mars, and Icy Moons: Their Effects in Combination with Temperature and Pressure on tRNA-Ligand Binding

The interactions of ligands with nucleic acids are central to numerous reactions in the biological cell. How such reactions are affected by harsh environmental conditions such as low temperatures, high pressures, and high concentrations of destructive ions is still largely unknown. To elucidate the ions’ role in shaping habitability in extraterrestrial environments and the deep subsurface of Earth with respect to fundamental biochemical processes, we investigated the effect of selected salts (MgCl₂, MgSO₄, and Mg(ClO₄)₂) and high hydrostatic pressure (relevant for the subsurface of that planet) on the complex formation between tRNA and the ligand ThT. The results show that Mg²⁺ salts reduce the binding tendency of ThT to tRNA. This effect is largely due to the interaction of ThT with the salt anions, which leads to a strong decrease in the activity of the ligand. However, at mM concentrations, binding is still favored. The ions alter the thermodynamics of binding, rendering complex formation that is more entropy driven. Remarkably, the pressure favors ligand binding regardless of the type of salt. Although the binding constant is reduced, the harsh conditions in the subsurface of Earth, Mars, and icy moons do not necessarily preclude nucleic acid–ligand interactions of the type studied here.

The Effects of Temperature and Pressure on Protein-Ligand Binding in the Presence of Mars-Relevant Salts

Simple Summary

Interactions of ligands with proteins are central to all reactions in the biological cell. How such reactions are affected by harsh environmental conditions, such as low temperatures, high pressures, and high concentrations of biologically destructive salts, is still largely unknown. Our work focused on specific salts found on Mars to understand whether the planet’s potentially liquid, water-rich subsurface harbors conditions that are theoretically favorable for life. Our data show that, while magnesium chloride and sulfate do not significantly alter protein–ligand interactions, the perchlorate ion strongly affects protein–ligand binding. However, the temperature and pressure conditions encountered on Mars do not necessarily preclude protein–ligand interactions of the type studied here.

Abstract

Protein–ligand interactions are fundamental to all biochemical processes. Generally, these processes are studied at ambient temperature and pressure conditions. We investigated the binding of the small ligand 8-anilinonaphthalene-1-sulfonic acid (ANS) to the multifunctional protein bovine serum albumin (BSA) at ambient and low temperatures and at high pressure conditions, in the presence of ions associated with the surface and subsurface of Mars, including the chaotropic perchlorate ion. We found that salts such as magnesium chloride and sulfate only slightly affect the protein–ligand complex formation. In contrast, magnesium perchlorate strongly affects the interaction between ANS and BSA at the single site level, leading to a change in stoichiometry and strength of ligand binding. Interestingly, both a decrease in temperature and an increase in pressure favor the ligand binding process, resulting in a negative change in protein–ligand binding volume. This suggests that biochemical reactions that are fundamental for the regulation of biological processes are theoretically possible outside standard temperature and pressure conditions, such as in the harsh conditions of the Martian subsurface.