Opening dynamics of 8-oxoguanine in DNA

Dynamics of 8-Oxoguanine in DNA: Decisive Effects of Base Pairing and Nucleotide Context

8-Oxo-7,8-dihydroguanine (oxoG), an abundant DNA lesion, can mispair with adenine and induce mutations. To prevent this, cells possess DNA repair glycosylases that excise either oxoG from oxoG:C pairs (bacterial Fpg, human OGG1) or A from oxoG:A mispairs (bacterial MutY, human MUTYH). Early lesion recognition steps remain murky and may include enforced base pair opening or capture of a spontaneously opened pair. We adapted the CLEANEX-PM NMR protocol to detect DNA imino proton exchange and analyzed the dynamics of oxoG:C, oxoG:A, and their undamaged counterparts in nucleotide contexts with different stacking energy. Even in a poorly stacking context, the oxoG:C pair did not open easier than G:C, arguing against extrahelical base capture by Fpg/OGG1. On the contrary, oxoG opposite A significantly populated the extrahelical state, which may assist recognition by MutY/MUTYH.

Shedding light on the base-pair opening dynamics of nucleic acids in living human cells

Base-pair opening is a fundamental property of nucleic acids that plays important roles in biological functions. However, studying the base-pair opening dynamics inside living cells has remained challenging. Here, to determine the base-pair opening kinetics inside living human cells, the exchange rate constant ([Formula: see text]) of the imino proton with the proton of solvent water involved in hairpin and G-quadruplex (GQ) structures is determined by the in-cell NMR technique. It is deduced on determination of [Formula: see text] values that at least some G-C base pairs of the hairpin structure and all G-G base-pairs of the GQ structure open more frequently in living human cells than in vitro. It is suggested that interactions with endogenous proteins could be responsible for the increase in frequency of base-pair opening. Our studies demonstrate a difference in dynamics of nucleic acids between in-cell and in vitro conditions.

Application of natural antioxidants from traditional Chinese medicine in the treatment of spinal cord injury

Spinal cord injury (SCI) is a devastating central nervous system disease, caused by physical traumas. With the characteristic of high disability rate, catastrophic dysfunction, and enormous burden on the patient’s family, SCI has become a tough neurological problem without efficient treatments. Contemporarily, the pathophysiology of SCI comprises complicated and underlying mechanisms, in which oxidative stress (OS) may play a critical role in contributing to a cascade of secondary injuries. OS substantively leads to ion imbalance, lipid peroxidation, inflammatory cell infiltration, mitochondrial disorder, and neuronal dysfunction. Hence, seeking the therapeutic intervention of alleviating OS and appropriate antioxidants is an essential clinical strategy. Previous studies have reported that traditional Chinese medicine (TCM) has antioxidant, anti-inflammatory, antiapoptotic and neuroprotective effects on alleviating SCI. Notably, the antioxidant effects of some metabolites and compounds of TCM have obtained numerous verifications, suggesting a potential therapeutic strategy for SCI. This review aims at investigating the mechanisms of OS in SCI and highlighting some TCM with antioxidant capacity used in the treatment of SCI.

Quantifying the stability of oxidatively damaged DNA by single-molecule DNA stretching

One of the most common DNA lesions is created when reactive oxygen alters guanine. 8-oxo-guanine may bind in the anti-conformation with an opposing cytosine or in the syn-conformation with an opposing adenine paired by transversion, and both conformations may alter DNA stability. Here we use optical tweezers to measure the stability of DNA hairpins containing 8-oxoguanine (8oxoG) lesions, comparing the results to predictive models of base-pair energies in the absence of the lesion. Contrasted with either a canonical guanine-cytosine or adenine-thymine pair, an 8oxoG-cytosine base pair shows significant destabilization of several k_BT. The magnitude of destabilization is comparable to guanine-thymine ‘wobble’ and cytosine-thymine mismatches. Furthermore, the measured energy of 8oxoG-adenine corresponds to theoretical predictions for guanine-adenine pairs, indicating that oxidative damage does not further destabilize this mismatch in our experiments, in contrast to some previous observations. These results support the hypothesis that oxidative damage to guanine subtly alters the direction of the guanine dipole, base stacking interactions, the local backbone conformation, and the hydration of the modified base. This localized destabilization under stress provides additional support for proposed mechanisms of enzyme repair.

Energetics of base flipping at a DNA mismatch site confined at the latch constriction of α-hemolysin

Unique, two-state modulating current signatures are observed when a cytosine-cytosine mismatch pair is confined at the 2.4 nm latch constriction of the α-hemolysin (αHL) nanopore. We have previously speculated that the modulation is due to base flipping at the mismatch site. Base flipping is a biologically significant mechanism in which a single base is rotated out of the DNA helical stack by 180°. It is the mechanism by which enzymes are able to access bases for repair operations without disturbing the global structure of the helix. Here, temperature dependent ion channel recordings of individual double-stranded DNA duplexes inside αHL are used to derive thermodynamic (ΔH, ΔS) and kinetic (E_A) parameters for base flipping of a cytosine at an unstable cytosine-cytosine mismatch site. The measured activation energy for flipping a cytosine located at the latch of αHL out of the helix (18 ± 1 kcal mol^-1) is comparable to that previously reported for base flipping at mismatch sites from NMR measurements and potential mean force calculations. We propose that the αHL nanopore is a useful tool for measuring conformational changes in dsDNA at the single molecule level.

Global deformation facilitates flipping of damaged 8-oxo-guanine and guanine in DNA

Oxidation of guanine (Gua) to form 7,8-dihydro-8-oxoguanine (8oxoG) is a frequent mutagenic DNA lesion. DNA repair glycosylases such as the bacterial MutM can effciently recognize and eliminate the 8oxoG damage by base excision. The base excision requires a 8oxoG looping out (flipping) from an intrahelical base paired to an extrahelical state where the damaged base is in the enzyme active site. It is still unclear how the damage is identified and flipped from an energetically stable stacked and paired state without any external energy source. Free energy simulations have been employed to study the flipping process for globally deformed DNA conformational states. DNA deformations were generated by systematically untwisting the DNA to mimic its conformation in repair enzyme encounter complex. The simulations indicate that global DNA untwisting deformation toward the enzyme bound form alone (without protein) significantly reduces the penalty for damage flipping to about half of the penalty observed in regular DNA. The finding offers a mechanistic explanation how binding free energy that is transformed to binding induced DNA deformation facilitates flipping and helps to rapidly detect a damaged base. It is likely of general relevance since repair enzyme binding frequently results in significant deformation of the target DNA.

Comparative Effects of Ions, Molecular Crowding, and Bulk DNA on the Damage Search Mechanisms of hOGG1 and hUNG

The energetic nature of the interactions of DNA base excision repair glycosylases with undamaged and damaged DNA and the nuclear environment are expected to significantly impact the time it takes for these enzymes to search for damaged DNA bases. In particular, the high concentration of monovalent ions, macromolecule crowding, and densely packed DNA chains in the cell nucleus could alter the search mechanisms of these enzymes as compared to findings in dilute buffers typically used in in vitro experiments. Here we utilize an in vitro system where the concerted effects of monovalent ions, macromolecular crowding, and high concentrations of bulk DNA chains on the activity of two paradigm human DNA glycosylases can be determined. We find that the energetic nature of the observed binding free energies of human 8-oxoguanine DNA glycosylase (hOGG1) and human uracil DNA glycosylase (hUNG) for undamaged DNA are derived from different sources. Although hOGG1 uses primarily nonelectrostatic binding interactions with nonspecific DNA, hUNG uses a salt-dependent electrostatic binding mode. Both enzymes turn to a nonelectrostatic mode in their specific complexes with damaged bases in DNA, which enhances damage site specificity at physiological ion concentrations. Neither enzyme was capable of efficiently locating and removing their respective damaged bases in the combined presence of physiological ions and a bulk DNA chain density approximating that found in the nucleus. However, the addition of an inert crowding agent to mimic macromolecular crowding in the nucleus largely restored their ability to track DNA chains and locate damaged sites. These findings suggest how the concerted action of monovalent ions and crowding could contribute to efficient DNA damage recognition in cells.

NMR analysis of base-pair opening kinetics in DNA

Base pairing in nucleic acids plays a crucial role in their structure and function. Differences in the base-pair opening and closing kinetics of individual double-stranded DNA sequences or between chemically modified base pairs provide insight into the recognition of these base pairs by DNA processing enzymes. This unit describes how to quantify the kinetics for localized base pairs by observing changes in the imino proton signals by nuclear magnetic resonance spectroscopy. The determination of all relevant parameters using state-of-the art techniques and NMR instrumentation, including cryoprobes, is discussed.