Structural and energetic heterogeneities of canonical and oxidized central guanine triad of B-DNA telomeric fragments

Structural and energetic properties of canonical and oxidized telomeric complexes studied by molecular dynamics simulations

The structural and energetic properties of native and oxidized telomeric complexes were defined by means of molecular dynamic (MD) simulations. As a starting point, the experimental conformation of B-DNA d(GpTpTpApGpGpGpTpTpApGpGpG) oligomer bound to human protein telomeric repeat binding factor 1 (TRF1) was used. The influence on the stability of the telomeric complex of the presence of 8-oxoguanine (8oxoG) in the central telomeric triad (CTT) was estimated based on trajectories collected during 130 ns MD runs. The data obtained indicate that the system analyzed is highly sensitive to the presence of oxidative damage in the CTT of the B-DNA telomeric sequence. The most important changes were observed in the immediate vicinity of the 8-oxoguanine molecule. The significantly higher mobility of arginine 425 interacting directly with the oxidized guanine molecule has a large influence on the structural, dynamic and energetic properties of neighboring amino acids. Local changes observed for individual hydrogen bonded interactions localized in the major groove of B-DNA also have significant impact on the properties of hydrophobic clusters, which are the second type of force responsible for stability of the studied bio-system. All the changes reported in detail here unambiguously indicate a significant decrease in telomer binding affinity after oxidation.

Structural and energetic consequences of oxidation of d(ApGpGpGpTpT) telomere repeat unit in complex with TRF1 protein

The configuration hyperspace of canonical and oxidized 14-mers of B-DNA comprising telomere repeat units d(ApGpGpGpTpT) was sampled over 40 ns via molecular dynamic (MD) simulations. The energetic and structural consequences of TRF1 binding to telomere B-DNA were compared with non-complexed systems. Energetic properties of analyzed pairs, di- and tri-nucleotide steps occurring in central telomere repeat unit were estimated by means of advanced quantum chemistry computations including not only BSSE corrections, electron correlation contributions but also non-negligible many-body terms. These data along with bases pair and base step parameters distributions allow for quantization of consequences of oxidation and/or TRF1 binding to telomere repeat units. Occurrence of 8-oxoguanine in central telomeric triad (CTT) is the source of high stiffness if compared to non-modified oligomer. The origin of this property comes from significantly alteration of intermolecular interactions introduced by 8-oxoguanine. The increased stability observed for base-base interactions are accumulated and characterizes also di- and tri-nucleotides. The observed changes in the intermolecular interactions originate from structural alterations imposed by TRF1 binding to canonical and oxidized telomere B-DNA. First and most direct consequence of TRF1 binding to oxidized telomere repeat unit is alteration of shift-slide correlations if compared to canonical system. This in turn leads to large differences in purine-purine overlapping in oxidized structures. Thus, oxidized telomere B-DNA double strands are sensitive to interactions with protein ligands and numerous structural and energetic changes are imposed on base pairs forming CTT.

Non-additive interactions of nucleobases in model dinucleotide steps occurring in B-DNA crystals

Non-additivity of base-base interactions in all ten possible model dinucleotide steps were analyzed on MP2/aug-cc-pvDZ quantum chemistry level. Conformations of four nucleobases exactly matched to ones occurring in B-DNA crystals. In most of thw 162 analyzed tetramers both three- and four-body contributions are negligible except for d(GpG) steps. However, in these dinucleotides both contributions are always of opposite signs and in all cases the sum of all non-additive part of intermolecular interactions do not exceed 2.6 kcal mol(-1). This stands for less than 5% of the overall binding energy of dinucleotide steps. Also replacements of guanine with 8-oxoguanine in d(GpG) systems introduces non-additivity of the same magnitude as for canonical dinucleotides. It is observed linear relationships between values of total binding energy obtained in the tetramer basis set and estimated energy exclusively in dimers basis sets with assumption of pairwise additivities. For all analyzed dinucleotides steps there are also linear correlations between amount of non-additive contributions and magnitude of pairs interactions. Based on differences in electrostatic contribution to the total binding energy of four nucleobases and polarity of dinucleotide steps three distinct classes of dinucleotide steps were identified.