Density functional theory (B3LYP) was employed to examine the hydrolysis of the canonical 2'-deoxynucleotides in varied environments (gas phase or water) using different computational models for the sugar residue (methyl or phosphate group at C5') and nucleophile (water activated through full or partial proton abstraction). Regardless of the degree of nucleophile activation, our results show that key geometrical parameters along the reaction pathway are notably altered upon direct inclusion of solvent effects in the optimization routine, which leads to significant changes in the reaction energetics and better agreement with experiment. Therefore, despite the wide use of gas-phase calculations in the literature, small model computational work, as well as large-scale enzyme models, that strive to understand nucleotide deglycosylation must adequately describe the environment. Alternatively, although inclusion of the phosphate group at C5' also affects the geometries of important stationary points, the effects cancel to yield unchanged deglycosylation barriers, and therefore smaller computational models can be used to estimate the energy associated with nucleotide deglycosylation, with the 5' phosphate group included if full (geometric) details of the reaction are desired. Hydrogen-bonding interactions with the nucleobase can significantly reduce the barrier to deglycosylation, which supports suggestions that discrete hydrogen-bonding interactions with active-site amino acid residues can play a significant role in enzyme-catalyzed nucleobase excision. Taken together with previous studies, the present work provides vital clues about the components that must be included in future studies of the deglycosylation of isolated noncanonical nucleotides, as well as the corresponding enzyme-catalyzed reactions.

5-Methylcytosine (5mC) in DNA CpG islands is an important epigenetic biomarker for mammalian gene regulation. It is oxidized to 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC), and 5-carboxylcytosine (5caC) by the ten-eleven translocation (TET) family enzymes, which are α-ketoglutarate (α-KG)/Fe(II)-dependent dioxygenases. In this work, we demonstrate that the epigenetic marker 5mC is modified to 5hmC, 5fC, and 5caC in vitro by another class of α-KG/Fe(II)-dependent proteins—the DNA repair enzymes in the AlkB family, which include ALKBH2, ALKBH3 in huamn and AlkB in Escherichia coli . Theoretical calculations indicate that these enzymes may bind 5mC in the syn -conformation, placing the methyl group comparable to 3-methylcytosine, the prototypic substrate of AlkB. This is the first demonstration of the AlkB proteins to oxidize a methyl group attached to carbon, instead of nitrogen, on a DNA base. These observations suggest a broader role in epigenetics for these DNA repair proteins.

Although DNA damage can have a variety of deleterious effects on cells (e.g., senescence, death, and rapid growth), the base excision repair (BER) pathway combats the effects by removing several types of damaged DNA. Since the first BER step involves cleavage of the bond between the damaged nucleobase and the DNA sugar-phosphate backbone, we have used density functional theory to compare the intrinsic stability of the glycosidic bond in a number of common DNA oxidation, deamination, and alkylation products to the corresponding natural nucleosides. Our calculations predict that the dissociative (SN1) and associative (SN2) pathways are nearly isoenergetic, with the dissociative pathway only slightly favored on the Gibbs reaction surface for all canonical and damaged nucleosides, which suggests that DNA damage does not affect the inherently most favorable deglycosylation pathway. More importantly, with the exception of thymine glycol, all DNA lesions exhibit reduced glycosidic bond stability relative to the undamaged nucleosides. Furthermore, the trend in the magnitude of the deglycosylation barrier reduction directly correlates with the relative nucleobase acidity (at N9 for purines or N1 for pyrimidines), which thereby provides a computationally efficient, qualitative measure of the glycosidic bond stability in DNA damage. The effect of nucleobase activation (protonation) at different sites predicts that the positions leading to the largest reductions in the deglycosylation barrier are typically used by DNA glycosylases to facilitate base excision. Finally, deaza purine derivatives are found to have greater glycosidic bond stability than the canonical counterparts, which suggests that alterations to excision rates measured using these derivatives to probe DNA glycosylase function must be interpreted in reference to the inherent differences in the nucleoside reactivity. Combined with previous studies of the deglycosylation of DNA nucleosides, the current study provides a greater fundamental understanding about the reactivity of the glycosidic bond in damaged DNA, which has direct implications to the function of critical DNA repair enzymes.

Human alkyladenine DNA glycosylase (AAG) functions as part of the base excision repair (BER) pathway by cleaving the N-glycosidic bond that connects nucleobases to the sugar-phosphate backbone in DNA. AAG targets a range of structurally diverse purine lesions using nonspecific DNA-protein π-π interactions. Nevertheless, the enzyme discriminates against the natural purines and is inhibited by pyrimidine lesions. This study uses molecular dynamics simulations and seven different neutral or charged substrates, inhibitors, or canonical purines to probe how the bound nucleotide affects the conformation of the AAG active site, and the role of active site residues in dictating substrate selectivity. The neutral substrates form a common DNA-protein hydrogen bond, which results in a consistent active site conformation that maximizes π-π interactions between the aromatic residues and the nucleobase required for catalysis. Nevertheless, subtle differences in DNA-enzyme contacts for different neutral substrates explain observed differential catalytic efficiencies. In contrast, the exocyclic amino groups of the natural purines clash with active site residues, which leads to catalytically incompetent DNA-enzyme complexes due to significant reorganization of active site water. Specifically, water resides between the A nucleobase and the active site aromatic amino acids required for catalysis, while a shift in the position of the general base (E125) repositions (potentially nucleophilic) water away from G. Despite sharing common amino groups, the methyl substituents in cationic purine lesions (3MeA and 7MeG) exhibit repulsion with active site residues, which repositions the damaged bases in the active site in a manner that promotes their excision. Overall, we provide a structural explanation for the diverse yet discriminatory substrate selectivity of AAG and rationalize key kinetic data available for the enzyme. Specifically, our results highlight the complex interplay of many different DNA-protein interactions used by AAG to facilitate BER, as well as the crucial role of the general base and water (nucleophile) positioning. The insights gained from our work will aid the understanding of the function of other enzymes that use flexible active sites to exhibit diverse substrate specificity.

Despite the inherent stability of glycosidic linkages in nucleic acids that connect the nucleobases to sugar-phosphate backbones, cleavage of these bonds is often essential for organism survival. The current study uses DFT (B3LYP) to provide a fundamental understanding of the hydrolytic deglycosylation of the natural RNA nucleosides (A, C, G, and U), offers a comparison to DNA hydrolysis, and examines the effects of acid, base, or simultaneous acid-base catalysis on RNA deglycosylation. By initially examining HCOO···HO mediated deglycosylation, the barriers for RNA hydrolysis were determined to be 30-38 kJ mol higher than the corresponding DNA barriers, indicating that the 2'-OH group stabilizes the glycosidic bond. Although the presence of HCOO as the base (i.e., to activate the water nucleophile) reduces the barrier for uncatalyzed RNA hydrolysis (i.e., unactivated HO nucleophile) by ∼15-20 kJ mol, the extreme of base catalysis as modeled using a fully deprotonated water molecule (i.e., OH nucleophile) decreases the uncatalyzed barriers by up to 65 kJ mol. Acid catalysis was subsequently examined by selectively protonating the hydrogen-bond acceptor sites of the RNA nucleobases, which results in an up to ∼80 kJ mol barrier reduction relative to the corresponding uncatalyzed pathway. Interestingly, the nucleobase proton acceptor sites that result in the greatest barrier reductions match sites typically targeted in enzyme-catalyzed reactions. Nevertheless, simultaneous acid and base catalysis is the most beneficial way to enhance the reactivity of the glycosidic bonds in RNA, with the individual effects of each catalytic approach being weakened, additive, or synergistic depending on the strength of the base (i.e., degree of water nucleophile activation), the nucleobase, and the hydrogen-bonding acceptor site on the nucleobase. Together, the current contribution provides a greater understanding of the reactivity of the glycosidic bond in natural RNA nucleosides, and has fundamental implications for the function of RNA-targeting enzymes.