A QM/QM investigation of the hUNG2 reaction surface: the untold tale of a catalytic residue

Preorganized Internal Electric Field Promotes a Double-Displacement Mechanism for the Adenine Excision Reaction by Adenine DNA Glycosylase

Adenine DNA glycosylase (MutY) is a monofunctional glycosylase, removing adenines (A) misinserted opposite 8-oxo-7,8-dihydroguanine (OG), a common product of oxidative damage to DNA. Through multiscale calculations, we decipher a detailed adenine excision mechanism of MutY that is consistent with all available experimental data, involving an initial protonation step and two nucleophilic displacement steps. During the first displacement step, N-glycosidic bond cleavage is accompanied by the attack of the carboxylate group of residue Asp144 at the anomeric carbon (C1'), forming a covalent glycosyl-enzyme intermediate to stabilize the fleeting oxocarbenium ion. After departure of the excised base, water nucleophiles can be recruited to displace Asp144, completing the catalytic cycle with retention of stereochemistry at the C1' position. The two displacement reactions are found to mostly involve the movement of the oxocarbenium ion, occurring with large charge reorganization and thus sensitive to the internal electric field (IEF) exerted by the polar protein environment. Intriguingly, we find that the negatively charged carboxylate group is a good nucleophile for the oxocarbenium ion, yet an unactivated water molecule is not, and that the electric field catalysis strategy is used by the enzyme to enable its unique double-displacement reaction mechanism. A strong IEF, pointing toward 5' direction of the substrate sugar ring, greatly facilitates the second displacement reaction at the expense of elevating the barrier of the first one, thereby allowing both reactions to occur. These findings not only increase our understanding of the strategies used by DNA glycosylases to repair DNA lesions, but also have important implications for how internal/external electric field can be applied to modulate chemical reactions.

Intrinsic Strand-Incision Activity of Human UNG: Implications for Nick Generation in Immunoglobulin Gene Diversification

Uracil arises in cellular DNA by cytosine (C) deamination and erroneous replicative incorporation of deoxyuridine monophosphate opposite adenine. The former generates C → thymine transition mutations if uracil is not removed by uracil-DNA glycosylase (UDG) and replaced by C by the base excision repair (BER) pathway. The primary human UDG is hUNG. During immunoglobulin gene diversification in activated B cells, targeted cytosine deamination by activation-induced cytidine deaminase followed by uracil excision by hUNG is important for class switch recombination (CSR) and somatic hypermutation by providing the substrate for DNA double-strand breaks and mutagenesis, respectively. However, considerable uncertainty remains regarding the mechanisms leading to DNA incision following uracil excision: based on the general BER scheme, apurinic/apyrimidinic (AP) endonuclease (APE1 and/or APE2) is believed to generate the strand break by incising the AP site generated by hUNG. We report here that hUNG may incise the DNA backbone subsequent to uracil excision resulting in a 3´-α,β-unsaturated aldehyde designated uracil-DNA incision product (UIP), and a 5´-phosphate. The formation of UIP accords with an elimination (E2) reaction where deprotonation of C2´ occurs via the formation of a C1´ enolate intermediate. UIP is removed from the 3´-end by hAPE1. This shows that the first two steps in uracil BER can be performed by hUNG, which might explain the significant residual CSR activity in cells deficient in APE1 and APE2.

Structural Insights into the Mechanism of Base Excision by MBD4

DNA glycosylases remove damaged or modified nucleobases by cleaving the N-glycosyl bond and the correct nucleotide is restored through subsequent base excision repair. In addition to excising threatening lesions, DNA glycosylases contribute to epigenetic regulation by mediating DNA demethylation and perform other important functions. However, the catalytic mechanism remains poorly defined for many glycosylases, including MBD4 (methyl-CpG binding domain IV), a member of the helix-hairpin-helix (HhH) superfamily. MBD4 excises thymine from G·T mispairs, suppressing mutations caused by deamination of 5-methylcytosine, and it removes uracil and modified uracils (e.g., 5-hydroxymethyluracil) mispaired with guanine. To investigate the mechanism of MBD4 we solved high-resolution structures of enzyme-DNA complexes at three stages of catalysis. Using a non-cleavable substrate analog, 2'-deoxy-pseudouridine, we determined the first structure of an enzyme-substrate complex for wild-type MBD4, which confirms interactions that mediate lesion recognition and suggests that a catalytic Asp, highly conserved in HhH enzymes, binds the putative nucleophilic water molecule and stabilizes the transition state. Observation that mutating the Asp (to Gly) reduces activity by 2700-fold indicates an important role in catalysis, but probably not one as the nucleophile in a double-displacement reaction, as previously suggested. Consistent with direct-displacement hydrolysis, a structure of the enzyme-product complex indicates a reaction leading to inversion of configuration. A structure with DNA containing 1-azadeoxyribose models a potential oxacarbenium-ion intermediate and suggests the Asp could facilitate migration of the electrophile towards the nucleophilic water. Finally, the structures provide detailed snapshots of the HhH motif, informing how these ubiquitous metal-binding elements mediate DNA binding.

DFT Study on the Deglycosylation of Methylated, Oxidized, and Canonical Pyrimidine Nucleosides in Water: Implications for Epigenetic Regulation and DNA Repair

Density functional theory (B3LYP) was used to characterize the kinetics and thermodynamics of the (nonenzymatic) deglycosylation in water for a variety of 2'-deoxycytidine (dC) and 2'-deoxyuridine (dU) nucleoside derivatives that differ in methylation and subsequent oxidation of the C5 substituent. A range of computational models are considered that combine implicit and explicit solvation of the nucleophile and nucleobase. Regardless of the model implemented, our calculations reveal that the glycosidic bond in dC is inherently more stable than that in dU. Furthermore, C5 methylation of either pyrimidine and subsequent oxidation of the methyl group yield overall small changes to the Gibbs reaction energy profiles and thereby preserve lower deglycosylation barriers for the dC compared to those for the dU nucleoside derivatives. However, hydrolytic deglycosylation becomes significantly more energetically favorable when 5-methyl-dC (5m-dC) undergoes two or three rounds of oxidation, with the Gibbs energy barrier decreasing and the reaction becoming more exergonic by up to 40 kJ/mol. In fact, two or three oxidation reactions from 5m-dC result in a deglycosylation barrier similar to that for dU, as well as those for the associated C5-methylated (2'-deoxythymidine) and oxidized (5-hydroxymethyl-dU) derivatives. These predicted trends in the inherent deglycosylation energetics in water directly correlate with the previously reported activity of thymine DNA glycosylase (TDG), which cleaves the glycosidic bond in select dC nucleosides as part of epigenetic regulation and in dU variants as part of DNA repair. Thus, our data suggests that fundamental differences in the intrinsic reactivity of the pyrimidine nucleosides help regulate the function of human enzymes that maintain cellular integrity.

Excision of uracil from DNA by hSMUG1 includes strand incision and processing

Uracil arises in DNA by hydrolytic deamination of cytosine (C) and by erroneous incorporation of deoxyuridine monophosphate opposite adenine, where the former event is devastating by generation of C → thymine transitions. The base excision repair (BER) pathway replaces uracil by the correct base. In human cells two uracil-DNA glycosylases (UDGs) initiate BER by excising uracil from DNA; one is hSMUG1 (human single-strand-selective mono-functional UDG). We report that repair initiation by hSMUG1 involves strand incision at the uracil site resulting in a 3′-α,β-unsaturated aldehyde designated uracil-DNA incision product (UIP), and a 5′-phosphate. UIP is removed from the 3′-end by human apurinic/apyrimidinic (AP) endonuclease 1 preparing for single-nucleotide insertion. hSMUG1 also incises DNA or processes UIP to a 3′-phosphate designated uracil-DNA processing product (UPP). UIP and UPP were indirectly identified and quantified by polyacrylamide gel electrophoresis and chemically characterised by matrix-assisted laser desorption/ionisation time-of-flight mass-spectrometric analysis of DNA from enzyme reactions using ¹⁸O- or ¹⁶O-water. The formation of UIP accords with an elimination (E2) reaction where deprotonation of C2′ occurs via the formation of a C1′ enolate intermediate. A three-phase kinetic model explains rapid uracil excision in phase 1, slow unspecific enzyme adsorption/desorption to DNA in phase 2 and enzyme-dependent AP site incision in phase 3.

QM/MM Study of the Uracil DNA Glycosylase Reaction Mechanism: A Competition between Asp145 and His148

Uracil DNA glycosylase catalyzes the N-glycosidic bond cleavage of uracil, thereby initiating the base excision repair mechanism for this DNA lesion. Here we employ hybrid quantum mechanics/molecular mechanics calculations to investigate the exact mechanism of the nucleophile attack and the role of the conserved His148 residue. Our calculations suggest that the C1'-N1 bond dissociation proceeds by a migration of the electrophilic sugar in the direction of the water nucleophile, resulting in a planar, oxocarbenium-like transition state. The subsequent nucleophile addition and proton transfer to a nearby base occur without a barrier. We assign the role of a proton acceptor to His148 and elucidate why mutations of this residue curtail the enzymatic activity but do not fully suppress it.

QM/MM Study of the Reaction Catalyzed by Alkyladenine DNA Glycosylase: Examination of the Substrate Specificity of a DNA Repair Enzyme

Human alkyladenine DNA glycosylase (AAG) functions as part of the base excision repair pathway to excise structurally diverse oxidized and alkylated DNA purines. Specifically, AAG uses a water molecule activated by a general base and a nonspecific active site lined with aromatic residues to cleave the N-glycosidic bond. Despite broad substrate specificity, AAG does not target the natural purines (adenine (A) and guanine (G)). Using the ONIOM(QM:MM) methodology, we provide fundamental atomic level details of AAG bound to DNA-containing a neutral substrate (hypoxanthine (Hx)), a nonsubstrate (G), or a cationic substrate (7-methylguanine (7MeG)) and probe changes in the reaction pathway that occur when AAG targets different nucleotides. We reveal that subtle differences in protein-DNA contacts upon binding different substrates within the flexible AAG active site can significantly affect the deglycosylation reaction. Notably, we predict that AAG excises Hx in a concerted mechanism that is facilitated through correct alignment of the (E125) general base due to hydrogen bonding with a neighboring aromatic amino acid (Y127). Hx departure is further stabilized by π-π interactions with aromatic amino acids and hydrogen bonds with active site water. Despite possessing a similar structure to Hx, G is not excised since the additional exocyclic amino group leads to misalignment of the general base due to disruption of the key E125-Y127 hydrogen bond, the catalytically unfavorable placement of water within the active site, and weakened π-contacts between aromatic amino acids and the nucleobase. In contrast, cationic 7MeG does not occupy the same position within the AAG active site as G due to steric clashes with the additional N7 methyl group, which results in the correct alignment of the general base and permits nucleobase excision as observed for neutral Hx. Overall, our structural data rationalizes the observed substrate specificity of AAG and contributes to our fundamental understanding of enzymes with flexible active sites and broad substrate specificities.

Cadmium(II) inhibition of human uracil-DNA glycosylase by catalytic water supplantation

Toxic metals are known to inhibit DNA repair but the underlying mechanisms of inhibition are still not fully understood. DNA repair enzymes such as human uracil-DNA glycosylase (hUNG) perform the initial step in the base excision repair (BER) pathway. In this work, we showed that cadmium [Cd(II)], a known human carcinogen, inhibited all activity of hUNG at 100 μM. Computational analyses based on 2 μs equilibrium, 1.6 μs steered molecular dynamics (SMD), and QM/MM MD determined that Cd(II) ions entered the enzyme active site and formed close contacts with both D145 and H148, effectively replacing the catalytic water normally found in this position. Geometry refinement by density functional theory (DFT) calculations showed that Cd(II) formed a tetrahedral structure with D145, P146, H148, and one water molecule. This work for the first time reports Cd(II) inhibition of hUNG which was due to replacement of the catalytic water by binding the active site D145 and H148 residues. Comparison of the proposed metal binding site to existing structural data showed that D145:H148 followed a general metal binding motif favored by Cd(II). The identified motif offered structural insights into metal inhibition of other DNA repair enzymes and glycosylases.

Evaluating the Substrate Selectivity of Alkyladenine DNA Glycosylase: The Synergistic Interplay of Active Site Flexibility and Water Reorganization

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.

Computational investigations on the catalytic mechanism of maleate isomerase: the role of the active site cysteine residues

The maleate isomerase (MI) catalysed isomerization of maleate to fumarate has been investigated using a wide range of computational modelling techniques, including small model DFT calculations, QM-cluster approach, quantum mechanical/molecular mechanical approach (QM/MM in the ONIOM formalism) and molecular dynamics simulations. Several fundamental questions regarding the mechanism were answered in detail, such as the activation and stabilization of the catalytic Cys in a rather hydrophobic active site. The two previously proposed mechanisms were considered, where either enediolate or succinyl-Cys intermediate forms. Small model calculations as well as an ONIOM-based approach suggest that an enediolate intermediate is too unstable. Furthermore, the formation of succinyl-Cys intermediate via the nucleophilic attack of Cys76(-) on the substrate C2 (as proposed experimentally) was found to be energetically unfeasible in both QM-cluster and ONIOM approaches. Instead, our results show that Cys194, upon activation via the substrate, acts as a nucleophile and Cys76 acts as an acid/base catalyst, forming a succinyl-Cys intermediate in a concerted fashion. Indeed, the calculated PA of Cys76 is always higher than that of Cys194 before or upon substrate binding in the active site. Furthermore, the mechanism proceeds via multiple steps by substrate rotation around C2-C3 with the assistance of the now negatively charged Cys76, leading to the formation of fumarate. Finally, our calculated barrier is in good agreement with experiment. These findings represent a novel mechanism in the racemase superfamily.

DNA-protein π-interactions in nature: abundance, structure, composition and strength of contacts between aromatic amino acids and DNA nucleobases or deoxyribose sugar

Four hundred twenty-eight high-resolution DNA-protein complexes were chosen for a bioinformatics study. Although 164 crystal structures (38% of those searched) contained no interactions, 574 discrete π-contacts between the aromatic amino acids and the DNA nucleobases or deoxyribose were identified using strict criteria, including visual inspection. The abundance and structure of the interactions were determined by unequivocally classifying the contacts as either π-π stacking, π-π T-shaped or sugar-π contacts. Three hundred forty-four nucleobase-amino acid π-π contacts (60% of all interactions identified) were identified in 175 of the crystal structures searched. Unprecedented in the literature, 230 DNA-protein sugar-π contacts (40% of all interactions identified) were identified in 137 crystal structures, which involve C-H···π and/or lone-pair···π interactions, contain any amino acid and can be classified according to sugar atoms involved. Both π-π and sugar-π interactions display a range of relative monomer orientations and therefore interaction energies (up to -50 (-70) kJ mol(-1) for neutral (charged) interactions as determined using quantum chemical calculations). In general, DNA-protein π-interactions are more prevalent than perhaps currently accepted and the role of such interactions in many biological processes may yet to be uncovered.

Standard role for a conserved aspartate or more direct involvement in deglycosylation? An ONIOM and MD investigation of adenine-DNA glycosylase

8-Oxoguanine (OG) is one of the most frequently occurring forms of DNA damage and is particularly deleterious since it forms a stable Hoogsteen base pair with adenine (A). The repair of an OG:A mispair is initiated by adenine-DNA glycosylase (MutY), which hydrolyzes the sugar-nucleobase bond of the adenine residue before the lesion is processed by other proteins. MutY has been proposed to use a two-part chemical step involving protonation of the adenine nucleobase, followed by SN1 hydrolysis of the glycosidic bond. However, differences between a recent (fluorine recognition complex, denoted as the FLRC) crystal structure and the structure on which most mechanistic conclusions have been based to date (namely, the lesion recognition complex or LRC) raise questions regarding the mechanism used by MutY and the discrete role of various active-site residues. The present work uses both molecular dynamics (MD) and quantum mechanical (ONIOM) models to compare the active-site conformational dynamics in the two crystal structures, which suggests that only the understudied FLRC leads to a catalytically competent reactant. Indeed, all previous computational studies on MutY have been initiated from the LRC structure. Subsequently, for the first time, various mechanisms are examined with detailed ONIOM(M06-2X:PM6) reaction potential energy surfaces (PES) based on the FLRC structure, which significantly extends the mechanistic picture. Specifically, our work reveals that the reaction proceeds through a different route than the commonly accepted mechanism and the catalytic function of various active-site residues (Geobacillus stearothermophilus numbering). Specifically, contrary to proposals based on the LRC, E43 is determined to solely be involved in the initial adenine protonation step and not the deglycosylation reaction as the general base. Additionally, a novel catalytic role is proposed for Y126, whereby this residue plays a significant role in stabilizing the highly charged active site, primarily through interactions with E43. More importantly, D144 is found to explicitly catalyze the nucleobase dissociation step through partial nucleophilic attack. Although this is a more direct role than previously proposed for any other DNA glycosylase, comparison to previous work on other glycosylases justifies the larger contribution in the case of MutY and allows us to propose a unified role for the conserved Asp/Glu in the DNA glycosylases, as well as other enzymes that catalyze nucleotide deglycosylation reactions.

Selecting DFT methods for use in optimizations of enzyme active sites: applications to ONIOM treatments of DNA glycosylases

When using a hybrid methodology to treat an enzymatic reaction, many factors contribute to selecting the method for the high-level region, which can be complicated by the presence of dispersion-driven interactions such as π–π stacking. In addition, the proper treatment of the reaction center often requires a large number of heavy atoms to be included in the high-level region, precluding the use of ab initio methods such as MP2 as well as large basis sets, in the optimization step. In the present work, popular DFT methods were tested to identify an appropriate functional for treating the high-level region in ONIOM optimizations of reactions catalyzed by nonmetalloenzymes. Eight different DFT methods (B3LYP, B97-2, MPW1K, MPWB1K, BB1K, B1B95, M06-2X, and ωB97X-D) in combination with four double-ζ quality Pople basis sets were tested for their ability to optimize noncovalent interactions (hydrogen bonding and π–π) and characterize reactions (proton transfer, SN2 hydrolysis, and unimolecular cleavage). Although the primary focus of this study is accurate structure determination, energetics were also examined at both the optimization level of theory, and with triple-ζ quality basis set and select (M06-2X or ωB97X-D) methods. If dispersion-driven interactions exist within the active site, then MPWB1K/6-31G(d,p) or M06-2X/6-31+G(d,p) are recommended for the optimization step with subsequent triple-ζ quality single-point energies. However, since dispersion-corrected functionals (M06-2X and ωB97X-D) generally require diffuse functions to yield appropriate geometries, the possible size of the high-level region is greatly limited with these methods. In contrast, if the model is large enough to recover steric constraints on π–π interactions, then B3LYP with a small basis set performs comparatively well for the optimization step and is significantly less computationally expensive. Interestingly, the functionals that afford the best geometries often do not yield the best energetics, which emphasizes the importance of structural benchmark studies.

Glycosidic bond cleavage in deoxynucleotides: effects of solvent and the DNA phosphate backbone in the computational model

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.

Combined effects of π-π stacking and hydrogen bonding on the (N1) acidity of uracil and hydrolysis of 2'-deoxyuridine

M06-2X/6-31+G(d,p) is used to study the simultaneous effects of π-π stacking interactions with phenylalanine (modeled as benzene) and hydrogen bonding with small molecules (HF, H(2)O, and NH(3)) on the N1 acidity of uracil and the hydrolytic deglycosylation of 2'-deoxyuridine (dU) (facilitated by fully (OH(-)) or partially (HCOO(-)···H(2)O) activated water). When phenylalanine is complexed with isolated uracil, the proton affinity of all acceptor sites significantly increases (by up to 28 kJ mol(-1)), while the N1 acidity slightly decreases (by ~6 kJ mol(-1)). When small molecules are hydrogen bound to uracil, addition of the phenylalanine ring can increase or decrease the acidity of uracil depending on the number and nature (acidity) of the molecules bound. Furthermore, a strong correlation between the effects of π-π stacking on the acidity of U and the dU deglycosylation reaction energetics is found, where the hydrolysis barrier can increase or decrease depending on the nature and number of small molecules bound, the nucleophile considered (which dictates the negative charge on U in the transition state), and the polarity of the (bulk) environment. These findings emphasize that the catalytic (or anticatalytic) role of the active-site aromatic amino acid residues is highly dependent on the situation under consideration. In the case of uracil-DNA glycosylase (UNG), which catalyzes the hydrolytic excision of uracil from DNA, the type of discrete hydrogen-bonding interactions with U, the nature of the nucleophile, and the anticipated weak, nonpolar environment in the active site suggest that phenylalanine will be slightly anticatalytic in the chemical step, and therefore experimentally observed contributions to catalysis may entirely result from associated structural changes that occur prior to deglycosylation.

Modeling the chemical step utilized by human alkyladenine DNA glycosylase: a concerted mechanism AIDS in selectively excising damaged purines

Human alkyladenine DNA glycosylase (AAG) initiates the repair of a wide variety of (neutral or cationic) alkylated and deaminated purines by flipping damaged nucleotides out of the DNA helix and catalyzing the hydrolytic N-glycosidic bond cleavage. Unfortunately, the limited number of studies on the catalytic pathway has left many unanswered questions about the hydrolysis mechanism. Therefore, detailed ONIOM(M06-2X/6-31G(d):AMBER) reaction potential energy surface scans are used to gain the first atomistic perspective of the repair pathway used by AAG. The lowest barrier for neutral 1,N(6)-ethenoadenine (εA) and cationic N(3)-methyladenine (3MeA) excision corresponds to a concerted (A(N)D(N)) mechanism, where our calculated ΔG(‡) = 87.3 kJ mol(-1) for εA cleavage is consistent with recent kinetic data. The use of a concerted mechanism supports previous speculations that AAG uses a nonspecific strategy to excise both neutral (εA) and cationic (3MeA) lesions. We find that AAG uses nonspecific active site DNA-protein π-π interactions to catalyze the removal of inherently more difficult to excise neutral lesions, and strongly bind to cationic lesions, which comes at the expense of raising the excision barrier for cationic substrates. Although proton transfer from the recently proposed general acid (protein-bound water) to neutral substrates does not occur, hydrogen-bond donation lowers the catalytic barrier, which clarifies the role of a general acid in the excision of neutral lesions. Finally, our work shows that the natural base adenine (A) is further inserted into the AAG active site than the damaged substrates, which results in the loss of a hydrogen bond with Y127 and misaligns the general base (E125) and water nucleophile to lead to poor nucleophile activation. Therefore, our work proposes how AAG discriminates against the natural purines in the chemical step and may also explain why some damaged pyrimidines are bound but are not excised by this enzyme.