Substrate specificity of human apurinic/apyrimidinic endonuclease APE1 in the nucleotide incision repair pathway

AND Logic Gate-Regulated DNAzyme Nanoflower for Monitoring the Activity of Multiple DNA Repair Enzymes

Despite the progress that has been made in diverse DNA-based nanodevices to in situ monitor the activity of the DNA repair enzymes in living cells, the significance of improving both the sensitivity and specificity has remained largely neglected and understudied. Herein, we propose a regulatable DNA nanodevice to specifically monitor the activity of DNA repair enzymes for early evaluation of cancer mediated by genomic instability. Concretely, an AND logic gate-regulated DNAzyme nanoflower was rationally designed by the self-assembly of the DNA duplex modified with both apurinic/apyrimidinic (AP) site and methyl lesion site. The DNAzyme nanoflower could be reconfigured under the repair of AP sites and O6-methylguanine sites by apurinic/apyrimidinic endonuclease 1 (APE1) and O6-methylguanine methyltransferase (MGMT) to produce a fluorescent signal, realizing the sensitive monitoring of the activity of APE1 and MGMT. Compared to the free DNAzyme duplex, the fluorescent response of the DNAzyme nanoflower increased by 60%, due to the effective enrichment of the DNA probes by the nanoflower structure. More importantly, we have demonstrated that the dual-enzyme activated strategy allows imaging of specific cancer cells in the AND logic gate manner using MCF-7 as a cancer cell model, improving the specificity of cancer cell imaging. This AND logic gate-regulated multifunctional DNAzyme nanoflower provides a simple tool for simultaneously visualizing multiple DNA repair enzymes, holding great potential in early clinical diagnosis and drug discovery.

Base Excision Repair: Mechanisms and Impact in Biology, Disease, and Medicine

Base excision repair (BER) corrects forms of oxidative, deamination, alkylation, and abasic single-base damage that appear to have minimal effects on the helix. Since its discovery in 1974, the field has grown in several facets: mechanisms, biology and physiology, understanding deficiencies and human disease, and using BER genes as potential inhibitory targets to develop therapeutics. Within its segregation of short nucleotide (SN-) and long patch (LP-), there are currently six known global mechanisms, with emerging work in transcription- and replication-associated BER. Knockouts (KOs) of BER genes in mouse models showed that single glycosylase knockout had minimal phenotypic impact, but the effects were clearly seen in double knockouts. However, KOs of downstream enzymes showed critical impact on the health and survival of mice. BER gene deficiency contributes to cancer, inflammation, aging, and neurodegenerative disorders. Medicinal targets are being developed for single or combinatorial therapies, but only PARP and APE1 have yet to reach the clinical stage.

Mechanistic insight into AP-endonuclease 1 cleavage of abasic sites at stalled replication fork mimics

Many types of damage, including abasic sites, block replicative DNA polymerases causing replication fork uncoupling and generating ssDNA. AP-Endonuclease 1 (APE1) has been shown to cleave abasic sites in ssDNA. Importantly, APE1 cleavage of ssDNA at a replication fork has significant biological implications by generating double strand breaks that could collapse the replication fork. Despite this, the molecular basis and efficiency of APE1 processing abasic sites at replication forks remain elusive. Here, we investigate APE1 cleavage of abasic substrates that mimic APE1 interactions at stalled replication forks or gaps. We determine that APE1 has robust activity on these substrates, like dsDNA, and report rates for cleavage and product release. X-ray structures visualize the APE1 active site, highlighting an analogous mechanism is used to process ssDNA substrates as canonical APE1 activity on dsDNA. However, mutational analysis reveals R177 to be uniquely critical for the APE1 ssDNA cleavage mechanism. Additionally, we investigate the interplay between APE1 and Replication Protein A (RPA), the major ssDNA-binding protein at replication forks, revealing that APE1 can cleave an abasic site while RPA is still bound to the DNA. Together, this work provides molecular level insights into abasic ssDNA processing by APE1, including the presence of RPA.

Inner Amino Acid Contacts Are Key Factors of Multistage Structural Rearrangements of DNA and Affect Substrate Specificity of Apurinic/Apyrimidinic Endonuclease APE1

Apurinic/apyrimidinic endonuclease 1 (APE1) is one of the most important enzymes in base excision repair. Studies on this enzyme have been conducted for a long time, but some aspects of its activity remain poorly understood. One such question concerns the mechanism of damaged-nucleotide recognition by the enzyme, and the answer could shed light on substrate specificity control in all enzymes of this class. In the present study, by pulsed electron-electron double resonance (DEER, also known as PELDOR) spectroscopy and pre-steady-state kinetic analysis along with wild-type (WT) APE1 from Danio rerio (zAPE1) or three mutants (carrying substitution N253G, A254G, or E260A), we aimed to elucidate the molecular events in the process of damage recognition. The data revealed that the zAPE1 mutant E260A has much higher activity toward DNA substrates containing 5,6-dihydro-2'-deoxyuridine (DHU), 2'-deoxyuridine (dU), alpha-2'-deoxyadenosine (αA), or 1,N6-ethenoadenosine (εA). Examination of conformational changes in DNA clearly revealed multistep DNA rearrangements during the formation of the catalytic complex. These structural rearrangements of DNA are directly associated with the capacity of damaged DNA for enzyme-induced bending and unwinding, which are required for eversion of the damaged nucleotide from the DNA duplex and for its placement into the active site of the enzyme. Taken together, the results experimentally prove the factors that control substrate specificity of the AP endonuclease zAPE1.

Individual Contributions of Amido Acid Residues Tyr122, Ile168, and Asp173 to the Activity and Substrate Specificity of Human DNA Dioxygenase ABH2

Human Fe(II)/α-ketoglutarate-dependent dioxygenase ABH2 plays a crucial role in the direct reversal repair of nonbulky alkyl lesions in DNA nucleobases, e.g., N¹-methyladenine (m¹A), N³-methylcytosine (m³C), and some etheno derivatives. Moreover, ABH2 is capable of a less efficient oxidation of an epigenetic DNA mark called 5-methylcytosine (m⁵C), which typically is a specific target of DNA dioxygenases from the TET family. In this study, to elucidate the mechanism of the substrate specificity of ABH2, we investigated the role of several active-site amino acid residues. Functional mapping of the lesion-binding pocket was performed through the analysis of the functions of Tyr122, Ile168, and Asp173 in the damaged base recognition mechanism. Interactions of wild-type ABH2, or its mutants Y122A, I168A, or D173A, with damaged DNA containing the methylated base m¹A or m³C or the epigenetic marker m⁵C were analyzed by molecular dynamics simulations and kinetic assays. Comparative analysis of the enzymes revealed an effect of the substitutions on DNA binding and on catalytic activity. Obtained data clearly demonstrate the effect of the tested amino acid residues on the catalytic activity of the enzymes rather than the DNA-binding ability. Taken together, these data shed light on the molecular and kinetic consequences of the substitution of active-site residues for the mechanism of the substrate recognition.

Novel mechanisms for the removal of strong replication-blocking HMCES- and thiazolidine-DNA adducts in humans

Apurinic/apyrimidinic (AP) sites are DNA lesions created under normal growth conditions that result in cytotoxicity, replication-blocks, and mutations. AP sites are susceptible to β-elimination and are liable to be converted to DNA strand breaks. HMCES (5-hydroxymethylcytosine binding, ES cell specific) protein interacts with AP sites in single stranded (ss) DNA exposed at DNA replication forks to generate a stable thiazolidine protein-DNA crosslink and protect cells against AP site toxicity. The crosslinked HMCES is resolved by proteasome-mediated degradation; however, it is unclear how HMCES-crosslinked ssDNA and the resulting proteasome-degraded HMCES adducts are processed and repaired. Here, we describe methods for the preparation of thiazolidine adduct-containing oligonucleotides and determination of their structure. We demonstrate that the HMCES-crosslink is a strong replication blocking adduct and that protease-digested HMCES adducts block DNA replication to a similar extent as AP sites. Moreover, we show that the human AP endonuclease APE1 incises DNA 5' to the protease-digested HMCES adduct. Interestingly, while HMCES-ssDNA crosslinks are stable, the crosslink is reversed upon the formation of dsDNA, possibly due to a catalytic reverse reaction. Our results shed new light on damage tolerance and repair pathways for HMCES-DNA crosslinks in human cells.

Disruption of energetic and dynamic base pairing cooperativity in DNA duplexes by an abasic site

DNA duplex stability arises from cooperative interactions between multiple adjacent nucleotides that favor base pairing and stacking when formed as a continuous stretch rather than individually. Lesions and nucleobase modifications alter this stability in complex manners that remain challenging to understand despite their centrality to biology. Here, we investigate how an abasic site destabilizes small DNA duplexes and reshapes base pairing dynamics and hybridization pathways using temperature-jump infrared spectroscopy and coarse-grained molecular dynamics simulations. We show how an abasic site splits the cooperativity in a short duplex into two segments, which destabilizes small duplexes as a whole and enables metastable half-dissociated configurations. Dynamically, it introduces an additional barrier to hybridization by constraining the hybridization mechanism to a step-wise process of nucleating and zipping a stretch on one side of the abasic site and then the other.

Fluorescently labeled human apurinic/apyrimidinic endonuclease APE1 reveals effects of DNA polymerase β on the APE1-DNA interaction

The base excision repair (BER) pathway involves sequential action of DNA glycosylases and apurinic/apyrimidinic (AP) endonucleases to incise damaged DNA and prepare DNA termini for incorporation of a correct nucleotide by DNA polymerases. It has been suggested that the enzymatic steps in BER include recognition of a product-enzyme complex by the next enzyme in the pathway, resulting in the "passing-the-baton" model of transfer of DNA intermediates between enzymes. To verify this model, in this work, we aimed to create a suitable experimental system. We prepared APE1 site-specifically labeled with a fluorescent reporter that is sensitive to stages of APE1-DNA binding, of formation of the catalytic complex, and of subsequent dissociation of the enzyme-product complex. Interactions of the labeled APE1 with various model DNA substrates (containing an abasic site) of varied lengths revealed that the enzyme remains mostly in complex with the DNA product. By means of the fluorescently labeled APE1 in combination with a stopped-flow fluorescence assay, it was found that Polβ stimulates both i) APE1 binding to an abasic-site-containing DNA duplex with the formation of a catalytically competent complex and ii) the dissociation of APE1 from its product. These findings confirm DNA-mediated coordination of APE1 and Polβ activities and suggest that Polβ is the key trigger of the DNA transfer between the enzymes participating in initial steps of BER.

The mechanism of damage recognition by apurinic/apyrimidinic endonuclease Nfo from Escherichia coli

Apurinic/apyrimidinic (AP) endonuclease Nfo from Escherichia coli recognises AP sites in DNA and catalyses phosphodiester bond cleavage on the 5' side of AP sites and some damaged or undamaged nucleotides. Here, the mechanism of target nucleotide recognition by Nfo was analysed by pulsed electron-electron double resonance (PELDOR, also known as DEER) spectroscopy and pre-steady-state kinetic analysis with Förster resonance energy transfer detection of DNA conformational changes during DNA binding. The efficiency of endonucleolytic cleavage of a target nucleotide in model DNA substrates was ranked as (2R,3S)-2-(hydroxymethyl)-3-hydroxytetrahydrofuran [F-site] > 5,6-dihydro-2'-deoxyuridine > α-anomer of 2'-deoxyadenosine >2'-deoxyuridine > undamaged DNA. Real-time conformational changes of DNA during interaction with Nfo revealed an increase of distances between duplex ends during the formation of the initial enzyme-substrate complex. The use of rigid-linker spin-labelled DNA duplexes in DEER measurements indicated that double-helix bending and unwinding by the target nucleotide itself is one of the key factors responsible for indiscriminate recognition of a target nucleotide by Nfo. The results for the first time show that AP endonucleases from different structural families utilise a common strategy of damage recognition, which globally may be integrated with the mechanism of searching for specific sites in DNA by other enzymes.

Pre-steady-state kinetic and mutational insights into mechanisms of endo- and exonuclease DNA processing by mutant forms of human AP endonuclease

Human apurinic/apyrimidinic endonuclease APE1 catalyzes endonucleolytic hydrolysis of phosphodiester bonds on the 5' side of structurally unrelated damaged nucleotides in DNA or native nucleotides in RNA. APE1 additionally possesses 3'-5'-exonuclease, 3'-phosphodiesterase, and 3'-phosphatase activities. According to structural data, endo- and exonucleolytic cleavage of DNA is executed in different complexes when the excised residue is everted from the duplex or placed within the intrahelical DNA cavity without nucleotide flipping. In this study, we investigated the functions of residues Arg177, Arg181, Tyr171 and His309 in the APE1 endo- and exonucleolytic reactions. The interaction between residues Arg177 and Met270, which was hypothesized recently to be a switch for endo- and exonucleolytic catalytic mode regulation, was verified by pre-steady-state kinetic analysis of the R177A APE1 mutant. The function of another DNA-binding-site residue, Arg181, was analyzed too; it changed its conformation when enzyme-substrate and enzyme-product complexes were compared. Mutation R181A significantly facilitated the product dissociation stage and only weakly affected DNA-binding affinity. Moreover, R181A reduced the catalytic rate constant severalfold due to a loss of contact with a phosphate group. Finally, the protonation/deprotonation state of residues Tyr171 and His309 in the catalytic reaction was verified by their substitution. Mutations Y171F and H309A inhibited the chemical step of the AP endonucleolytic reaction by several orders of magnitude with retention of capacity for (2R,3S)-2-(hydroxymethyl)-3-hydroxytetrahydrofuran-containing-DNA binding and without changes in the pH dependence profile of AP endonuclease activity, indicating that deprotonation of these residues is likely not important for the catalytic reaction.

Insights into Mechanisms of Damage Recognition and Catalysis by APE1-like Enzymes

Apurinic/apyrimidinic (AP) endonucleases are the key DNA repair enzymes in the base excision repair (BER) pathway, and are responsible for hydrolyzing phosphodiester bonds on the 5′ side of an AP site. The enzymes can recognize not only AP sites but also some types of damaged bases, such as 1,N⁶-ethenoadenosine, α-adenosine, and 5,6-dihydrouridine. Here, to elucidate the mechanism underlying such a broad substrate specificity as that of AP endonucleases, we performed a computational study of four homologous APE1-like endonucleases: insect (Drosophila melanogaster) Rrp1, amphibian (Xenopus laevis) APE1 (xAPE1), fish (Danio rerio) APE1 (zAPE1), and human APE1 (hAPE1). The contact between the amino acid residues of the active site of each homologous APE1-like enzyme and the set of damaged DNA substrates was analyzed. A comparison of molecular dynamic simulation data with the known catalytic efficiency of these enzymes allowed us to gain a deep insight into the differences in the efficiency of the cleavage of various damaged nucleotides. The obtained data support that the amino acid residues within the “damage recognition” loop containing residues Asn222–Ala230 significantly affect the catalytic-complex formation. Moreover, every damaged nucleotide has its unique position and a specific set of interactions with the amino acid residues of the active site.

Comparative Analysis of Exo- and Endonuclease Activities of APE1-like Enzymes

Apurinic/apyrimidinic (AP)-endonucleases are multifunctional enzymes that are required for cell viability. AP-endonucleases incise DNA 5′ to an AP-site; can recognize and process some damaged nucleosides; and possess 3′-phosphodiesterase, 3′-phosphatase, and endoribonuclease activities. To elucidate the mechanism of substrate cleavage in detail, we analyzed the effect of mono- and divalent metal ions on the exo- and endonuclease activities of four homologous APE1-like endonucleases (from an insect (Rrp1), amphibian (xAPE1), fish (zAPE1), and from humans (hAPE1)). It was found that the enzymes had similar patterns of dependence on metal ions’ concentrations in terms of AP-endonuclease activity, suggesting that the main biological function (AP-site cleavage) was highly conserved among evolutionarily distant species. The efficiency of the 3′-5′ exonuclease activity was the highest in hAPE1 among these enzymes. In contrast, the endoribonuclease activity of the enzymes could be ranked as hAPE1 ≈ zAPE1 ≤ xAPE1 ≤ Rrp1. Taken together, the results revealed that the tested enzymes differed significantly in their capacity for substrate cleavage, even though the most important catalytic and substrate-binding amino acid residues were conserved. It can be concluded that substrate specificity and cleavage efficiency were controlled by factors external to the catalytic site, e.g., the N-terminal domain of these enzymes.

Is Cys(MTSL) the Best α-Amino Acid Residue to Electron Spin Labeling of Synthetically Accessible Peptide Molecules with Nitroxides?

Electron paramagnetic resonance spectroscopy, particularly its pulse technique double electron-electron resonance (DEER) (also termed PELDOR), is rapidly becoming an extremely useful tool for the experimental determination of side chain-to-side chain distances between free radicals in molecules fundamental for life, such as polypeptides. Among appropriate probes, the most popular are undoubtedly nitroxide electron spin labels. In this context, suitable biosynthetically derived, helical regions of proteins, along with synthetic peptides with amphiphilic properties and antibacterial activities, are the most extensively investigated compounds. A strict requirement for a precise distance measurement has been identified in a minimal dynamic flexibility of the two nitroxide-bearing α-amino acid side chains. To this end, in this study, we have experimentally compared in detail the side-chain mobility properties of the two currently most widely utilized residues, namely, Cys(MTSL) and 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC). In particular, two double-labeled, chemically synthesized 20-mer peptide molecules have been adopted as appropriate templates for our investigation on the determination of the model intramolecular separations. These double-Cys(MTSL) and double-TOAC compounds are both analogues of the almost completely rigid backbone peptide ruler which we have envisaged and 3D structurally analyzed as our original, unlabeled compound. Here, we have clearly found that the TOAC side-chain labels are largely more 3D structurally restricted than the MTSL labels. From this result, we conclude that the TOAC residue offers more precise information than the Cys(MTSL) residue on the side chain-to-side chain distance distribution in synthetically accessible peptide molecules.

A Peptide-Based Trap for Metal Ions Studied by Electron Paramagnetic Resonance

Peptide-based materials provide a versatile platform for sensing and ion sequestration since peptides are endowed with stimuli-responsive properties. The mechanism of molecular sensing is often based on peptide structural changes (or switching), caused by the binding of the target molecule. One scope of sensing applications is the selection of a specific analyte, which may be achieved by adjusting the structure of the peptide binding site. Therefore, exact knowledge of peptide properties and 3D-structure in the ‘switched’ state is desirable for tuning the detection and for further molecular construction. Hence, here we demonstrate the performance of Electron Paramagnetic Resonance (EPR) spectroscopy in the identification of metal ion binding by the antimicrobial peptide trichogin GA IV. Na(I), Ca(II), and Cu(II) ions were probed as analytes to evaluate the impact of coordination number, ionic radii, and charge. Conclusions drawn by EPR are in line with literature data, where other spectroscopic techniques were exploited to study peptide-ion interactions for trichogin GA IV, and the structural switch from an extended helix to a hairpin structure, wrapped around the metal ion upon binding of divalent cations was proposed.

Direct Measurement of Initial Rate of Enzyme Reaction by Electrokinetic Filtration Using a Hydrogel-plugged Capillary Device

A novel electrokinetic filtration device using a plugged hydrogel was developed to directly measure the initial rate of enzyme reactions. In the proposed method, the enzyme reaction proceeded only for a short time when the substrate was passed through a thin layer of enzyme trapped by the hydrogel without any lag times for mixing and detection. In experimental conditions, alkaline phosphatase (enzyme) was filtrated at a cathodic-side interface of the plugged hydrogel by molecular sieving effect, providing the thin enzyme zone whose thickness was approximately 100 μm. When 4-methylumberiferyl phosphate (substrate) was electrokinetically introduced into the device after trapping the enzyme, 4-methylumberiferone (product) was generated by the enzyme reaction for only 1.26 s as the substrate passed through the trapped enzyme zone. As a result, the initial rate of the enzyme reaction could be directly calculated to 31.0 μM/s by simply dividing the concentration of the product by the tunable reaction time. Compared to the initial rate obtained by mixing the enzyme and substrate solutions, the value of the maximum velocity of the enzyme reaction was 30-fold larger than that in the mixing method due to the preconcentration of the enzyme by trapping. The Michaelis-Menten constant in the proposed method was 2.7-fold larger than that in the mixing method, suggesting the variation of changes in the equilibrium of complex formation under the experimental conditions.

Kinetic Analysis of the Interaction of Nicking Endonuclease BspD6I with DNA

Nicking endonucleases (NEs) are enzymes that incise only one strand of the duplex to produce a DNA molecule that is 'nicked' rather than cleaved in two. Since these precision tools are used in genetic engineering and genome editing, information about their mechanism of action at all stages of DNA recognition and phosphodiester bond hydrolysis is essential. For the first time, fast kinetics of the Nt.BspD6I interaction with DNA were studied by the stopped-flow technique, and changes of optical characteristics were registered for the enzyme or DNA molecules. The role of divalent metal cations was estimated at all steps of Nt.BspD6I-DNA complex formation. It was demonstrated that divalent metal ions are not required for the formation of a non-specific complex of the protein with DNA. Nt.BspD6I bound five-fold more efficiently to its recognition site in DNA than to a random DNA. DNA bending was confirmed during the specific binding of Nt.BspD6I to a substrate. The optimal size of Nt.BspD6I's binding site in DNA as determined in this work should be taken into account in methods of detection of nucleic acid sequences and/or even various base modifications by means of NEs.

Common Kinetic Mechanism of Abasic Site Recognition by Structurally Different Apurinic/Apyrimidinic Endonucleases

Apurinic/apyrimidinic (AP) endonucleases Nfo (Escherichia coli) and APE1 (human) represent two conserved structural families of enzymes that cleave AP-site–containing DNA in base excision repair. Nfo and APE1 have completely different structures of the DNA-binding site, catalytically active amino acid residues and catalytic metal ions. Nonetheless, both enzymes induce DNA bending, AP-site backbone eversion into the active-site pocket and extrusion of the nucleotide located opposite the damage. All these stages may depend on local stability of the DNA duplex near the lesion. Here, we analysed effects of natural nucleotides located opposite a lesion on catalytic-complex formation stages and DNA cleavage efficacy. Several model DNA substrates that contain an AP-site analogue [F-site, i.e., (2R,3S)-2-(hydroxymethyl)-3-hydroxytetrahydrofuran] opposite G, A, T or C were used to monitor real-time conformational changes of the tested enzymes during interaction with DNA using changes in the enzymes’ intrinsic fluorescence intensity mainly caused by Trp fluorescence. The extrusion of the nucleotide located opposite F-site was recorded via fluorescence intensity changes of two base analogues. The catalytic rate constant slightly depended on the opposite-nucleotide nature. Thus, structurally different AP endonucleases Nfo and APE1 utilise a common strategy of damage recognition controlled by enzyme conformational transitions after initial DNA binding.

Non-uniform sampling in pulse dipolar spectroscopy by EPR: the redistribution of noise and the optimization of data acquisition

Pulse dipolar spectroscopy (PDS) in Electron Paramagnetic Resonance (EPR) is the method of choice for determining the distance distribution function for mono-, bi- or multi- spin-labeled macromolecules and nanostructures. PDS acquisition schemes conventionally use uniform sampling of the dipolar trace, but non-uniform sampling (NUS) schemes can decrease the total measurement time or increase the accuracy of the resulting distance distributions. NUS requires optimization of the data acquisition scheme, as well as changes in data processing algorithms to accommodate the non-uniformly sampled data. We investigate in silico the applicability of the NUS approach in PDS, considering its effect on random, truncation and sampling noise in the experimental data. Each type of noise in the time-domain data propagates differently and non-uniformly into the distance spectrum as errors in the distance distribution. NUS schemes seem to be a valid approach for increasing sensitivity and/or throughput in PDS by decreasing and redistributing noise in the distance spectrum so that it has less impact on the distance spectrum.

The Enigma of Substrate Recognition and Catalytic Efficiency of APE1-Like Enzymes

Despite significant achievements in the elucidation of the nature of protein-DNA contacts that control the specificity of nucleotide incision repair (NIR) by apurinic/apyrimidinic (AP) endonucleases, the question on how a given nucleotide is accommodated by the active site of the enzyme remains unanswered. Therefore, the main purpose of our study was to compare kinetics of conformational changes of three homologous APE1-like endonucleases (insect Drosophila melanogaster Rrp1, amphibian Xenopus laevis xAPE1, and fish Danio rerio zAPE1) during their interaction with various damaged DNA substrates, i.e., DNA containing an F-site (an uncleavable by DNA-glycosylases analog of an AP-site), 1,N⁶-ethenoadenosine (εA), 5,6-dihydrouridine (DHU), uridine (U), or the α-anomer of adenosine (αA). Pre-steady-state analysis of fluorescence time courses obtained for the interaction of the APE1-like enzymes with DNA substrates containing various lesions allowed us to outline a model of substrate recognition by this class of enzymes. It was found that the differences in rates of DNA substrates’ binding do not lead to significant differences in the cleavage efficiency of DNA containing a damaged base. The results suggest that the formation of enzyme–substrate complexes is not the key factor that limits enzyme turnover; the mechanisms of damage recognition and cleavage efficacy are related to fine conformational tuning inside the active site.

Spectroscopic sensing and quantification of AP-endonucleases using fluorescence-enhancement by cis–trans isomerization of cyanine dyes

Apurinic/apyrimidinic (AP) endonucleases are vital DNA repair enzymes, and proposed to be a prognostic biomarker for various types of cancer in humans. Numerous DNA sensors have been developed to evaluate the extent of nuclease activity but their DNA termini are not protected against other nucleases, hampering accurate quantification. Here we developed a new fluorescence enhancement (FE)-based method as an enzyme-specific DNA biosensor with nuclease-protection by three functional units (an AP-site, Cy3 and termini that are protected from exonucleolytic cleavage). A robust FE signal arises from the fluorescent cis–trans isomerization of a cyanine dye (e.g., Cy3) upon the enzyme-triggered structural change from double-stranded (ds)DNA to single-stranded (ss)DNA that carries Cy3. The FE-based assay reveals a linear dependency on sub-nanomolar concentrations as low as 10⁻¹¹ M for the target enzyme and can be also utilized as a sensitive readout of other nuclease activities.

Apurinic/apyrimidinic (AP) endonucleases are vital DNA repair enzymes, and proposed to be a prognostic biomarker for various types of cancer in humans.

Probing the E/K Peptide Coiled-Coil Assembly by Double Electron-Electron Resonance and Circular Dichroism

Double electron-electron resonance (DEER, also known as PELDOR) and circular dichroism (CD) spectroscopies were explored for the purpose of studying the specificity of the conformation of peptides induced by their assembly into a self-recognizing system. The E and K peptides are known to form a coiled-coil heterodimer. Two paramagnetic TOAC α-amino acid residues were incorporated into each of the peptides (denoted as K** and E**), and a three-dimensional structural investigation in the presence or absence of their unlabeled counterparts E and K was performed. The TOAC spin-labels, replacing two Ala residues in each compound, are covalently and quasi-rigidly connected to the peptide backbone. They are known not to disturb the native structure, so that any conformational change can easily be monitored and assigned. DEER spectroscopy enables the measurement of the intramolecular electron spin-spin distance distribution between the two TOAC labels, within a length range of 1.5-8 nm. This method allows the individual conformational changes for the K**, K**/E, E**, and E**/K molecules to be investigated in glassy frozen solutions. Our data reveal that the conformations of the E** and K** peptides are strongly influenced by the presence of their counterparts. The results are discussed with those from CD spectroscopy and with reference to the already reported nuclear magnetic resonance data. We conclude that the combined DEER/TOAC approach allows us to obtain accurate and reliable information about the conformation of the peptides before and after their assembly into coiled-coil heterodimers. Applications of this induced fit method to other two-component, but more complex, systems, like a receptor and antagonists, a receptor and a hormone, and an enzyme and a ligand, are discussed.

Activity of Human Apurinic/Apyrimidinic Endonuclease APE1 Toward Damaged DNA and Native RNA With Non-canonical Structures

The primary role of apurinic/apyrimidinic (AP) endonuclease APE1 in human cells is the cleavage of the sugar phosphate backbone 5' to an AP site in DNA to produce a single-strand break with a 5'-deoxyribose phosphate and 3'-hydroxyl end groups. APE1 can also recognize and incise some damaged or modified nucleotides and possesses some minor activities: 3'-5' exonuclease, 3'-phosphodiesterase, 3'-phosphatase, and RNase H. A molecular explanation for the discrimination of structurally different substrates by the single active site of the enzyme remains elusive. Here, we report a mechanism of target nucleotide recognition by APE1 as revealed by the results of an analysis of the APE1 process involving damaged DNA and native RNA substrates with non-canonical structures. The mechanism responsible for substrate specificity proved to be directly related to the ability of a target nucleotide to get into the active site of APE1 in response to an enzyme-induced DNA distortion.

Kuznetsov N. The Role of Active-Site Plasticity in Damaged-Nucleotide Recognition by Human Apurinic/Apyrimidinic Endonuclease APE1

Human apurinic/apyrimidinic (AP) endonuclease APE1 hydrolyzes phosphodiester bonds on the 5′ side of an AP-site, and some damaged nucleotides such as 1,N6-ethenoadenosine (εA), α-adenosine (αA), and 5,6-dihydrouridine (DHU). To investigate the mechanism behind the broad substrate specificity of APE1, we analyzed pre-steady-state kinetics of conformational changes in DNA and the enzyme during DNA binding and damage recognition. Molecular dynamics simulations of APE1 complexes with one of damaged DNA duplexes containing εA, αA, DHU, or an F-site (a stable analog of an AP-site) revealed the involvement of residues Asn229, Thr233, and Glu236 in the mechanism of DNA lesion recognition. The results suggested that processing of an AP-site proceeds faster in comparison with nucleotide incision repair substrates because eversion of a small abasic site and its insertion into the active site do not include any unfavorable interactions, whereas the insertion of any target nucleotide containing a damaged base into the APE1 active site is sterically hindered. Destabilization of the α-helix containing Thr233 and Glu236 via a loss of the interaction between these residues increased the plasticity of the damaged-nucleotide binding pocket and the ability to accommodate structurally different damaged nucleotides. Nonetheless, the optimal location of εA or αA in the binding pocket does not correspond to the optimal conformation of catalytic amino acid residues, thereby significantly decreasing the cleavage efficacy for these substrates.

The role of active-site amino acid residues in the cleavage of DNA and RNA substrates by human apurinic/apyrimidinic endonuclease APE1

BACKGROUND

Human apurinic/apyrimidinic endonuclease APE1 is one of participants of the DNA base excision repair pathway. APE1 processes AP-sites and many other types of DNA damage via hydrolysis of the phosphodiester bond on the 5' side of the lesion. APE1 also acts as an endoribonuclease, i.e., can cleave undamaged RNA.

METHODS

Using pre-steady-state kinetic analysis we examined the role of certain catalytically important amino acids in APE1 enzymatic pathway and described their involvement in the mechanism of the target nucleotide recognition.

RESULTS

Comparative analysis of the cleavage efficiency of damaged DNAs containing an abasic site, 5,6-dihydrouridine, or α-anomer of adenosine as well as 3'-5'-exonuclease degradation of undamaged DNA and endonuclease hydrolysis of RNA substrates by mutant APE1 enzymes containing a substitution of an active-site amino acid residue (D210N, N212A, T268D, M270A, or D308A) was performed. Detailed pre-steady-state kinetics of conformational changes of the enzyme and of DNA substrate molecules during recognition and cleavage of the abasic site were studied.

CONCLUSIONS

It was revealed that substitution T268D significantly disturbed initial DNA binding, whereas Asn212 is critical for the DNA-bending stage and catalysis. Substitution D210N increased the binding efficacy and blocked the catalytic reaction, but D308A decreased the binding efficacy owing to disruption of Mg²⁺ coordination. Finally, the substitution of Met270 also destabilized the enzyme-substrate complex but did not affect the catalytic reaction.

SIGNIFICANCE

It was found that the tested substitutions of the active-site amino acid residues affected different stages of the complex formation process as well as the catalytic reaction.

Structural and Functional Characterization of a Unique AP Endonuclease From Deinococcus radiodurans

Various endogenous and exogenous agents cause DNA damage, including apurinic/apyrimidinic (AP) sites. Due to their cytotoxic effects, AP sites are usually cleaved by AP endonuclease through the base excision repair (BER) pathway. Deinococcus radiodurans, an extraordinary radiation-resistant bacterium, is known as an ideal model organism for elucidating DNA repair processes. Here, we have investigated a unique AP endonuclease (DrXth) from D. radiodurans and found that it possesses AP endonuclease, 3'-phosphodiesterase, 3'-phosphatase, and 3'-5' exonuclease but has no nucleotide incision repair (NIR) activity. We also found that Mg²⁺ and Mn²⁺ were the preferred divalent metals for endonuclease and exonuclease activities, respectively. In addition, DrXth were crystallized and the crystals diffracted to 1.5 Å. Structural and biochemical analyses demonstrated that residue Gly198 is the key residue involved in the substrate DNA binding and cleavage. Deletion of the drxth gene in D. radiodurans caused elevated sensitivity to DNA damage agents and increased spontaneous mutation frequency. Overall, our results indicate that DrXth is an important AP endonuclease involved in BER pathway and functions in conjunction with other DNA repair enzymes to maintain the genome stability.

An effective molecular blocker of ion channel of M2 protein as anti-influenza a drug

Design of a drug compound that can effectively bind to the M2 ion channel and block the diffusion of hydrogen ions (H⁺) through and inhibit influenza A virus replication is an important task. Known anti-influenza drugs amantadine and rimantadine have a weak effect on influenza A virus. A new class of positively charged, +2 e.u., molecules is proposed here to block diffusion of H⁺ ion through the M2 channel. Several drug candidates, derivatives of a lead compound (diazabicyclooctane), were proposed and investigated. Molecular dynamics of thermal fluctuations of M2 protein structure and ionization-conformation coupling of all the ionizable residues were simulated at physiological pH. The influence of the most probable mutations of key drug-binding amino acid residues in the M2 ion channel were investigated too. It is shown that the suggested new blocker has high binding affinity for the M2 channel. There are two in-channel binding sites of high affinity, the first one has H-bonds with two of four serine residues Ser-31A (B) or Ser-31C(D), and the second one has H-bonds with two of four histidine residues His-37A (B), or His-37C(D). The main advantage of the new drug molecule is the positive charge, +2 e.u., which creates a positive electrostatic potential barrier (in addition to a steric one) for a transfer of H⁺ ion through M2 channel and may serve as an effective anti-influenza A virus drug.Communicated by Ramaswamy H. Sarma.

Effect of the Substrate Structure and Metal Ions on the Hydrolysis of Undamaged RNA by Human AP Endonuclease APE1

Human apurinic/apyrimidinic (AP) endonuclease APE1 is one of the participants in the DNA base excision repair. The main biological function of APE1 is to hydrolyze the phosphodiester bond on the 5'-side of the AP sites. It has been shown recently that APE1 acts as an endoribonuclease and can cleave mRNA, thereby controlling the level of some transcripts. The sequences of CA, UA, and UG dinucleotides are the cleavage sites in RNA. In the present work, we performed a comparative analysis of the cleavage efficiency of model RNA substrates with short hairpin structures in which the loop size and the location of the pyrimidine-purine dinucleotide sequence were varied. The effect of various divalent metal ions and pH on the efficiency of the endoribonuclease reaction was analyzed. It was shown that site-specific hydrolysis of model RNA substrates depends on the spatial structure of the substrate. In addition, RNA cleavage occured in the absence of divalent metal ions, which proves that hydrolysis of DNA- and RNA substrates occurs via different catalytic mechanisms.

DNA complexes with human apurinic/apyrimidinic endonuclease 1: structural insights revealed by pulsed dipolar EPR with orthogonal spin labeling

A DNA molecule is under continuous influence of endogenous and exogenous damaging factors, which produce a variety of DNA lesions. Apurinic/apyrimidinic sites (abasic or AP sites) are among the most common DNA lesions. In this work, we applied pulse dipolar electron paramagnetic resonance (EPR) spectroscopy in combination with molecular dynamics (MD) simulations to investigate in-depth conformational changes in DNA containing an AP site and in a complex of this DNA with AP endonuclease 1 (APE1). For this purpose, triarylmethyl (TAM)-based spin labels were attached to the 5' ends of an oligonucleotide duplex, and nitroxide spin labels were introduced into APE1. In this way, we created a system that enabled monitoring the conformational changes of the main APE1 substrate by EPR. In addition, we were able to trace substrate-to-product transformation in this system. The use of different (orthogonal) spin labels in the enzyme and in the DNA substrate has a crucial advantage allowing for detailed investigation of local damage and conformational changes in AP-DNA alone and in its complex with APE1.

Ultrasensitive Apurinic/Apyrimidinic Site-Specific Ratio Fluorescent Rotor for Real-Time Highly Selective Evaluation of mtDNA Oxidative Damage in Living Cells

The unrepaired apurinic/apyrimidinic site (AP site) in mitochondrial DNA (mtDNA) promotes misincorporation of nucleotides and further causes serious damage for the living organism. Thus, accurate quantitative detection of AP sites in mtDNA in a rapid, highly sensitive, and highly selective fashion is important for the real-time evaluation of mtDNA oxidative damage. In this study, a targeting mtDNA ultrasensitive AP site-specific fluorescent rotor (BTBM-CN₂) was designed by the strategy of molecular conformation torsion adjustment ratio fluorescent signal. The specific recognition reaction is activated when it encountered AP sites in mtDNA within 20 s, and BTBM-CN₂ presented a "turn-on" red fluorescence signal at 598 nm. Then, about 100 s later, BTBM-CN₂ emitted a new green fluorescence signal at 480 nm, which is mainly due to the activation of the rate-limiting reaction. With increasing numbers of AP sites (1-40 in 1 × 10⁵ bp of mtDNA), the fluorescence emission at 598 nm decreased gradually, and the new emission at 480 nm increased. Intracellular experiments indicated that BTBM-CN₂ could detect AP sites in mtDNA in a rapid and quantitative fashion with high selectivity and ultrasensitivity. On the basis of the emergence of the fluorescence signal at 480 nm and its signal strength, the cell whose mtDNA was damaged could be screened by flow cytometry and its degree of damage could be evaluated in real time by comet assay. Hence, the rotor may have potential applications varying from accurate and ultrasensitive detection of AP sites to the real-time evaluation of the oxidative damage in living cells.

The impact of single-nucleotide polymorphisms of human apurinic/apyrimidinic endonuclease 1 on specific DNA binding and catalysis

Human apurinic/apyrimidinic (AP) endonuclease APE1 is a crucial enzyme of the base excision repair (BER) pathway, which is in charge of recognition and initiation of removal of AP-sites in DNA. It is known that some single-nucleotide polymorphism (SNP) variants of APE1 have a reduced activity as compared to wild-type APE1. It has been hypothesized that genetic variation in APE1 might be responsible for an increased risk of some types of cancer. In the present work, analysis of SNPs of the APE1 gene was performed to select the set of variants having substitutions of amino acid residues on the surface of the enzyme globule and in the DNA-binding site, thereby affecting protein-protein interactions or the catalytic reaction, respectively. For seven APE1 variants (R221C, N222H, R237A, G241R, M270T, R274Q, and P311S), conformational dynamics and catalytic activities were examined. The conformational changes in the molecules of APE1 variants and in a DNA substrate were recorded as fluorescence changes of Trp and 2-aminopurine residues, respectively, using the stopped-flow technique. The results made it possible to determine the kinetic mechanism underlying the interactions of the APE1 variants with DNA substrates, to calculate the rate constants of the elementary stages, and to identify the stages of the process affected by mutation.