Structural characterization of Clostridium acetobutylicum 8-oxoguanine DNA glycosylase in its apo form and in complex with 8-oxodeoxyguanosine

Differential Binding of NLRP3 to non-oxidized and Ox-mtDNA mediates NLRP3 Inflammasome Activation

The NLRP3 inflammasome is a key mediator of the innate immune response to sterile tissue injury and is involved in many chronic and acute diseases. Physically and chemically diverse agents activate the NLRP3 inflammasome. Here, we show that NLRP3 binds non-oxidized and Ox-mtDNA differentially, with a half maximum inhibitory concentration (IC50) for non-oxidized and Ox-mtDNA of 4 nM and 247.2 nM, respectively. The NLRP3 Neonatal-Onset Multisystem Inflammatory Disease (NOMIDFCAS) gain of function mutant could bind non-oxidized mtDNA but had higher affinity for Ox-mtDNA compared to WT with an IC50 of 8.1 nM. NLRP3 lacking the pyrin domain can bind both oxidized and non-oxidized mtDNA. Isolated pyrin domain prefers Ox-mtDNA. The NLRP3 pyrin domain shares a protein fold with DNA glycosylases and generate a model for DNA binding based on the structure and sequence alignment to Clostridium acetobutylicum and human OGG1, an inhibitor of Ox-mtDNA generation, 8-oxoguanine DNA glycosylases. We provide a new model for how NLRP3 interacts with Ox-mtDNA supported by DNA binding in the presence of a monoclonal antibody against the pyrin domain. These results give new insights into the mechanism of inflammasome assembly, and into the function of reactive oxygen species in establishing a robust immune response.

A thermophilic 8-oxoguanine DNA glycosylase from Thermococcus barophilus Ch5 is a new member of AGOG DNA glycosylase family

8-Oxoguanine (8oxoG) in DNA is a major oxidized base that poses a severe threat to genome stability. To counteract the mutagenic effect generated by 8oxoG in DNA, cells have evolved 8oxoG DNA glycosylase (OGG) that can excise this oxidized base from DNA. Currently, OGG enzymes have been divided into three families: OGG1, OGG2 and AGOG (archaeal 8oxoG DNA glycosylase). Due to the limited reports, our understanding on AGOG enzymes remains incomplete. Herein, we present evidence that an AGOG from the hyperthermophilic euryarchaeon Ch5 (Tb-AGOG) excises 8oxoG from DNA at high temperature. The enzyme displays maximum efficiency at 75°C-95°C and at pH 9.0. As expected, Tb-AGOG is a bifunctional glycosylase that harbors glycosylase activity and AP (apurinic/apyrimidinic) lyase activity. Importantly, we reveal for the first time that residue D41 in Tb-AGOG is essential for 8oxoG excision and intermediate formation, but not essential for DNA binding or AP cleavage. Furthermore, residue E79 in Tb-AGOG is essential for 8oxoG excision and intermediate formation, and is partially involved in DNA binding and AP cleavage, which has not been described among the reported AGOG members to date. Overall, our work provides new insights into catalytic mechanism of AGOG enzymes.

Biochemical characterization and mutational studies of the 8-oxoguanine DNA glycosylase from the hyperthermophilic and radioresistant archaeon Thermococcus gammatolerans

8-oxoguanine (GO) is a major lesion found in DNA that arises from guanine oxidation. The hyperthermophilic and radioresistant euryarchaeon Thermococcus gammatolerans encodes an archaeal GO DNA glycosylase (Tg-AGOG). Here, we characterized biochemically Tg-AGOG and probed its GO removal mechanism by mutational studies. Tg-AGOG can remove GO from DNA at high temperature through a β-elimination reaction. The enzyme displays an optimal temperature, ca.85-95 °C, and an optimal pH, ca.7.0-8.5. In addition, Tg-AGOG activity is independent on a divalent metal ion. However, both Co²⁺ and Cu²⁺ inhibit its activity. The enzyme activity is also inhibited by NaCl. Furthermore, Tg-AGOG specifically cleaves GO-containing dsDNA in the order: GO:C, GO:T, GO:A, and GO:G. Moreover, the temperature dependence of cleavage rates of the enzyme was determined, and from this, the activation energy for GO removal from DNA was first estimated to be 16.9 ± 0.9 kcal/mol. In comparison with the wild-type Tg-AGOG, the R197A mutant has a reduced cleavage activity for GO-containing DNA, whereas both the P193A and F167A mutants exhibit similar cleavage activities for GO-containing DNA. While the mutations of P193 and F167 to Ala lead to increased binding, the mutation of R197 to Ala had no significant effect on binding. These observations suggest that residue R197 is involved in catalysis, and residues P193 and F167 are flexible for conformational change.

Reading and Misreading 8-oxoguanine, a Paradigmatic Ambiguous Nucleobase

7,8-Dihydro-8-oxoguanine (oxoG) is the most abundant oxidative DNA lesion with dual coding properties. It forms both Watson–Crick (anti)oxoG:(anti)C and Hoogsteen (syn)oxoG:(anti)A base pairs without a significant distortion of a B-DNA helix. DNA polymerases bypass oxoG but the accuracy of nucleotide incorporation opposite the lesion varies depending on the polymerase-specific interactions with the templating oxoG and incoming nucleotides. High-fidelity replicative DNA polymerases read oxoG as a cognate base for A while treating oxoG:C as a mismatch. The mutagenic effects of oxoG in the cell are alleviated by specific systems for DNA repair and nucleotide pool sanitization, preventing mutagenesis from both direct DNA oxidation and oxodGMP incorporation. DNA translesion synthesis could provide an additional protective mechanism against oxoG mutagenesis in cells. Several human DNA polymerases of the X- and Y-families efficiently and accurately incorporate nucleotides opposite oxoG. In this review, we address the mutagenic potential of oxoG in cells and discuss the structural basis for oxoG bypass by different DNA polymerases and the mechanisms of the recognition of oxoG by DNA glycosylases and dNTP hydrolases.

Probing the activity of NTHL1 orthologs by targeting conserved amino acid residues

The base excision repair DNA glycosylases, EcoNth and hNTHL1, are homologous, with reported overlapping yet different substrate specificities. The catalytic amino acid residues are known and are identical between the two enzymes although the exact structures of the substrate binding pockets remain to be determined. We sought to explore the sequence basis of substrate differences using a phylogeny-based design of site-directed mutations. Mutations were made for each enzyme in the vicinity of the active site and we examined these variants for glycosylase and lyase activity. Single turnover kinetics were done on a subgroup of these, comparing activity on two lesions, 5,6-dihydrouracil and 5,6-dihydrothymine, with different opposite bases. We report that wild type hNTHL1 and EcoNth are remarkably alike with respect to the specificity of the glycosylase reaction, and although hNTHL1 is a much slower enzyme than EcoNth, the tighter binding of hNTHL1 compensates, resulting in similar kcat/Kd values for both enzymes with each of the substrates tested. For the hNTHL1 variant Gln287Ala, the specificity for substrates positioned opposite G is lost, but not that of substrates positioned opposite A, suggesting a discrimination role for this residue. The EcoNth Thr121 residue influences enzyme binding to DNA, as binding is significantly reduced with the Thr121Ala variant. Finally, we present evidence that hNTHL1 Asp144, unlike the analogous EcoNth residue Asp44, may be involved in resolving the glycosylase transition state.

Zinc finger oxidation of Fpg/Nei DNA glycosylases by 2-thioxanthine: biochemical and X-ray structural characterization

DNA glycosylases from the Fpg/Nei structural superfamily are base excision repair enzymes involved in the removal of a wide variety of mutagen and potentially lethal oxidized purines and pyrimidines. Although involved in genome stability, the recent discovery of synthetic lethal relationships between DNA glycosylases and other pathways highlights the potential of DNA glycosylase inhibitors for future medicinal chemistry development in cancer therapy. By combining biochemical and structural approaches, the physical target of 2-thioxanthine (2TX), an uncompetitive inhibitor of Fpg, was identified. 2TX interacts with the zinc finger (ZnF) DNA binding domain of the enzyme. This explains why the zincless hNEIL1 enzyme is resistant to 2TX. Crystal structures of the enzyme bound to DNA in the presence of 2TX demonstrate that the inhibitor chemically reacts with cysteine thiolates of ZnF and induces the loss of zinc. The molecular mechanism by which 2TX inhibits Fpg may be generalized to all prokaryote and eukaryote ZnF-containing Fpg/Nei-DNA glycosylases. Cell experiments show that 2TX can operate in cellulo on the human Fpg/Nei DNA glycosylases. The atomic elucidation of the determinants for the interaction of 2TX to Fpg provides the foundation for the future design and synthesis of new inhibitors with high efficiency and selectivity.

Crystal structures of MBOgg1 in complex with two abasic DNA ligands

7,8-Dihydro-8-oxoguanine (8-oxoG) is one of the most common oxidative DNA lesions. 8-oxoguanine DNA glycosylases (Oggs) detect and excise 8-oxoG through a multiple-step process. To better understand the basis for estranged base recognition, we have solved the crystal structures of MBOgg1, the 8-oxoguanine DNA glycosylase of Thermoanaerobacter tengcongensis, in complex with DNA containing a tetrahydrofuranyl site (THF, a stable abasic site analog) paired with an estranged cytosine (MBOgg1/DNA(THF:C)) or thymine (MBOgg1/DNA(THF:T)). Different states of THF (extrahelical or intrahelical) are observed in the two complexes of the ASU of MBOgg1/DNA(THF:C) structure. Analyses of their different interaction modes reveal that variable contacts on the 5' region flanking the THF abasic site are correlated with the states of the THF. Comparison of MBOgg1/DNA(THF:T) with MBOgg1/DNA(THF:C) indicates that the non-preferred estranged T may affect MBOgg1's contacts with the 5' flank of the lesion strand. Furthermore, we identified a region in MBOgg1 that is rich in positive charges and interacts with the 5' region flanking the lesion. This region is conserved only in non-eukaryotic Oggs, and additional mutagenesis and biochemical assays reveal that it may contribute to the distinct estranged base specificities between eukaryotic and non-eukaryotic Oggs.

Recent advances in the structural mechanisms of DNA glycosylases

DNA glycosylases safeguard the genome by locating and excising a diverse array of aberrant nucleobases created from oxidation, alkylation, and deamination of DNA. Since the discovery 28years ago that these enzymes employ a base flipping mechanism to trap their substrates, six different protein architectures have been identified to perform the same basic task. Work over the past several years has unraveled details for how the various DNA glycosylases survey DNA, detect damage within the duplex, select for the correct modification, and catalyze base excision. Here, we provide a broad overview of these latest advances in glycosylase mechanisms gleaned from structural enzymology, highlighting features common to all glycosylases as well as key differences that define their particular substrate specificities.

8-oxoguanine DNA glycosylases: one lesion, three subfamilies

Amongst the four bases that form DNA, guanine is the most susceptible to oxidation, and its oxidation product, 7,8-dihydro-8-oxoguanine (8-oxoG) is the most prevalent base lesion found in DNA. Fortunately, throughout evolution cells have developed repair mechanisms, such as the 8-oxoguanine DNA glycosylases (OGG), which recognize and excise 8-oxoG from DNA thereby preventing the accumulation of deleterious mutations. OGG are divided into three subfamilies, OGG1, OGG2 and AGOG, which are all involved in the base excision repair (BER) pathway. The published structures of OGG1 and AGOG, as well as the recent availability of OGG2 structures in both apo- and liganded forms, provide an excellent opportunity to compare the structural and functional properties of the three OGG subfamilies. Among the observed differences, the three-dimensional fold varies considerably between OGG1 and OGG2 members, as the latter lack the A-domain involved in 8-oxoG binding. In addition, all three OGG subfamilies bind 8-oxoG in a different manner even though the crucial interaction between the enzyme and the protonated N7 of 8-oxoG is conserved. Finally, the three OGG subfamilies differ with respect to DNA binding properties, helix-hairpin-helix motifs, and specificity for the opposite base.

The C-terminal lysine of Ogg2 DNA glycosylases is a major molecular determinant for guanine/8-oxoguanine distinction

7,8-Dihydro-8-oxoguanine (8-oxoG) is a major oxidative lesion found in DNA. The 8-oxoguanine DNA glycosylases (Ogg) responsible for the removal of 8-oxoG are divided into three families Ogg1, Ogg2 and AGOG. The Ogg2 members are devoid of the recognition loop used by Ogg1 to discriminate between 8-oxoG and guanine and it was unclear until recently how Ogg2 enzymes recognize the oxidized base. We present here the first crystallographic structure of an Ogg2 member, Methanocaldococcus janischii Ogg, in complex with a DNA duplex containing the 8-oxoG lesion. This structure highlights the crucial role of the C-terminal lysine, strictly conserved in Ogg2, in the recognition of 8-oxoG. The structure also reveals that Ogg2 undergoes a conformational change upon DNA binding similar to that observed in Ogg1 glycosylases. Furthermore, this work provides a structural rationale for the lack of opposite base specificity in this family of enzymes.

Structural basis for the lack of opposite base specificity of Clostridium acetobutylicum 8-oxoguanine DNA glycosylase

7,8-Dihydro-8-oxoguanine (8-oxoG) is the major oxidative product of guanine and the most prevalent base lesion observed in DNA molecules. Because 8-oxoG has the capability to form a Hoogsteen pair with adenine (8-oxoG:A) in addition to a normal Watson-Crick pair with cytosine (8-oxoG:C), this lesion can lead to a G:C-->T:A transversion after replication. However, 8-oxoG is recognized and excised by the 8-oxoguanine DNA glycosylase (Ogg) of the base excision repair pathway. Members of the Ogg1 family usually display a strong preference for a C opposite the lesion. In contrast, the atypical Ogg1 from Clostridium actetobutylicum (CacOgg) can excise 8-oxoG when paired with either one of the four bases, albeit with a preference for C and A. Here we describe the first high-resolution crystal structures of CacOgg in complex with duplex DNA containing the 8-oxoG lesion paired to cytosine and to adenine. A structural comparison with human OGG1 provides a rationale for the lack of opposite base specificity displayed by the bacterial Ogg.

Crystal structures of two archaeal 8-oxoguanine DNA glycosylases provide structural insight into guanine/8-oxoguanine distinction

Among the four DNA bases, guanine is particularly vulnerable to oxidative damage and the most common oxidative product, 7,8-dihydro-8-oxoguanine (8-oxoG), is the most prevalent lesion observed in DNA molecules. Fortunately, 8-oxoG is recognized and excised by the 8-oxoguanine DNA glycosylase (Ogg) of the base excision repair pathway. Ogg enzymes are divided into three separate families, namely, Ogg1, Ogg2, and archaeal GO glycosylase (AGOG). To date, structures of members of both Ogg1 and AGOG families are known but no structural information is available for members of Ogg2. Here we describe the first crystal structures of two archaeal Ogg2: Methanocaldococcus janischii Ogg and Sulfolobus solfataricus Ogg. A structural comparison with OGG1 and AGOG suggested that the C-terminal lysine of Ogg2 may play a key role in discriminating between guanine and 8-oxoG. This prediction was substantiated by measuring the glycosylase/lyase activity of a C-terminal deletion mutant of MjaOgg.