[Show abstract][Hide abstract] ABSTRACT: DNA apurinic-apyrimidinic (AP) sites are prevalent noncoding threats to genomic stability and are processed by AP endonuclease 1 (APE1). APE1 incises the AP-site phosphodiester backbone, generating a DNA-repair intermediate that is potentially cytotoxic. The molecular events of the incision reaction remain elusive, owing in part to limited structural information. We report multiple high-resolution human APE1-DNA structures that divulge new features of the APE1 reaction, including the metal-binding site, the nucleophile and the arginine clamps that mediate product release. We also report APE1-DNA structures with a T-G mismatch 5' to the AP site, representing a clustered lesion occurring in methylated CpG dinucleotides. These structures reveal that APE1 molds the T-G mismatch into a unique Watson-Crick-like geometry that distorts the active site, thus reducing incision. These snapshots provide mechanistic clarity for APE1 while affording a rational framework to manipulate biological responses to DNA damage.
[Show abstract][Hide abstract] ABSTRACT: DNA polymerases facilitate faithful insertion of nucleotides, a central reaction occurring during DNA replication and repair. DNA synthesis (forward reaction) is "balanced," as dictated by the chemical equilibrium by the reverse reaction of pyrophosphorolysis. Two closely spaced divalent metal ions (catalytic and nucleotide-binding metals) provide the scaffold for these reactions. The catalytic metal lowers the pKa of O3' of the growing primer terminus, and the nucleotide-binding metal facilitates substrate binding. Recent time-lapse crystallographic studies of DNA polymerases have identified an additional metal ion (product metal) associated with pyrophosphate formation, leading to the suggestion of its possible involvement in the reverse reaction. Here, we establish a rationale for a role of the product metal using quantum mechanical/molecular mechanical calculations of the reverse reaction in the confines of the DNA polymerase β active site. Additionally, site-directed mutagenesis identifies essential residues and metal-binding sites necessary for pyrophosphorolysis. The results indicate that the catalytic metal site must be occupied by a magnesium ion for pyrophosphorolysis to occur. Critically, the product metal site is occupied by a magnesium ion early in the pyrophosphorolysis reaction path but must be removed later. The proposed dynamic nature of the active site metal ions is consistent with crystallographic structures. The transition barrier for pyrophosphorolysis was estimated to be significantly higher than that for the forward reaction, consistent with kinetic activity measurements of the respective reactions. These observations provide a framework to understand how ions and active site changes could modulate the internal chemical equilibrium of a reaction that is central to genome stability.
Full-text · Article · Sep 2015 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Although the two B-family human DNA polymerases, pol delta and pol epsilon, are responsible for the bulk of nuclear genome replication, at least 14 additional polymerases have roles in nuclear DNA repair and replication. In this issue, newly reported crystal structures of two specialized A-family polymerases, pol nu and pol theta, expose these enzymes' strategies for handling aberrant DNA ends.
[Show abstract][Hide abstract] ABSTRACT: Oxidative stress promotes genomic instability and human diseases. A common oxidized nucleoside is 8-oxo-7,8-dihydro-2'-deoxyguanosine, which is found both in DNA (8-oxo-G) and as a free nucleotide (8-oxo-dGTP). Nucleotide pools are especially vulnerable to oxidative damage. Therefore cells encode an enzyme (MutT/MTH1) that removes free oxidized nucleotides. This cleansing function is required for cancer cell survival and to modulate Escherichia coli antibiotic sensitivity in a DNA polymerase (pol)-dependent manner. How polymerases discriminate between damaged and non-damaged nucleotides is not well understood. This analysis is essential given the role of oxidized nucleotides in mutagenesis, cancer therapeutics, and bacterial antibiotics. Even with cellular sanitizing activities, nucleotide pools contain enough 8-oxo-dGTP to promote mutagenesis. This arises from the dual coding potential where 8-oxo-dGTP(anti) base pairs with cytosine and 8-oxo-dGTP(syn) uses its Hoogsteen edge to base pair with adenine. Here we use time-lapse crystallography to follow 8-oxo-dGTP insertion opposite adenine or cytosine with human pol β, to reveal that insertion is accommodated in either the syn- or anti-conformation, respectively. For 8-oxo-dGTP(anti) insertion, a novel divalent metal relieves repulsive interactions between the adducted guanine base and the triphosphate of the oxidized nucleotide. With either templating base, hydrogen-bonding interactions between the bases are lost as the enzyme reopens after catalysis, leading to a cytotoxic nicked DNA repair intermediate. Combining structural snapshots with kinetic and computational analysis reveals how 8-oxo-dGTP uses charge modulation during insertion that can lead to a blocked DNA repair intermediate.
[Show abstract][Hide abstract] ABSTRACT: DNA polymerases and substrates undergo conformational changes upon forming protein-ligand complexes. These conformational adjustments can hasten or deter DNA synthesis and influence substrate discrimination. From structural comparison of binary DNA and ternary DNA/dNTP complexes of DNA polymerase β, several side-chains have been implicated in facilitating formation of an active ternary complex poised for chemistry. Site-directed mutagenesis of these highly conserved residues (Asp192, Arg258, Phe272, Glu295, and Tyr296) and kinetic characterization provides insight into the role these residues play during correct and incorrect insertion as well as their role in conformational activation. The catalytic efficiencies for correct nucleotide insertion for alanine mutants was wild type ≈ R258A > F272A ≈ Y296A > E295A > D192. Since the efficiencies for incorrect insertion was affected to about the same extent for each mutant, effects on fidelity were modest (<5-fold). The R258A mutant exhibited an increase in the single-turnover rate of correct nucleotide insertion. This suggests that the wild-type Arg258 side-chain generates a population of non-productive ternary complexes. Structures of binary and ternary substrate complexes of the R258A mutant and a mutant associated with gastric carcinomas, E295K, provide molecular insight into intermediate structural conformations not appreciated previously. While the R258A mutant crystal structures were similar to wild-type enzyme, the open ternary complex structure of E295K indicates that Arg258 stabilizes a non-productive conformation of the primer terminus that would decrease catalysis. Significantly, the open E295K ternary complex binds two metal ions indicating that metal binding cannot overcome the modified interactions that have interrupted the closure of the N-subdomain.
No preview · Article · Sep 2014 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: Mammalian DNA polymerase (pol) β is the founding member of a large group of DNA polymerases now termed the X-family. DNA polymerase β has been kinetically, structurally, and biologically well characterized and can serve as a phylogenetic reference. Accordingly, we have performed a phylogenetic analysis to understand the relationship between pol β and other members of the X-family of DNA polymerases. The bacterial X-family DNA polymerases, Saccharomyces cerevisiae pol IV, and four mammalian X-family polymerases appear to be directly related. These enzymes originated from an ancient common ancestor characterized in two Bacillus species. Understanding distinct functions for each of the X-family polymerases, evolving from a common bacterial ancestor is of significant interest in light of the specialized roles of these enzymes in DNA metabolism.
[Show abstract][Hide abstract] ABSTRACT: Cytosine methylation and demethylation in tracks of CpG dinucleotides is an epigenetic mechanism for control of gene expression. The initial step in the demethylation process can be deamination of 5-methylcytosine producing the TpG alteration and T:G mispair, and this step is followed by thymine DNA glycosylase (TDG) initiated base excision repair (BER). A further consideration is that guanine in the CpG dinucleotide may become oxidized to 7,8-dihydro-8-oxoguanine (8-oxoG), and this could affect the demethylation process involving TDG-initiated BER. However, little is known about the enzymology of BER of tandemly altered CpG dinucleotides; e.g., Tp8-oxoG. Here, we investigated interactions between this altered dinucleotide and purified BER enzymes, the DNA glycosylases TDG and 8-oxoG DNA glycosylase 1 (OGG1), apurinic/apyrimidinic (AP) endonuclease 1, DNA polymerase β and DNA ligases. The overall TDG-initiated BER of the Tp8-oxoG dinucleotide is significantly reduced. Specifically, TDG and DNA ligase activities are reduced by a 3[prime]-flanking 8-oxoG. In contrast, OGG1-initiated BER pathway is blocked due to the 5[prime]-flanking T:G mispair; this reduces OGG1, AP endonuclease 1, and DNA polymerase β activities. Further, in TDG-initiated BER, TDG remains bound to its product AP site blocking OGG1 access to the adjacent 8-oxoG. These results reveal BER enzyme specificities enabling suppression of OGG1-initiated BER and coordination of TDG-initiated BER at this tandem alteration in the CpG dinucleotide.
[Show abstract][Hide abstract] ABSTRACT: DNA polymerase (pol) β is a multi-domain enzyme with two enzymatic activities that plays a central role in the overlapping base excision repair and single strand break repair pathways. The high frequency of pol β variants identified in tumor-derived tissues suggests a possible role in the progression of cancer, making it of interest to determine the functional consequences of these variants. Pol β containing a proline substitution for leucine 22 in the lyase domain (LD), identified in gastric tumors, has been reported to exhibit severe impairment of both lyase and polymerase activities. NMR spectroscopic evaluations of both pol β and the isolated LD containing the L22P mutation demonstrate destabilization sufficient to result in LD-selective unfolding with minimal structural perturbations to the polymerase domain. Unexpectedly, addition of single-stranded or hairpin DNA resulted in partial refolding of the mutated lyase domain, both in isolation and for the full-length enzyme. Further, formation of an abortive ternary complex using Ca2+ and a complementary dNTP indicates that the fraction of pol β(L22P) containing the folded LD undergoes conformational activation similar to that of the wild-type enzyme. Kinetic characterization of the polymerase activity of L22P pol β indicates that the L22P mutation compromises DNA binding, but nearly wild-type catalytic rates can be observed at elevated substrate concentrations. The organic osmolyte trimethylamine N-oxide (TMAO) is similarly able to induce folding and kinetic activation of both polymerase and lyase activities of the mutant. Kinetic data indicate synergy between the TMAO cosolvent and substrate binding. NMR data indicate that the effect of the DNA results primarily from interaction with the folded LD(L22P), while the effect of the TMAO results primarily from destabilization of the unfolded LD(L22P). These studies illustrate that substrate-induced catalytic activation of pol β provides an optimal enzyme conformation even in the presence of a strongly destabilizing point mutation. Accordingly, it remains to be determined whether this mutation alters the threshold of cellular repair activity needed for routine genome maintenance or whether the 'inactive' variant interferes with DNA repair.
[Show abstract][Hide abstract] ABSTRACT: Kinetics studies of dNTP analogs having pyrophosphate-mimicking β,γ-pCXYp leaving groups with variable X,Y-substitution reveal striking differences in the chemical transition-state energy for DNA polymerase β that depend on all aspects of base pairing configurations, including whether the incoming dNTP is a purine or pyrimidine and if base pairings are right (T•A, G•C) or wrong (T•G, G•T). Brønsted plots of the catalytic rate constant (log(kpol)) vs pKa4 for the leaving group exhibit Linear Free Energy Relationships (LFERs) with negative slopes ranging from -0.6 to -2.0, consistent with chemical rate-determining transition-states in which the active site adjusts to charge stabilization demand during chemistry depending on base-pair configuration. The Brønsted slopes and also intercepts differ dramatically, and provide the first direct evidence that dNTP base recognition by the enzyme-primer-template complex triggers a conformational change in the catalytic region of the active site, that significantly modifies the rate-determining chemical step.
[Show abstract][Hide abstract] ABSTRACT: To interpret recent structures of the human DNA repair enzyme DNA polymerase β (pol β) differing in the number of Mg(2+) ions, we apply transition path sampling (TPS) to assess the effect of differing ion placement on the transition from the open one-metal to the closed two-metal state. We find that the closing pathway depends on the initial ion position, both in terms of the individual transition states and associated energies. The energy barrier of the conformational pathway varies from 25 to 58 kJ/mol, compared to the conformational energy barrier of 42 kJ/mol for the wild-type pol β reported previously. Moreover, we find a preferred ion route located in the center of the enzyme, parallel to the DNA. Within this route, the conformational pathway is similar to that of the overall open to closed transition of pol β but outside it, especially when ion starts near active site residues Arg258 and Asp190, the conformational pathway diverges significantly. We hypothesize that our findings should apply generally to pol β, since R283 is relatively far from the active site. Our hypothesis suggests further experimental and computational work. Our studies also underscore the common feature that less active mutants have less stable closed states than their open states, in marked contrast to the wild-type enzyme, where the closed state is significantly more stable than the open form.
Full-text · Article · Feb 2014 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: DNA polymerase β (pol β) plays a crucial role in the base-excision repair (BER) pathway that cleanses the genome of apurinic/apyrimidinic (AP) sites. AP sites arise in DNA from spontaneous base loss (depurination) and DNA damage-specific glycosylases that hydrolyze the N-glycosidic bond between the deoxyribose and damaged base. AP sites represent a potentially dangerous lesion to a cell since they can be mutagenic or cytotoxic. Pol β is composed of two specialized domains that contribute two enzymatic activities, DNA synthesis and lyase, during the repair of AP sites. The amino-terminal domain of pol β possesses the lyase activity necessary to remove the 5′-deoxyribose phosphate intermediate generated during BER. Failure to remove this group may initiate alternate BER pathways. The larger polymerase domain has nucleotidyl transferase activity. This domain has a modular organization with subdomains that bind duplex DNA, catalytic metals, and the correct nucleoside triphosphate in a template-dependent manner. Detailed crystallographic structures of pre-catalytic complexes and mutagenic intermediates of pol β have provided insights into the strategies utilized by polymerases to ensure high-fidelity DNA synthesis.
[Show abstract][Hide abstract] ABSTRACT: The review demonstrates that structural comparison of DNA polymerase architecture suggests a nucleotide gateway to the polymerase active site. The identity, crystallographic structures, accessory domains, and biological function of the polymerases have been analyzed. Investigations reveal that the polymerase domain of most DNA polymerases is comprised of three subdomains. The architectural arrangement of the subdomains of A-, B-, and Y-family DNA polymerases has been compared to a right hand and is referred to as fingers, palm, and thumb. The palm subdomains are homologous despite the structures of the fingers and thumb subdomains being distinct among these families. The active site acidic residues that coordinate two divalent metals necessary for nucleotidyl transfer are found in the palm subdomain.
No preview · Article · Dec 2013 · Chemical Reviews
[Show abstract][Hide abstract] ABSTRACT: To aid in the characterization of the relationship of structure and function for human immunodeficiency virus type-1 reverse transcriptase (HIV-1 RT), this investigation utilized DNAs containing benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide (BPDE)-modified primers and templates as a probe of the architecture of this complex. BPDE lesions that differed in their stereochemistry around the C10 position were covalently linked to N (6)-adenine and positioned in either the primer or template strand of a duplex template-primer. HIV-1 RT exhibited a stereoisomer-specific and strand-specific difference in replication when the BPDE-lesion was placed in the template versus the primer strand. When the C10 R-BPDE adduct was positioned in the primer strand in duplex DNA, 5 nucleotides from the 3΄ end of the primer terminus, HIV-1 RT could not fully replicate the template, producing truncated products; this block to further synthesis did not affect rates of dissociation or DNA binding affinity. Additionally, when the adducts were in the same relative position, but located in the template strand, similar truncated products were observed with both the C10 R and C10 S BPDE adducts. These data suggest that the presence of covalently-linked intercalative DNA adducts distant from the active site can lead to termination of DNA synthesis catalyzed by HIV-1 RT.
[Show abstract][Hide abstract] ABSTRACT: Human 8-oxoguanine DNA glycosylase (OGG1) excises the mutagenic oxidative DNA lesion 8-oxo-7,8-dihydroguanine (8-oxoG) from DNA. Kinetic characterization of OGG1 is undertaken to measure the rates of 8-oxoG excision and product release. When the OGG1 concentration is lower than substrate DNA, time courses of product formation are biphasic; a rapid exponential phase (i.e. burst) of product formation is followed by a linear steady-state phase. The initial burst of product formation corresponds to the concentration of enzyme properly engaged on the substrate, and the burst amplitude depends on the concentration of enzyme. The first-order rate constant of the burst corresponds to the intrinsic rate of 8-oxoG excision and the slower steady-state rate measures the rate of product release (product DNA dissociation rate constant, koff). Here, we describe steady-state, pre-steady-state, and single-turnover approaches to isolate and measure specific steps during OGG1 catalytic cycling. A fluorescent labeled lesion-containing oligonucleotide and purified OGG1 are used to facilitate precise kinetic measurements. Since low enzyme concentrations are used to make steady-state measurements, manual mixing of reagents and quenching of the reaction can be performed to ascertain the steady-state rate (koff). Additionally, extrapolation of the steady-state rate to a point on the ordinate at zero time indicates that a burst of product formation occurred during the first turnover (i.e. y-intercept is positive). The first-order rate constant of the exponential burst phase can be measured using a rapid mixing and quenching technique that examines the amount of product formed at short time intervals (<1 sec) before the steady-state phase and corresponds to the rate of 8-oxoG excision (i.e. chemistry). The chemical step can also be measured using a single-turnover approach where catalytic cycling is prevented by saturating substrate DNA with enzyme (E>S). These approaches can measure elementary rate constants that influence the efficiency of removal of a DNA lesion.
No preview · Article · Sep 2013 · Journal of Visualized Experiments
[Show abstract][Hide abstract] ABSTRACT: Human DNA polymerase (pol) β is essential for base excision repair. We previously reported a triple somatic mutant of pol β (p.P261L/T292A/I298T) found in an early onset prostate tumor. This mutation abolishes polymerase activity, and the wild-type allele was not present in the tumor, indicating a complete deficiency in pol β function. The effect on polymerase activity is unexpected because the point mutations that comprise the triple mutant are not part of the active site. Herein, we demonstrate the mechanism of this loss-of-function. In order to understand the effect of the individual point mutations we biochemically analyzed all single and double mutants that comprise the triple mutant. We found that the p.I298T mutation is responsible for a marked instability of the triple mutant protein at 37˚C. At room temperature the triple mutant's low efficiency is also due to a decrease in the apparent binding affinity for the dNTP substrate, which is due to the p.T292A mutation. Furthermore, the triple mutant displays lower fidelity for transversions in vitro, due to the p.T292A mutation. We conclude that distinct mutations of the triple pol β mutant are responsible for the loss of activity, lower fidelity, and instability observed in vitro.
Full-text · Article · Jul 2013 · International Journal of Oncology
[Show abstract][Hide abstract] ABSTRACT: DNA polymerase (pol) β is a model polymerase involved in gap-filling DNA synthesis utilizing two metals to facilitate nucleotidyl transfer. Previous structural studies have trapped catalytic intermediates by utilizing substrate analogs (dideoxy-terminated primer or nonhydrolysable incoming nucleotide). To identify additional intermediates during catalysis, we now employ natural substrates (correct and incorrect nucleotides) and follow product formation in real time with 15 different crystal structures. We are able to observe molecular adjustments at the active site that hasten correct nucleotide insertion and deter incorrect insertion not appreciated previously. A third metal binding site is transiently formed during correct, but not incorrect, nucleotide insertion. Additionally, long incubations indicate that pyrophosphate more easily dissociates after incorrect, compared to correct, nucleotide insertion. This appears to be coupled to subdomain repositioning that is required for catalytic activation/deactivation. The structures provide insights into a fundamental chemical reaction that impacts polymerase fidelity and genome stability.