The role of Asn-212 in the catalytic mechanism of human endonuclease APE1: stopped-flow kinetic study of incision activity on a natural AP site and a tetrahydrofuran analogue

The Impact of DFT Functional, Cluster Model Size, and Implicit Solvation on the Structural Description of Single-Metal-Mediated DNA Phosphodiester Bond Cleavage: The Case Study of APE1

Phosphodiester bond hydrolysis in nucleic acids is a ubiquitous reaction that can be facilitated by enzymes called nucleases, which often use metal ions to achieve catalytic function. While a two-metal-mediated pathway has been well established for many enzymes, there is growing support that some enzymes require only one metal for the catalytic step. Using human apurinic/apyrimidinic endonuclease (APE1) as a prototypical example and cluster models, this study clarifies the impact of DFT functional, cluster model size, and implicit solvation on single-metal-mediated phosphodiester bond cleavage and provides insight into how to efficiently model this chemistry. Initially, a model containing 69 atoms built from a high-resolution X-ray crystal structure is used to explore the reaction pathway mapped by a range of DFT functionals and basis sets, which provides support for the use of standard functionals (M06-2X and B3LYP-D3) to study this reaction. Subsequently, systematically increasing the model size to 185 atoms by including additional amino acids and altering residue truncation points highlights that small models containing only a few amino acids or β carbon truncation points introduce model strains and lead to incorrect metal coordination. Indeed, a model that contains all key residues (general base and acid, residues that stabilize the substrate, and amino acids that maintain the metal coordination) is required for an accurate structural depiction of the one-metal-mediated phosphodiester bond hydrolysis by APE1, which results in 185 atoms. The additional inclusion of the broader enzyme environment through continuum solvation models has negligible effects. The insights gained in the present work can be used to direct future computational studies of other one-metal-dependent nucleases to provide a greater understanding of how nature achieves this difficult chemistry.

The metal dependence of single-metal mediated phosphodiester bond cleavage: a QM/MM study of a multifaceted human enzyme

Nucleases catalyze the cleavage of phosphodiester bonds in nucleic acids using a range of metal cofactors. Although it is well accepted that many nucleases rely on two metal ions, the one-metal mediated pathway is debated. Furthermore, one-metal mediated nucleases maintain activity in the presence of many different metals, but the underlying reasons for this broad metal specificity are unknown. The human apurinic/apyrimidinic endonuclease (APE1), which plays a key role in DNA repair, transcription regulation, and gene expression, is a prototypical example of a one-metal dependent nuclease. Although Mg²⁺ is the native metal cofactor, APE1 remains catalytically active in the presence of several metals, with the rate decreasing as Mg²⁺ > Mn²⁺ > Ni²⁺ > Zn²⁺, while Ca²⁺ completely abolished the activity. The present work uses quantum mechanics-molecular mechanics techniques to map APE1-facilitated phosphodiester bond hydrolysis in the presence of these metals. The structural differences in stationary points along the reaction pathway shed light on the interplay between several factors that allow APE1 to remain catalytically active for various metals, with the trend in the barrier heights correlating with the experimentally reported APE1 catalytic activity. In contrast, Ca²⁺ significantly changes the metal coordination and active site geometry, and thus completely inhibits catalysis. Our work thereby provides support for the controversial single-metal mediated phosphodiester bond cleavage and clarifies uncertainties regarding the role of the metal and metal identity in this important reaction. This information is key for future medicinal and biotechnological applications including disease diagnosis and treatment, and protein engineering.

Oxidative stress-mediated epigenetic regulation by G-quadruplexes

Many cancer-associated genes are regulated by guanine (G)-rich sequences that are capable of refolding from the canonical duplex structure to an intrastrand G-quadruplex. These same sequences are sensitive to oxidative damage that is repaired by the base excision repair glycosylases OGG1 and NEIL1–3. We describe studies indicating that oxidation of a guanosine base in a gene promoter G-quadruplex can lead to up- and downregulation of gene expression that is location dependent and involves the base excision repair pathway in which the first intermediate, an apurinic (AP) site, plays a key role mediated by AP endonuclease 1 (APE1/REF1). The nuclease activity of APE1 is paused at a G-quadruplex, while the REF1 capacity of this protein engages activating transcription factors such as HIF-1α, AP-1 and p53. The mechanism has been probed by in vitro biophysical studies, whole-genome approaches and reporter plasmids in cellulo. Replacement of promoter elements by a G-quadruplex sequence usually led to upregulation, but depending on the strand and precise location, examples of downregulation were also found. The impact of oxidative stress-mediated lesions in the G-rich sequence enhanced the effect, whether it was positive or negative.

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.

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.

Role of Ionizing Amino Acid Residues in the Process of DNA Binding by Human AP Endonuclease 1 and in Its Catalysis

In the repair of the damage to bases, human apurinic/apyrimidinic (AP) endonuclease 1 (APE1) is a key participant via the DNA base excision repair pathway. APE1 cleaves AP sites in DNA, which are potentially cytotoxic and highly mutagenic if left unrepaired. According to existing structural data, this enzyme's active site contains many polar amino acid residues, which form extensive contacts with a DNA substrate. A few alternative catalytic mechanisms of the phosphodiester bond hydrolysis by APE1 have been reported. Here, the kinetics of conformational changes of the enzyme and of DNA substrate molecules were studied during the recognition and cleavage of the abasic site in the pH range from 5.5 to 9.0 using stopped-flow fluorescence techniques. The activity of APE1 increased with an increase in pH because of acceleration of the rates of catalytic complex formation and of the catalytic reaction. Molecular dynamics simulation uncovered a significant increase in the pK_a of His-309 located in the active site of the enzyme. This finding revealed that the observed enhancement of enzymatic activity with pH could be associated with deprotonation of not only Tyr-171 but also His-309. The obtained data allowed us to hypothesize that the ionized state of these residues could be a molecular switch between the alternative catalytic mechanisms, which involve different functionalities of these residues throughout the reaction.

Apurinic/apyrimidinic endonuclease Apn1 from Saccharomyces cerevisiae is recruited to the nucleotide incision repair pathway: Kinetic and structural features

Apurinic/apyrimidinic endonuclease Apn1 of Saccharomyces cerevisiae is known as a key player of the base excision DNA repair (BER) pathway in yeast. BER is initiated by DNA glycosylases, whereas Apn1 can start DNA repair individually in the nucleotide incision repair (NIR) pathway. The aim of this research was to elucidate kinetic and structural dynamic aspects of Apn1 involvement in the NIR process. One of the key characteristics of AP endonuclease's interactions is known to be divalent metal ions playing a part of a cofactor. Well-studied human APE1 employs Mg²⁺ ions, with metal ion concentration's affecting enzymatic activity exerted by APE1. In our study, we aimed to test the effect of the Mg²⁺ ion on Apn1's NIR catalysis by examining structural dynamics of DNA during the interaction in real time using the stopped-flow technique. To test NIR activity of Apn1, deoxyribooligonucleotide duplexes containing a 5,6-dihydro-2'-deoxyuridine (DHU) residue were employed as substrates. A 2-aminopurine (2-aPu) residue was a reporter group fluorescence intensity of which was detected during Apn1-DNA interactions. NIR activity of both WT and H83A Apn1 was found to be arrested during the interaction with a DNA duplex containing the 2-aPu residue upstream of DHU. We conducted molecular dynamics simulations to elucidate the structural features of complexes of the enzyme with DHU-containing DNAs. The NIR recruiting S. cerevisiae Apn1 proceeds via multistep rearrangements of the complex of Apn1 with a DHU-containing DNA substrate and results in the incised product of the reaction. For wild-type Apn1, the catalytic rate constants do not depend on the Mg²⁺ concentration, i.e., they are equal in NIR and BER buffers, with equilibrium association constant K_a being 10-fold higher in NIR buffer. Our data reveal more delicate regulation of Apn1's NIR activity due to the more complicated kinetic mechanism, as compared to BER.

Role of AP-endonuclease (Ape1) active site residues in stabilization of the reactant enzyme-DNA complex

Apurinic/apyrimidinic endonuclease 1 (Ape1) is an important metal-dependent enzyme in the base excision repair mechanism, responsible for the backbone cleavage of abasic DNA through a phosphate hydrolysis reaction. Molecular dynamics simulations of Ape1 complexed to its substrate DNA performed for models containing 1 or 2 Mg²⁺ -ions as cofactor located at different positions show a complex with 1 metal ion bound on the leaving group site of the scissile phosphate to be the most likely reaction-competent conformation. Active-site residue His309 is found to be protonated based on pKa calculations and the higher conformational stability of the Ape1-DNA substrate complex compared to scenarios with neutral His309. Simulations of the D210N mutant further support the prevalence of protonated His309 and strongly suggest Asp210 as the general base for proton acceptance by a nucleophilic water molecule.

Processing of the abasic sites clustered with the benzo[a]pyrene adducts by the base excision repair enzymes

The major enzyme in eukaryotic cells that catalyzes the cleavage of apurinic/apyrimidinic (AP or abasic) sites is AP endonuclease 1 (APE1) that cleaves the phosphodiester bond on the 5'-side of AP sites. We found that the efficiency of AP site cleavage by APE1 was affected by the benzo[a]pyrenyl-DNA adduct (BPDE-dG) in the opposite strand. AP sites directly opposite of the modified dG or shifted toward the 5' direction were hydrolyzed by APE1 with an efficiency moderately lower than the AP site in the control DNA duplex, whereas AP sites shifted toward the 3' direction were hydrolyzed significantly less efficiently. For all DNA structures except DNA with the AP site shifted by 3 nucleotides in the 3' direction (AP₊₃-BP-DNA), hydrolysis was more efficient in the case of (+)-trans-BPDE-dG. Using molecular dynamic simulation, we have shown that in the complex of APE1 with the AP₊₃-BP-DNA, the BP residue is located within the DNA bend induced by APE1 and contacts the amino acids in the enzyme catalytic center and the catalytic metal ion. The geometry of the APE1 active site is perturbed more significantly by the trans-isomer of BPDE-dG that intercalates into the APE1-DNA complex near the cleaved phosphodiester bond. The ability of DNA polymerases β (Polβ), λ and ι to catalyze gap-filling synthesis in cooperation with APE1 was also analyzed. Polβ was shown to inhibit the 3'→5' exonuclease activity of APE1 when both enzymes were added simultaneously and to insert the correct nucleotide into the gap arising after AP site hydrolysis. Therefore, further evidence for the functional cooperation of APE1 and Polβ in base excision repair was obtained.

Capturing snapshots of APE1 processing DNA damage

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.

The role of His-83 of yeast apurinic/apyrimidinic endonuclease Apn1 in catalytic incision of abasic sites in DNA

BACKGROUND

The apurinic/apyrimidinic (AP) endonuclease Apn1 from Saccharomyces cerevisiae is a key enzyme involved in the base excision repair (BER) at the cleavage stage of abasic sites (AP sites) in DNA. The crystal structure of Apn1 from S. cerevisiae is unresolved. Based on its high amino acid homology to Escherichia coli Endo IV, His-83 is believed to coordinate one of three Zn2+ ions in Apn1's active site similar to His-69 in Endo IV. Substituting His-83 with Ala is proposed to decrease the AP endonuclease activity of Apn1 owing to weak coordination of Zn2+ ions involved in enzymatic catalysis.

METHODS

The kinetics of recognition, binding, and incision of DNA substrates with the H83A Apn1 mutant was investigated. The stopped-flow method detecting fluorescence intensity changes of 2-aminopurine (2-aPu) was used to monitor the conformational dynamics of DNA at pre-steady-state conditions.

RESULTS

We found substituting His-83 with Ala influenced catalytic complex formation and further incision of the damaged DNA strand. The H83A Apn1 catalysis depends not only on the location of the mismatch relative to the abasic site in DNA, but also on the nature of damage.

CONCLUSIONS

We consider His-83 properly coordinates the active site Zn2+ ion playing a crucial role in catalytic incision stage. Our data prove suppressed enzymatic activity of H83A Apn1 results from the reduced number of active site Zn2+ ions.

GENERAL SIGNIFICANCE

Our study provides insights into mechanistic specialty of AP site repair by yeast AP endonuclease Apn1 of Endo IV family, which members are not found in mammals, but are present in many microorganisms. The results will provide useful guidelines for design of new anti-fungal and anti-malarial agents.