DNA-mediated charge transport as a probe of MutY/DNA interaction

Correlation between Charge Transport and Base Excision Repair in the MutY-DNA Glycosylase

Experimental evidence suggests that DNA-mediated redox signaling between high-potential [Fe₄S₄] proteins is relevant to DNA replication and repair processes, and protein-mediated charge transfer (CT) between [Fe₄S₄] clusters and nucleic acids is a fundamental process of the signaling and repair mechanisms. We analyzed the dominant CT pathways in the base excision repair glycosylase MutY using molecular dynamics simulations and hole hopping pathway analysis. We find that the adenine nucleobase of the mismatched A·oxoG DNA base pair facilitates [Fe₄S₄]-DNA CT prior to adenine excision by MutY. We also find that the R153L mutation in MutY (linked to colorectal adenomatous polyposis) influences the dominant [Fe4S4]-DNA CT pathways and appreciably decreases their effective CT rates.

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.

Sensing DNA through DNA Charge Transport

DNA charge transport chemistry involves the migration of charge over long molecular distances through the aromatic base pair stack within the DNA helix. This migration depends upon the intimate coupling of bases stacked one with another, and hence any perturbation in that stacking, through base modifications or protein binding, can be sensed electrically. In this review, we describe the many ways DNA charge transport chemistry has been utilized to sense changes in DNA, including the presence of lesions, mismatches, DNA-binding proteins, protein activity, and even reactions under weak magnetic fields. Charge transport chemistry is remarkable in its ability to sense the integrity of DNA.

The Oxidation State of [4Fe4S] Clusters Modulates the DNA-Binding Affinity of DNA Repair Proteins

A central question important to understanding DNA repair is how certain proteins are able to search for, detect, and fix DNA damage on a biologically relevant time scale. A feature of many base excision repair proteins is that they contain [4Fe4S] clusters that may aid their search for lesions. In this paper, we establish the importance of the oxidation state of the redox-active [4Fe4S] cluster in the DNA damage detection process. We utilize DNA-modified electrodes to generate repair proteins with [4Fe4S] clusters in the 2+ and 3+ states by bulk electrolysis under an O2-free atmosphere. Anaerobic microscale thermophoresis results indicate that proteins carrying [4Fe4S]3+ clusters bind to DNA 550 times more tightly than those with [4Fe4S]2+ clusters. The measured increase in DNA-binding affinity matches the calculated affinity change associated with the redox potential shift observed for [4Fe4S] cluster proteins upon binding to DNA. We further devise an electrostatic model that shows this change in DNA-binding affinity of these proteins can be fully explained by the differences in electrostatic interactions between DNA and the [4Fe4S] cluster in the reduced versus oxidized state. We then utilize atomic force microscopy (AFM) to demonstrate that the redox state of the [4Fe4S] clusters regulates the ability of two DNA repair proteins, Endonuclease III and DinG, to bind preferentially to DNA duplexes containing a single site of DNA damage (here a base mismatch) which inhibits DNA charge transport. Together, these results show that the reduction and oxidation of [4Fe4S] clusters through DNA-mediated charge transport facilitates long-range signaling between [4Fe4S] repair proteins. The redox-modulated change in DNA-binding affinity regulates the ability of [4Fe4S] repair proteins to collaborate in the lesion detection process.

Sulfur K-Edge XAS Studies of the Effect of DNA Binding on the [Fe4S4] Site in EndoIII and MutY

S K-edge X-ray absorption spectroscopy (XAS) was used to study the [Fe₄S₄] clusters in the DNA repair glycosylases EndoIII and MutY to evaluate the effects of DNA binding and solvation on Fe-S bond covalencies (i.e., the amount of S 3p character mixed into the Fe 3d valence orbitals). Increased covalencies in both iron-thiolate and iron-sulfide bonds would stabilize the oxidized state of the [Fe₄S₄] clusters. The results are compared to those on previously studied [Fe₄S₄] model complexes, ferredoxin (Fd), and to new data on high-potential iron-sulfur protein (HiPIP). A limited decrease in covalency is observed upon removal of solvent water from EndoIII and MutY, opposite to the significant increase observed for Fd, where the [Fe₄S₄] cluster is solvent exposed. Importantly, in EndoIII and MutY, a large increase in covalency is observed upon DNA binding, which is due to the effect of its negative charge on the iron-sulfur bonds. In EndoIII, this change in covalency can be quantified and makes a significant contribution to the observed decrease in reduction potential found experimentally in DNA repair proteins, enabling their HiPIP-like redox behavior.

MUTYH DNA glycosylase: the rationale for removing undamaged bases from the DNA

Maintenance of genetic stability is crucial for all organisms in order to avoid the onset of deleterious diseases such as cancer. One of the many proveniences of DNA base damage in mammalian cells is oxidative stress, arising from a variety of endogenous and exogenous sources, generating highly mutagenic oxidative DNA lesions. One of the best characterized oxidative DNA lesion is 7,8-dihydro-8-oxoguanine (8-oxo-G), which can give rise to base substitution mutations (also known as point mutations). This mutagenicity is due to the miscoding potential of 8-oxo-G that instructs most DNA polymerases (pols) to preferentially insert an Adenine (A) opposite 8-oxo-G instead of the appropriate Cytosine (C). If left unrepaired, such A:8-oxo-G mispairs can give rise to CG→AT transversion mutations. A:8-oxo-G mispairs are proficiently recognized by the MutY glycosylase homologue (MUTYH). MUTYH can remove the mispaired A from an A:8-oxo-G, giving way to the canonical base-excision repair (BER) that ultimately restores undamaged Guanine (G). The importance of this MUTYH-initiated pathway is illustrated by the fact that biallelic mutations in the MUTYH gene are associated with a hereditary colorectal cancer syndrome termed MUTYH-associated polyposis (MAP). In this review, we will focus on MUTYH, from its discovery to the most recent data regarding its cellular roles and interaction partners. We discuss the involvement of the MUTYH protein in the A:8-oxo-G BER pathway acting together with pol λ, the pol that can faithfully incorporate C opposite 8-oxo-G and thus bypass this lesion in a correct manner. We also outline the current knowledge about the regulation of MUTYH itself and the A:8-oxo-G repair pathway by posttranslational modifications (PTM). Finally, to achieve a clearer overview of the literature, we will briefly touch on the rather confusing MUTYH nomenclature. In short, MUTYH is a unique DNA glycosylase that catalyzes the excision of an undamaged base from DNA.

Probing the effects of DNA-protein interactions on DNA hole transport: the N-terminal histone tails modulate the distribution of oxidative damage and chemical lesions in the nucleosome core particle

The ability of DNA to transport positive charges, or holes, over long distances is well-established, but the mechanistic details of how this process is influenced by packaging into DNA-protein complexes have not been fully delineated. In eukaryotes, genomic DNA is packaged into chromatin through its association with the core histone octamer to form the nucleosome core particle (NCP), a complex whose structure can be modulated through changes in the local environment and the histone proteins. Because (i) varying the salt concentration and removing the histone tails influence the structure of the NCP in known ways and (ii) previous studies have shown that DNA hole transport (HT) occurs in the nucleosome, we have used our previously described 601 sequence NCPs to test the hypothesis that DNA HT dynamics can be modulated by structural changes in a DNA-protein complex. We show that at low salt concentrations there is a sharp increase in long-range DNA HT efficiency in the NCP as compared to naked DNA. This enhancement of HT can be negated by either removal of the histone tails at low salt concentrations or disruption of the interaction of the packaged DNA and the histone tails by increasing the buffer's ionic strength. Association of the histone tails with 601 DNA at low salt concentrations shifts the guanine damage spectrum to favor lesions like 8-oxoguanine in the NCP, most likely through modulation of the rate of the reaction of the guanine radical cation with oxygen. These experimental results indicate that for most genomic DNA, the influence of DNA-protein interactions on DNA HT will depend strongly on the level of protection of the DNA nucleobases from oxygen. Further, these results suggest that the oxidative damage arising from DNA HT may vary in different genomic regions depending on the presence of either euchromatin or heterochromatin.

Metal Complexes for DNA-Mediated Charge Transport

In all organisms, oxidation threatens the integrity of the genome. DNA-mediated charge transport (CT) may play an important role in the generation and repair of this oxidative damage. In studies involving long-range CT from intercalating Ru and Rh complexes to 5'-GG-3' sites, we have examined the efficiency of CT as a function of distance, temperature, and the electronic coupling of metal oxidants bound to the base stack. Most striking is the shallow distance dependence and the sensitivity of DNA CT to how the metal complexes are stacked in the helix. Experiments with cyclopropylamine-modified bases have revealed that charge occupation occurs at all sites along the bridge. Using Ir complexes, we have seen that the process of DNA-mediated reduction is very similar to that of DNA-mediated oxidation. Studies involving metalloproteins have, furthermore, shown that their redox activity is DNA-dependent and can be DNA-mediated. Long range DNA-mediated CT can facilitate the oxidation of DNA-bound base excision repair proteins to initiate a redox-active search for DNA lesions. DNA CT can also activate the transcription factor SoxR, triggering a cellular response to oxidative stress. Indeed, these studies show that within the cell, redox-active proteins may utilize the same chemistry as that of synthetic metal complexes in vitro, and these proteins may harness DNA-mediated CT to reduce damage to the genome and regulate cellular processes.

Hole transfer energetics in structurally distorted DNA: the nucleosome core particle

The dynamics of long-range hole transport (HT) through DNA are critically dependent on the relative energies of guanine radical cation states. Electrostatic contacts with protein fragments and changes in the secondary structure of the DNA helix are expected to directly influence the stability of a guanine radical cation. This expectation is especially relevant when considering DNA HT in the eukaryotic nucleus, where DNA is condensed into nucleosome core particles (NCPs), the fundamental building blocks of chromatin. Using quantum-chemical calculations, we consider how the electrostatic interactions between the DNA nucleobases and the surrounding protein and water atoms and the structural changes in DNA arising from compaction into a NCP affect the energetics of hole transfer between guanine sites. We find that structural distortions of DNA can have dramatic consequences for the stability of a guanine radical cation, and therefore, these effects must be taken into account during the modeling of in vivo DNA HT and in the interpretation of experimental findings. When the electrostatic potential arising from the water and basic histone proteins is included we find that DNA-histone contacts, particularly between arginine residues and the DNA minor groove, destabilize the hole state on specific guanine residues. Therefore, contacts between the DNA nucleobases and basic amino acids have the potential to perturb the sites of preferred hole stability in DNA.

Reaction intermediates in the catalytic mechanism of Escherichia coli MutY DNA glycosylase

The Escherichia coli adenine DNA glycosylase, MutY, plays an important role in the maintenance of genomic stability by catalyzing the removal of adenine opposite 8-oxo-7,8-dihydroguanine or guanine in duplex DNA. Although the x-ray crystal structure of the catalytic domain of MutY revealed a mechanism for catalysis of the glycosyl bond, it appeared that several opportunistically positioned lysine side chains could participate in a secondary beta-elimination reaction. In this investigation, it is established via site-directed mutagenesis and the determination of a 1.35-A structure of MutY in complex with adenine that the abasic site (apurinic/apyrimidinic) lyase activity is alternatively regulated by two lysines, Lys142 and Lys20. Analyses of the crystallographic structure also suggest a role for Glu161 in the apurinic/apyrimidinic lyase chemistry. The beta-elimination reaction is structurally and chemically uncoupled from the initial glycosyl bond scission, indicating that this reaction occurs as a consequence of active site plasticity and slow dissociation of the product complex. MutY with either the K142A or K20A mutation still catalyzes beta and beta-delta elimination reactions, and both mutants can be trapped as covalent enzyme-DNA intermediates by chemical reduction. The trapping was observed to occur both pre- and post-phosphodiester bond scission, establishing that both of these intermediates have significant half-lives. Thus, the final spectrum of DNA products generated reflects the outcome of a delicate balance of closely related equilibrium constants.