Non-cognate Enzyme–DNA Complex: Structural and Kinetic Analysis of EcoRV Endonuclease Bound to the EcoRI Recognition Site GAATTC

Is Metal Stabilization of the Leaving Group Required or Can Lysine Facilitate Phosphodiester Bond Cleavage in Nucleic Acids? A Computational Study of EndoV

Endonuclease V (EndoV) is a single-metal-dependent enzyme that repairs deaminated DNA nucleobases in cells by cleaving the phosphodiester bond, and this enzyme has proven to be a powerful tool in biotechnology and medicine. The catalytic mechanism used by EndoV must be understood to design new disease detection and therapeutic solutions and further exploit the enzyme in interdisciplinary applications. This study has used a mixed molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) approach to compare eight distinct catalytic pathways and provides the first proposed mechanism for bacterial EndoV. The calculations demonstrate that mechanisms involving either direct or indirect metal coordination to the leaving group of the substrate previously proposed for other nucleases are unlikely for EndoV, regardless of the general base (histidine, aspartate, and substrate phosphate moiety). Instead, distinct catalytic pathways are characterized for EndoV that involve K139 stabilizing the leaving group, a metal-coordinated water stabilizing the transition structure, and either H214 or a substrate phosphate group activating the water nucleophile. In silico K139A and H214A mutational results support the newly proposed roles of these residues. Although this is a previously unseen combination of general base, general acid, and metal-binding architecture for a one-metal-dependent endonuclease, our proposed catalytic mechanisms are fully consistent with experimental kinetic, structural, and mutational data. In addition to substantiating a growing body of literature, suggesting that one metal is enough to catalyze P-O bond cleavage in nucleic acids, this new fundamental understanding of the catalytic function will promote the exploration of new and improved applications of EndoV.

Electrochemical DNA Cleavage Sensing for EcoRV Activity and Inhibition with an ERGO Electrode

An electrochemically reduced graphene oxide (ERGO) electrode-based electrochemical assay was developed for rapid, sensitive, and straightforward analysis of both activity and inhibition of the endonuclease EcoRV. The procedure uses a DNA substrate designed for EcoRV, featuring a double-stranded DNA (dsDNA) region labeled with methylene blue (MB) and a single-stranded DNA (ssDNA) region immobilized on the ERGO surface. The ERGO electrode, immobilized with the DNA substrate, was subsequently exposed to a sample containing EcoRV. Upon enzymatic hydrolysis, the cleaved dsDNA fragments were detached from the ERGO surface, leading to a decrease in the MB concentration near the electrode. This diminished the electron transfer efficiency for MB reduction, resulting in a decreased reduction current. This assay demonstrates excellent specificity and high sensitivity, with a limit of detection (LOD) of 9.5 × 10-3 U mL-1. Importantly, it can also measure EcoRV activity in the presence of aurintricarboxylic acid, a known inhibitor, highlighting its potential for drug discovery and clinical diagnostic applications.

Structural predictions of protein-DNA binding: MELD-DNA

Structural, regulatory and enzymatic proteins interact with DNA to maintain a healthy and functional genome. Yet, our structural understanding of how proteins interact with DNA is limited. We present MELD-DNA, a novel computational approach to predict the structures of protein-DNA complexes. The method combines molecular dynamics simulations with general knowledge or experimental information through Bayesian inference. The physical model is sensitive to sequence-dependent properties and conformational changes required for binding, while information accelerates sampling of bound conformations. MELD-DNA can: (i) sample multiple binding modes; (ii) identify the preferred binding mode from the ensembles; and (iii) provide qualitative binding preferences between DNA sequences. We first assess performance on a dataset of 15 protein-DNA complexes and compare it with state-of-the-art methodologies. Furthermore, for three selected complexes, we show sequence dependence effects of binding in MELD predictions. We expect that the results presented herein, together with the freely available software, will impact structural biology (by complementing DNA structural databases) and molecular recognition (by bringing new insights into aspects governing protein-DNA interactions).

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.

High Free-Energy Barrier of 1D Diffusion Along DNA by Architectural DNA-Binding Proteins

Architectural DNA-binding proteins function to regulate diverse DNA reactions and have the defining property of significantly changing DNA conformation. Although the 1D movement along DNA by other types of DNA-binding proteins has been visualized, the mobility of architectural DNA-binding proteins on DNA remains unknown. Here, we applied single-molecule fluorescence imaging on arrays of extended DNA molecules to probe the binding dynamics of three structurally distinct architectural DNA-binding proteins: Nhp6A, HU, and Fis. Each of these proteins was observed to move along DNA, and the salt concentration independence of the 1D diffusion implies sliding with continuous contact to DNA. Nhp6A and HU exhibit a single sliding mode, whereas Fis exhibits two sliding modes. Based on comparison of the diffusion coefficients and sizes of many DNA binding proteins, the architectural proteins are categorized into a new group distinguished by an unusually high free-energy barrier for 1D diffusion. The higher free-energy barrier for 1D diffusion by architectural proteins can be attributed to the large DNA conformational changes that accompany binding and impede rotation-coupled movement along the DNA grooves.

Structural basis for substrate discrimination byE. colirepair enzyme, AlkB

Positive charge on methylated nucleotides is a prime criterion for substrate recognition byE. coliAlkB.

Metal Ion Binding at the Catalytic Site Induces Widely Distributed Changes in a Sequence Specific Protein-DNA Complex

Metal ion cofactors can alter the energetics and specificity of sequence specific protein–DNA interactions, but it is unknown if the underlying effects on structure and dynamics are local or dispersed throughout the protein–DNA complex. This work uses EcoRV endonuclease as a model, and catalytically inactive lanthanide ions, which replace the Mg²⁺ cofactor. Nuclear magnetic resonance (NMR) titrations indicate that four Lu³⁺ or two La³⁺ cations bind, and two new crystal structures confirm that Lu³⁺ binding is confined to the active sites. NMR spectra show that the metal-free EcoRV complex with cognate (GATATC) DNA is structurally distinct from the nonspecific complex, and that metal ion binding sites are not assembled in the nonspecific complex. NMR chemical shift perturbations were determined for ¹H–¹⁵N amide resonances, for ¹H–¹³C Ile-δ-CH₃ resonances, and for stereospecifically assigned Leu-δ-CH₃ and Val-γ-CH₃ resonances. Many chemical shifts throughout the cognate complex are unperturbed, so metal binding does not induce major conformational changes. However, some large perturbations of amide and side chain methyl resonances occur as far as 34 Å from the metal ions. Concerted changes in specific residues imply that local effects of metal binding are propagated via a β-sheet and an α-helix. Both amide and methyl resonance perturbations indicate changes in the interface between subunits of the EcoRV homodimer. Bound metal ions also affect amide hydrogen exchange rates for distant residues, including a distant subdomain that contacts DNA phosphates and promotes DNA bending, showing that metal ions in the active sites, which relieve electrostatic repulsion between protein and DNA, cause changes in slow dynamics throughout the complex.

Functional Studies of DNA-Protein Interactions Using FRET Techniques

Protein-DNA interactions underpin life and play key roles in all cellular processes and functions including DNA transcription, packaging, replication, and repair. Identifying and examining the nature of these interactions is therefore a crucial prerequisite to understand the molecular basis of how these fundamental processes take place. The application of fluorescence techniques and in particular fluorescence resonance energy transfer (FRET) to provide structural and kinetic information has experienced a stunning growth during the past decade. This has been mostly promoted by new advances in the preparation of dye-labeled nucleic acids and proteins and in optical sensitivity, where its implementation at the level of individual molecules has opened a new biophysical frontier. Nowadays, the application of FRET-based techniques to the analysis of protein-DNA interactions spans from the classical steady-state and time-resolved methods averaging over large ensembles to the analysis of distances, conformational changes, and enzymatic reactions in individual protein-DNA complexes. This chapter introduces the practical aspects of applying these methods for the study of protein-DNA interactions.

Distinct facilitated diffusion mechanisms by E. coli Type II restriction endonucleases

The passive search by proteins for particular DNA sequences involving nonspecific DNA is essential for gene regulation, DNA repair, phage defense, and diverse epigenetic processes. Distinct mechanisms contribute to these searches, and it remains unresolved as to which mechanism or blend of mechanisms best suits a particular protein and, more importantly, its biological role. To address this, we compare the translocation properties of two well-studied bacterial restriction endonucleases (ENases), EcoRI and EcoRV. These dimeric, magnesium-dependent enzymes hydrolyze related sites (EcoRI ENase, 5'-GAATTC-3'; EcoRV ENase, 5'-GATATC-3'), leaving overhangs and blunt DNA segments, respectively. Here, we demonstrate that the extensive sliding by EcoRI ENase, involving sliding up to ∼600 bp prior to dissociating from the DNA, contrasts with a larger reliance on hopping mechanism(s) by EcoRV ENase. The mechanism displayed by EcoRI ENase results in a highly thorough search of DNA, whereas the EcoRV ENase mechanism results in an extended, yet less rigorous, interrogation of DNA sequence space. We describe how these mechanistic distinctions are complemented by other aspects of these endonucleases, such as the 10-fold higher in vivo concentrations of EcoRI ENase compared to that of EcoRV ENase. Further, we hypothesize that the highly diverse enzyme arsenal that bacteria employ against foreign DNA involves seemingly similar enzymes that rely on distinct but complementary search mechanisms. Our comparative approach reveals how different proteins utilize distinct site-locating strategies.

Type II restriction endonucleases--a historical perspective and more

This article continues the series of Surveys and Summaries on restriction endonucleases (REases) begun this year in Nucleic Acids Research. Here we discuss 'Type II' REases, the kind used for DNA analysis and cloning. We focus on their biochemistry: what they are, what they do, and how they do it. Type II REases are produced by prokaryotes to combat bacteriophages. With extreme accuracy, each recognizes a particular sequence in double-stranded DNA and cleaves at a fixed position within or nearby. The discoveries of these enzymes in the 1970s, and of the uses to which they could be put, have since impacted every corner of the life sciences. They became the enabling tools of molecular biology, genetics and biotechnology, and made analysis at the most fundamental levels routine. Hundreds of different REases have been discovered and are available commercially. Their genes have been cloned, sequenced and overexpressed. Most have been characterized to some extent, but few have been studied in depth. Here, we describe the original discoveries in this field, and the properties of the first Type II REases investigated. We discuss the mechanisms of sequence recognition and catalysis, and the varied oligomeric modes in which Type II REases act. We describe the surprising heterogeneity revealed by comparisons of their sequences and structures.

Thermodynamic and structural basis for relaxation of specificity in protein-DNA recognition

As a novel approach to the structural and functional properties that give rise to extremely stringent sequence specificity in protein-DNA interactions, we have exploited "promiscuous" mutants of EcoRI endonuclease to study the detailed mechanism by which changes in a protein can relax specificity. The A138T promiscuous mutant protein binds more tightly to the cognate GAATTC site than does wild-type EcoRI yet displays relaxed specificity deriving from tighter binding and faster cleavage at EcoRI* sites (one incorrect base pair). AAATTC EcoRI* sites are cleaved by A138T up to 170-fold faster than by wild-type enzyme if the site is abutted by a 5'-purine-pyrimidine (5'-RY) motif. When wild-type protein binds to an EcoRI* site, it forms structurally adapted complexes with thermodynamic parameters of binding that differ markedly from those of specific complexes. By contrast, we show that A138T complexes with 5'-RY-flanked AAATTC sites are virtually indistinguishable from wild-type-specific complexes with respect to the heat capacity change upon binding (∆C°P), the change in excluded macromolecular volume upon association, and contacts to the phosphate backbone. While the preference for the 5'-RY motif implicates contacts to flanking bases as important for relaxed specificity, local effects are not sufficient to explain the large differences in ∆C°P and excluded volume, as these parameters report on global features of the complex. Our findings therefore support the view that specificity does not derive from the additive effects of individual interactions but rather from a set of cooperative events that are uniquely associated with specific recognition.

Ca(2+) binding to the ExDxD motif regulates the DNA cleavage specificity of a promiscuous endonuclease

Most of the restriction endonucleases (REases) are dependent on Mg(2+) for DNA cleavage, and in general, Ca(2+) inhibits their activity. R.KpnI, an HNH active site containing ββα-Me finger nuclease, is an exception. In presence of Ca(2+), the enzyme exhibits high-fidelity DNA cleavage and complete suppression of Mg(2+)-induced promiscuous activity. To elucidate the mechanism of unusual Ca(2+)-mediated activity, we generated alanine variants in the putative Ca(2+) binding motif, E(132)xD(134)xD(136), of the enzyme. Mutants showed decreased levels of DNA cleavage in the presence of Ca(2+). We demonstrate that ExDxD residues are involved in Ca(2+) coordination; however, the invariant His of the catalytic HNH motif acts as a general base for nucleophile activation, and the other two active site residues, D148 and Q175, also participate in Ca(2+)-mediated cleavage. Insertion of a 10-amino acid linker to disrupt the spatial organization of the ExDxD and HNH motifs impairs Ca(2+) binding and affects DNA cleavage by the enzyme. Although ExDxD mutant enzymes retained efficient cleavage at the canonical sites in the presence of Mg(2+), the promiscuous activity was greatly reduced, indicating that the carboxyl residues of the acidic triad play an important role in sequence recognition by the enzyme. Thus, the distinct Ca(2+) binding motif that confers site specific cleavage upon Ca(2+) binding is also critical for the promiscuous activity of the Mg(2+)-bound enzyme, revealing its role in metal ion-mediated modulation of DNA cleavage.

Towards artificial metallonucleases for gene therapy: recent advances and new perspectives

The process of DNA targeting or repair of mutated genes within the cell, induced by specifically positioned double-strand cleavage of DNA near the mutated sequence, can be applied for gene therapy of monogenic diseases. For this purpose, highly specific artificial metallonucleases are developed. They are expected to be important future tools of modern genetics. The present state of art and strategies of research are summarized, including protein engineering and artificial ‘chemical’ nucleases. From the results, we learn about the basic role of the metal ions and the various ligands, and about the DNA binding and cleavage mechanism. The results collected provide useful guidance for engineering highly controlled enzymes for use in gene therapy.

The elastic network model reveals a consistent picture on intrinsic functional dynamics of type II restriction endonucleases

The vibrational dynamics of various type II restriction endonucleases, in complex with cognate/non-cognate DNA and in the apo form, are investigated with the elastic network model in order to reveal common functional mechanisms in this enzyme family. Scissor-like and tong-like motions observed in the slowest modes of all enzymes and their complexes point to common DNA recognition and cleavage mechanisms. Normal mode analysis further points out that the scissor-like motion has an important role in differentiating between cognate and non-cognate sequences at the recognition site, thus implying its catalytic relevance. Flexible regions observed around the DNA-binding site of the enzyme usually concentrate on the highly conserved β-strands, especially after DNA binding. These β-strands may have a structurally stabilizing role in functional dynamics for target site recognition and cleavage. In addition, hot spot residues based on high-frequency modes reveal possible communication pathways between the two distant cleavage sites in the enzyme family. Some of these hot spots also exist on the shortest path between the catalytic sites and are highly conserved.

Functional studies of DNA-protein interactions using FRET techniques

Protein-DNA interactions underpin life and play key roles in all cellular processes and functions including DNA transcription, packaging, replication, and repair. Identifying and examining the nature of these interactions is therefore a crucial prerequisite to understand the molecular basis of how these fundamental processes take place. The application of fluorescence techniques and in particular fluorescence resonance energy transfer (FRET) to provide structural and kinetic information has experienced a stunning growth during the past decade. This has been mostly promoted by new advances in the preparation of dye-labeled nucleic acids and proteins and in optical sensitivity, where its implementation at the level of individual molecules has opened a new biophysical frontier. Nowadays, the application of FRET-based techniques to the analysis of protein-DNA interactions spans from the classical steady-state and time-resolved methods averaging over large ensembles to the analysis of distances, conformational changes, and enzymatic reactions in individual Protein-DNA complexes. This chapter introduces the practical aspects of applying these methods for the study of Protein-DNA interactions.

How DNA coiling enhances target localization by proteins

Many genetic processes depend on proteins interacting with specific sequences on DNA. Despite the large excess of nonspecific DNA in the cell, proteins can locate their targets rapidly. After initial nonspecific binding, they are believed to find the target site by 1D diffusion ("sliding") interspersed by 3D dissociation/reassociation, a process usually referred to as facilitated diffusion. The 3D events combine short intrasegmental "hops" along the DNA contour, intersegmental "jumps" between nearby DNA segments, and longer volume "excursions." The impact of DNA conformation on the search pathway is, however, still unknown. Here, we show direct evidence that DNA coiling influences the specific association rate of EcoRV restriction enzymes. Using optical tweezers together with a fast buffer exchange system, we obtained association times of EcoRV on single DNA molecules as a function of DNA extension, separating intersegmental jumping from other search pathways. Depending on salt concentration, targeting rates almost double when the DNA conformation is changed from fully extended to a coiled configuration. Quantitative analysis by an extended facilitated diffusion model reveals that only a fraction of enzymes are ready to bind to DNA. Generalizing our results to the crowded environment of the cell we predict a major impact of intersegmental jumps on target localization speed on DNA.

Determinants of sequence-specific DNA methylation: target recognition and catalysis are coupled in M.HhaI

Sequence specificity studies of the wild-type bacterial DNA cytosine C5 methyltransferase HhaI were carried out with cognate (5'GCGC3') and noncognate DNA substrates containing single base pair changes at the first and the fourth position (underlined). Specificity for noncognate site methylation at the level of kcat/KDDNA is decreased 9000-80000-fold relative to the cognate site, manifested through changes in methylation, or a prior step, and changes in KDDNA. Analysis of a new high-resolution enzyme-DNA cocrystal structure provides a partial mechanistic understanding of this discrimination. To probe the significance of conformational transitions occurring prior to catalysis in determining specificity, we analyzed the double mutant (H127A/T132A). These amino acid substitutions disrupt the interface between the flexible loop (residues 80-99), which interacts with the DNA minor groove, and the active site. The mutant's methylation of the cognate site is essentially unchanged, yet its methylation of noncognate sites is decreased up to 460-fold relative to the wild-type enzyme. We suggest that a significant contribution to M.HhaI's specificity involves the stabilization of reaction intermediates prior to methyl transfer, mediated by DNA minor groove-protein flexible loop interactions.

R.KpnI, an HNH superfamily REase, exhibits differential discrimination at non-canonical sequences in the presence of Ca2+ and Mg2+

KpnI REase recognizes palindromic sequence, GGTAC↓C, and forms complex in the absence of divalent metal ions, but requires the ions for DNA cleavage. Unlike most other REases, R.KpnI shows promiscuous DNA cleavage in the presence of Mg²⁺. Surprisingly, Ca²⁺ suppresses the Mg²⁺-mediated promiscuous activity and induces high fidelity cleavage. To further analyze these unique features of the enzyme, we have carried out DNA binding and kinetic analysis. The metal ions which exhibit disparate pattern of DNA cleavage have no role in DNA recognition. The enzyme binds to both canonical and non-canonical DNA with comparable affinity irrespective of the metal ions used. Further, Ca²⁺-imparted exquisite specificity of the enzyme is at the level of DNA cleavage and not at the binding step. With the canonical oligonucleotides, the cleavage rate of the enzyme was comparable for both Mg²⁺- and Mn²⁺-mediated reactions and was about three times slower with Ca²⁺. The enzyme discriminates non-canonical sequences poorly from the canonical sequence in Mg²⁺-mediated reactions unlike any other Type II REases, accounting for the promiscuous behavior. R.KpnI, thus displays properties akin to that of typical Type II REases and also endonucleases with degenerate specificity in its DNA recognition and cleavage properties.

BstYI Bound to Noncognate DNA Reveals a “Hemispecific” Complex: Implications for DNA Scanning

DNA recognition by proteins is essential for specific expression of genes in a living organism. En route to a target DNA site, a protein will often sample noncognate DNA sites through nonspecific protein-DNA interactions, resulting in a variety of conformationally different binding states. We present here the crystal structure of endonuclease BstYI bound to a noncognate DNA. Surprisingly, the structure reveals the enzyme in a "hemispecific" binding state on the pathway between nonspecific and specific recognition. A single base pair change in the DNA abolishes binding of only one monomer, with the second monomer bound specifically. We show that the enzyme binds essentially as a rigid body, and that one end of the DNA is accommodated loosely in the binding cleft while the other end is held tightly. Another intriguing feature of the structure is Ser172, which has a dual role in establishing nonspecific and specific contacts. Taken together, the structure provides a snapshot of an enzyme in a "paused" intermediate state that may be part of a more general mechanism of scanning DNA.

Positively charged C-terminal subdomains of EcoRV endonuclease: contributions to DNA binding, bending, and cleavage

The carboxy-terminal subdomains of the homodimeric EcoRV restriction endonuclease each bear a net charge of +4 and are positioned on the inner concave surface of the 50 degree DNA bend that is induced by the enzyme. A complete kinetic and structural analysis of a truncated EcoRV mutant lacking these domains was performed to assess the importance of this diffuse charge in facilitating DNA binding, bending, and cleavage. At the level of formation of an enzyme-DNA complex, the association rate for the dimeric mutant enzyme was sharply decreased by 10(3)-fold, while the equilibrium dissociation constant was weakened by nearly 10(6)-fold compared with that of wild-type EcoRV. Thus, the C-terminal subdomains strongly stabilize the enzyme-DNA ground-state complex in which the DNA is known to be bent. Further, the extent of DNA bending as observed by fluorescence resonance energy transfer was also significantly decreased. The crystal structure of the truncated enzyme bound to DNA and calcium ions at 2.4 A resolution reveals that the global fold is preserved and suggests that a divalent metal ion crucial to catalysis is destabilized in the active site. This may explain the 100-fold decrease in the rate of metal-dependent phosphoryl transfer observed for the mutant. These results show that diffuse positive charge associated with the C-terminal subdomains of EcoRV plays a key role in DNA association, bending, and cleavage.

Conformational transitions as determinants of specificity for the DNA methyltransferase EcoRI

Changes in DNA bending and base flipping in a previously characterized specificity-enhanced M.EcoRI DNA adenine methyltransferase mutant suggest a close relationship between precatalytic conformational transitions and specificity (Allan, B. W., Garcia, R., Maegley, K., Mort, J., Wong, D., Lindstrom, W., Beechem, J. M., and Reich, N. O. (1999) J. Biol. Chem. 274, 19269-19275). The direct measurement of the kinetic rate constants for DNA bending, intercalation, and base flipping with cognate and noncognate substrates (GAATTT, GGATTC) of wild type M.EcoRI using fluorescence resonance energy transfer and 2-aminopurine fluorescence studies reveals that DNA bending precedes both intercalation and base flipping, and base flipping precedes intercalation. Destabilization of these intermediates provides a molecular basis for understanding how conformational transitions contribute to specificity. The 3500- and 23,000-fold decreases in sequence specificity for noncognate sites GAATTT and GGATTC are accounted for largely by an approximately 2500-fold increase in the reverse rate constants for intercalation and base flipping, respectively. Thus, a predominant contribution to specificity is a partitioning of enzyme intermediates away from the Michaelis complex prior to catalysis. Our results provide a basis for understanding enzyme specificity and, in particular, sequence-specific DNA modification. Because many DNA methyltransferases and DNA repair enzymes induce similar DNA distortions, these results are likely to be broadly relevant.