Noncysteinyl coordination to the [4Fe-4S]2+ cluster of the DNA repair adenine glycosylase MutY introduced via site-directed mutagenesis. Structural characterization of an unusual histidinyl-coordinated cluster

The Escherichia coli DNA repair enzyme MutY plays an important role in the recognition and repair of 7,8-dihydro-8-oxo-2'-deoxyguanosine-2'-deoxyadenosine (OG*A) mismatches in DNA. MutY prevents DNA mutations caused by the misincorporation of A opposite OG by catalyzing the deglycosylation of the aberrant adenine. MutY is representative of a unique subfamily of DNA repair enzymes that also contain a [4Fe-4S]2+ cluster, which has been implicated in substrate recognition. Previously, we have used site-directed mutagenesis to individually replace the cysteine ligands to the [4Fe-4S]2+ cluster of E. coli MutY with serine, histidine, or alanine. These experiments suggested that histidine coordination to the iron-sulfur cluster may be accommodated in MutY at position 199. Purification and enzymatic analysis of C199H and C199S forms indicated that these forms behave nearly identical to the WT enzyme. Furthermore, introduction of the C199H mutation in a truncated form of MutY (C199HT) allowed for crystallization and structural characterization of the modified [4Fe-4S] cluster coordination. The C199HT structure showed that histidine coordinated to the iron cluster although comparison to the structure of the WT truncated enzyme indicated that the occupancy of iron at the modified position had been reduced to 60%. Electron paramagnetic resonance (EPR) spectroscopy on samples of C199HT indicates that a significant percentage (15-30%) of iron clusters were of the [3Fe-4S]1+ form. Oxidation of the C199HT enzyme with ferricyanide increases the amount of the 3Fe cluster by approximately 2-fold. Detailed kinetic analysis on samples containing a mixture of [3Fe-4S]1+ and [4Fe-4S]2+ forms indicated that the reactivity of the [3Fe-4S]1+ C199HT enzyme does not differ significantly from that of the WT truncated enzyme. The relative resistance of the [4Fe-4S]2+ cluster toward oxidation, as well as the retention of activity of the [3Fe-4S]1+ form, may be an important aspect of the role of MutY in repair of DNA damage resulting from oxidative stress.     

DNA-bound redox activity of DNA repair glycosylases containing [4Fe-4S] clusters

MutY and endonuclease III, two DNA glycosylases from Escherichia coli, and AfUDG, a uracil DNA glycosylase from Archeoglobus fulgidus, are all base excision repair enzymes that contain the [4Fe-4S](2+) cofactor. Here we demonstrate that, when bound to DNA, these repair enzymes become redox-active; binding to DNA shifts the redox potential of the [4Fe-4S](3+/2+) couple to the range characteristic of high-potential iron proteins and activates the proteins toward oxidation. Electrochemistry on DNA-modified electrodes reveals potentials for Endo III and AfUDG of 58 and 95 mV versus NHE, respectively, comparable to 90 mV for MutY bound to DNA. In the absence of DNA modification of the electrode, no redox activity can be detected, and on electrodes modified with DNA containing an abasic site, the redox signals are dramatically attenuated; these observations show that the DNA base pair stack mediates electron transfer to the protein, and the potentials determined are for the DNA-bound protein. In EPR experiments at 10 K, redox activation upon DNA binding is also evident to yield the oxidized [4Fe-4S](3+) cluster and the partially degraded [3Fe-4S](1+) cluster. EPR signals at g = 2.02 and 1.99 for MutY and g = 2.03 and 2.01 for Endo III are seen upon oxidation of these proteins by Co(phen)(3)(3+) in the presence of DNA and are characteristic of [3Fe-4S](1+) clusters, while oxidation of AfUDG bound to DNA yields EPR signals at g = 2.13, 2.04, and 2.02, indicative of both [4Fe-4S](3+) and [3Fe-4S](1+) clusters. On the basis of this DNA-dependent redox activity, we propose a model for the rapid detection of DNA lesions using DNA-mediated electron transfer among these repair enzymes; redox activation upon DNA binding and charge transfer through well-matched DNA to an alternate bound repair protein can lead to the rapid redistribution of proteins onto genome sites in the vicinity of DNA lesions. This redox activation furthermore establishes a functional role for the ubiquitous [4Fe-4S] clusters in DNA repair enzymes that involves redox chemistry and provides a means to consider DNA-mediated signaling within the cell.     

Site-directed mutagenesis of the cysteine ligands to the [4Fe-4S] cluster of Escherichia coli MutY.

The Escherichia coli DNA repair enzyme MutY plays an important role in the recognition and repair of 7, 8-dihydro-8-oxo-2'-deoxyguanosine:2'-deoxyadenosine (OG:A) mismatches in DNA [Michaels et al. (1992) Proc. Natl. Acad. Sci. U.S. A. 89, 7022-7025]. MutY prevents DNA mutations resulting from the misincorporation of A opposite OG by using N-glycosylase activity to remove the adenine base. An interesting feature of MutY is that it contains a [4Fe-4S]2+ cluster that has been shown to play an important role in substrate recognition [Porello, S. L., Cannon, M. J., David, S. S. (1998) Biochemistry 37, 6465-6475]. Herein, we have used site-directed mutagenesis to individually replace the cysteine ligands to the [4Fe-4S]2+ cluster of E. coli MutY with serine, histidine, and alanine. The extent to which the various mutations reduce the levels of protein overexpression suggests that coordination of the [4Fe-4S]2+ cluster provides stability to MutY in vivo. The ability of the mutated enzymes to bind to a substrate analogue DNA duplex and their in vivo activity were evaluated. Remarkably, the effects are both substitution and position dependent. For example, replacement of cysteine 199 with histidine provides a mutated enzyme that is expressed at high levels and exhibits DNA binding and in vivo activity similar to the WT enzyme. These results suggest that histidine coordination to the iron-sulfur cluster may be accommodated at this position in MutY. In contrast, replacement of cysteine 192 with histidine results in less efficient DNA binding and in vivo activity compared to the WT enzyme without affecting levels of overexpression. The results from the site-directed mutagenesis suggest that the structural properties of the iron-sulfur cluster coordination domain are important for both substrate DNA recognition and the in vivo activity of MutY.     

DNA repair glycosylases with a [4Fe-4S] cluster: a redox cofactor for DNA-mediated charge transport?

The [4Fe-4S] cluster is ubiquitous to a class of base excision repair enzymes in organisms ranging from bacteria to man and was first considered as a structural element, owing to its redox stability under physiological conditions. When studied bound to DNA, two of these repair proteins (MutY and Endonuclease III from Escherichia coli) display DNA-dependent reversible electron transfer with characteristics typical of high potential iron proteins. These results have inspired a reexamination of the role of the [4Fe-4S] cluster in this class of enzymes. Might the [4Fe-4S] cluster be used as a redox cofactor to search for damaged sites using DNA-mediated charge transport, a process well known to be highly sensitive to lesions and mismatched bases? Described here are experiments demonstrating the utility of DNA-mediated charge transport in characterizing these DNA-binding metalloproteins, as well as efforts to elucidate this new function for DNA as an electronic signaling medium among the proteins.     

Characterization of an Escherichia coli mutant MutY with a cysteine to alanine mutation at the iron-sulfur cluster domain

Escherichia coli MutY is an adenine and a weak guanine DNA glycosylase involved in reducing mutagenic effects of 7,8-dihydro-8-oxoguanine (8-oxoG). The [4Fe-4S] cluster of MutY is ligated by four conserved cysteine residues and has been shown to be important in substrate recognition. Here, we show that the C199A mutant MutY is very insoluble and can be denatured and renatured to regain activity only if iron and sulfur are present in the renaturation steps. The solubility of C199A-MutY can be improved substantially as a fusion protein containing streptococcal protein G (GB1 domain) at its N-terminus. Here, we describe the first biochemical characterization of the purified GB1-C199A-MutY protein which contains a [3Fe-4S] cluster. The apparent dissociation constant (K(d)) values of GB1-C199A-MutY with both A/G and A/8-oxoG mismatches are slightly higher than that of the wild-type protein. The DNA glycosylase activity of GB1-C199A-MutY is comparable to that of the wild-type enzyme. Interestingly, the major difference between the C199A-MutY and wild-type proteins is their trapping activities (formation of Schiff base intermediates). The GB1-C199A-MutY mutant has a weaker trapping activity than the wild-type enzyme. Importantly, highly expressed GB1-C199A-MutY and untagged C199A-MutY can complement mutY mutants; however, GB1-C199A-MutY and untagged C199A-MutY cannot complement mutY mutants in vivo when both proteins are poorly expressed. Therefore, an intact [4Fe-4S] cluster domain is critical for MutY stability and activity.     

A residue in MutY important for catalysis identified by photocross-linking and mass spectrometry

MutY is an adenine glycosylase in the base excision repair (BER) superfamily that is involved in the repair of 7,8-dihydro-8-oxo-2'-deoxyguanosine (OG):A and G:A mispairs in DNA. MutY contains a [4Fe-4S]2+ cluster that is part of a novel DNA binding motif, referred to as the iron-sulfur cluster loop (FCL) motif. This motif is found in a subset of members of the BER glycosylase superfamily, defining the endonuclease III-like subfamily. Site-specific cross-linking was successfully employed to investigate the DNA-protein interface of MutY. The photoreactive nucleotide 4-thiothymidine (4ST) incorporated adjacent to the OG:A mismatch formed a specific cross-link between the substrate DNA and MutY. The amino acid participating in the cross-linking reaction was characterized by positive ion electrospray ionization (ESI) tandem mass spectrometry. This analysis revealed Arg 143 as the site of modification in MutY. Arg 143 and nearby Arg 147 are conserved throughout the endo III-like subfamily. Replacement of Arg 143 and Arg 147 with alanine by site-directed mutagenesis reduces adenine glycosylase activity of MutY toward OG:A and G:A mispairs. In addition, the R143A and R147A enzymes exhibit a reduced affinity for duplexes containing the substrate analogue 2'-deoxy-2'-fluoroadenosine opposite OG and G. Modeling of MutY bound to DNA using an endonuclease III-DNA complex structure shows that these two conserved arginines are located within close proximity to the DNA backbone. The insight from mass spectrometry experiments combined with functional mutagenesis results indicate that these two amino acids in the [4Fe-4S]2+ cluster-containing subfamily play an important role in recognition of the damaged DNA substrate.     

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

MutY is an Escherichia coli DNA repair enzyme that binds to 8-oxo-G:A and G:A mismatches and catalyzes the deglycosylation of the mismatched 2'-deoxyadenosine. We have applied DNA-mediated charge transport to probe the interaction of MutY with its DNA substrate. Oligonucleotides synthesized with a tethered rhodium intercalator and guanine doublets placed before and after the MutY binding site are used to assay for base flipping activity by MutY. On the basis of this assay, we find no evidence that MutY uses progressive base flipping as a means to find its binding site; protein binding does not perturb long-range DNA charge transport. DNA-mediated charge transport can be utilized to promote protein-DNA cross-linking from a distance. Long-range oxidation of 8-oxo-G within the MutY binding site using tethered rhodium intercalators promoted cross-linking and yielded information on MutY side chains that interact with this base. On the basis of photooxidative cross-linking of the wild type but not K142A mutant, it is evident that, within the protein complex, lysine 142 makes important contacts with 8-oxo-G.     

Positively charged residues within the iron-sulfur cluster loop of E. coli MutY participate in damage recognition and removal

Escherichia coli MutY is an adenine glycosylase involved in base excision repair that recognizes OG:A (where OG = 7, 8-dihydro-8-oxo-2'-deoxyguanosine) and G:A mismatches in DNA. MutY contains a solvent-exposed polypeptide loop between two of the cysteine ligands to the [4Fe-4S](2+) cluster, referred to as the iron-sulfur cluster loop (FCL) motif. The FCL is located adjacent to the proposed active site pocket and has been suggested to be part of the DNA binding surface of MutY (Y. Guan et al., 1998, Nat. Struct. Biol. 5, 1058-1064). In order to investigate the role of specific residues within the FCL motif, we have determined the effects of replacing arginine 194, lysine 196, and lysine 198 with alanine on the enzymatic properties of MutY. The properties of the R194A, K196A, and K198A enzymes were also compared to the properties of mutated enzymes in which lysine residues near the active site pocket were replaced with alanine or glycine. Substrate recognition was evaluated using a duplex containing a 2'-deoxyadenosine analog in a base pair opposite G or OG. These results indicate that removal of positively charged amino acids within the FCL and the active site compromise the ability of the enzyme to bind to the substrate analog. However, only the K198A enzyme exhibited a significant reduction (15-fold) of the rate of adenine removal from a G:A base pair-containing duplex. This is the first direct evidence that Lys 198 within the FCL motif of MutY has a role in specific damage recognition and removal. Furthermore, these results suggest that the FCL motif is intimately involved in the base removal process.     

An iron-sulfur cluster loop motif in the Archaeoglobus fulgidus uracil-DNA glycosylase mediates efficient uracil recognition and removal

The family 4 uracil-DNA glycosylase from the hyperthermophilic organism Archaeoglobus fulgidus (AFUDG) is responsible for the removal of uracil in DNA as the first step in the base excision repair (BER) pathway. AFUDG contains a large solvent-exposed peptide region containing an α helix and loop anchored on each end via ligation of two cysteine thiolates to a [4Fe-4S](2+) cluster. We propose that this region plays a similar role in DNA damage recognition as a smaller iron-sulfur cluster loop (FCL) motif in the structurally unrelated BER glycosylases MutY and Endonuclease III and therefore refer to this region as the "pseudo-FCL" in AFUDG. In order to evaluate the importance of this region, three positively charged residues (Arg 86, Arg 91, Lys 100) and the anchoring Cys residues (Cys 85, Cys 101) within this motif were replaced with alanine, and the effects of these replacements on uracil excision in single- and double-stranded DNA were evaluated. These results show that this region participates and allows for efficient recognition and excision of uracil within DNA. Notably, R86A AFUDG exhibited reduced activity for uracil removal only within double-stranded DNA, suggesting an importance in duplex disruption and extrusion of the base as part of the excision process. In addition, mutation of the [4Fe-4S](2+) cluster cysteine ligands at the ends of the pseudo-FCL to alanine reduced the uracil excision efficiency, suggesting the importance of anchoring the loop via coordination to the cluster. In contrast, K100A AFUDG exhibited enhanced uracil excision activity, providing evidence for the importance of the loop conformation and flexibility. Taken together, the results herein provide evidence that the pseudo-FCL motif is involved in DNA binding and catalysis, particularly in duplex DNA contexts. This work underscores the requirement of an ensemble of interactions, both distant and in proximity to the damaged site, for accurate and efficient uracil excision.     

Mutants of the base excision repair glycosylase, endonuclease III: DNA charge transport as a first step in lesion detection

Endonuclease III (EndoIII) is a base excision repair glycosylase that targets damaged pyrimidines and contains a [4Fe-4S] cluster. We have proposed a model where BER proteins that contain redox-active [4Fe-4S] clusters utilize DNA charge transport (CT) as a first step in the detection of DNA lesions. Here, several mutants of EndoIII were prepared to probe their efficiency of DNA/protein charge transport. Cyclic voltammetry experiments on DNA-modified electrodes show that aromatic residues F30, Y55, Y75, and Y82 help mediate charge transport between DNA and the [4Fe-4S] cluster. On the basis of circular dichroism studies to measure protein stability, mutations at residues W178 and Y185 are found to destabilize the protein; these residues may function to protect the [4Fe-4S] cluster. Atomic force microscopy studies furthermore reveal a correlation in the ability of mutants to carry out protein/DNA CT and their ability to relocalize onto DNA strands containing a single base mismatch; EndoIII mutants that are defective in carrying out DNA/protein CT do not redistribute onto mismatch-containing strands, consistent with our model. These results demonstrate a link between the ability of the repair protein to carry out DNA CT and its ability to relocalize near lesions, thus pointing to DNA CT as a key first step in the detection of base damage in the genome.     

[4Fe-4S] cluster trafficking mediated by Arabidopsis mitochondrial ISCA and NFU proteins

Numerous iron-sulfur (Fe-S) proteins with diverse functions are present in the matrix and respiratory chain complexes of mitochondria. Although [4Fe-4S] clusters are the most common type of Fe-S cluster in mitochondria, the molecular mechanism of [4Fe-4S] cluster assembly and insertion into target proteins by the mitochondrial iron-sulfur cluster (ISC) maturation system is not well-understood. Here we report a detailed characterization of two late-acting Fe-S cluster-carrier proteins from Arabidopsis thaliana, NFU4 and NFU5. Yeast two-hybrid and bimolecular fluorescence complementation studies demonstrated interaction of both the NFU4 and NFU5 proteins with the ISCA class of Fe-S carrier proteins. Recombinant NFU4 and NFU5 were purified as apo-proteins after expression in Escherichia coli. In vitro Fe-S cluster reconstitution led to the insertion of one [4Fe-4S]²⁺ cluster per homodimer as determined by UV-visible absorption/CD, resonance Raman and EPR spectroscopy, and analytical studies. Cluster transfer reactions, monitored by UV-visible absorption and CD spectroscopy, showed that a [4Fe-4S]²⁺ cluster-bound ISCA1a/2 heterodimer is effective in transferring [4Fe-4S]²⁺ clusters to both NFU4 and NFU5 with negligible back reaction. In addition, [4Fe-4S]²⁺ cluster-bound ISCA1a/2, NFU4, and NFU5 were all found to be effective [4Fe-4S]²⁺ cluster donors for maturation of the mitochondrial apo-aconitase 2 as assessed by enzyme activity measurements. The results demonstrate rapid, unidirectional, and quantitative [4Fe-4S]²⁺ cluster transfer from ISCA1a/2 to NFU4 or NFU5 that further delineates their respective positions in the plant ISC machinery and their contributions to the maturation of client [4Fe-4S] cluster-containing proteins.     

Structural Plasticity of NFU1 Upon Interaction with Binding Partners: Insights into the Mitochondrial [4Fe-4S] Cluster Pathway

In humans, the biosynthesis and trafficking of mitochondrial [4Fe-4S]²⁺ clusters is a highly coordinated process that requires a complex protein machinery. In a mitochondrial pathway among various proposed to biosynthesize nascent [4Fe-4S]²⁺ clusters, two [2Fe-2S]²⁺ clusters are converted into a [4Fe-4S]²⁺ cluster on a ISCA1-ISCA2 complex. Along this pathway, this cluster is then mobilized from this complex to mitochondrial apo recipient proteins with the assistance of accessory proteins. NFU1 is the accessory protein that first receives the [4Fe-4S]²⁺ cluster from ISCA1-ISCA2 complex. A structural view of the protein–protein recognition events occurring along the [4Fe-4S]²⁺ cluster trafficking as well as how the globular N-terminal and C-terminal domains of NFU1 act in such process is, however, still elusive. Here, we applied small-angle X-ray scattering coupled with on-line size-exclusion chromatography and paramagnetic NMR to disclose structural snapshots of ISCA1-, ISCA2- and NFU1-containing apo complexes as well as the coordination of [4Fe-4S]²⁺ cluster bound to the ISCA1-NFU1 complex, which is the terminal stable species of the [4Fe-4S]²⁺ cluster transfer pathway involving ISCA1-, ISCA2- and NFU1 proteins. The structural modelling of ISCA1-ISCA2, ISCA1-ISCA2-NFU1 and ISCA1-NFU1 apo complexes, here reported, reveals that the structural plasticity of NFU1 domains is crucial to drive protein partner recognition and modulate [4Fe-4S]²⁺ cluster transfer from the cluster-assembly site in the ISCA1-ISCA2 complex to a cluster-binding site in the ISCA1-NFU1 complex. These structures allowed us to provide a first rational for the molecular function of the N-domain of NFU1, which can act as a modulator in the [4Fe-4S]²⁺ cluster transfer.     

Tuning the redox properties of a [4Fe-4S] center to modulate the activity of Mo-bisPGD periplasmic nitrate reductase

Molybdoenzymes are ubiquitous in living organisms and catalyze, for most of them, oxidation-reduction reactions using a large range of substrates. Periplasmic nitrate reductase (NapAB) from Rhodobacter sphaeroides catalyzes the 2-electron reduction of nitrate into nitrite. Its active site is a Mo bis-(pyranopterin guanine dinucleotide), or Mo-bisPGD, found in most prokaryotic molybdoenzymes. A [4Fe-4S] cluster and two c-type hemes form an intramolecular electron transfer chain that deliver electrons to the active site. Lysine 56 is a highly conserved amino acid which connects, through hydrogen-bonds, the [4Fe-4S] center to one of the pyranopterin ligands of the Mo-cofactor. This residue was proposed to be involved in the intramolecular electron transfer, either defining an electron transfer pathway between the two redox cofactors, and/or modulating their redox properties.In this work, we investigated the role of this lysine by combining site-directed mutagenesis, activity assays, redox titrations, EPR and HYSCORE spectroscopies. Removal of a positively-charged residue at position 56 strongly decreased the redox potential of the [4Fe-4S] cluster at pH?8 by 230?mV to 400?mV in the K56H and K56M mutants, respectively, thus affecting the kinetics of electron transfer from the hemes to the [4Fe-4S] center up to 5 orders of magnitude. This effect was partly reversed at acidic pH in the K56H mutant likely due to protonation of the imidazole ring of the histidine. Overall, our study demonstrates the critical role of a charged residue from the second coordination sphere in tuning the reduction potential of the [4Fe-4S] cluster in RsNapAB and related molybdoenzymes.     

Modifications of Protein Environment of the [2Fe-2S] Cluster of the bc1 Complex: EFFECTS ON THE BIOPHYSICAL PROPERTIES OF THE RIESKE IRON-SULFUR PROTEIN AND ON THE KINETICS OF THE COMPLEX*

The rate-determining step in the overall turnover of the bc₁ complex is electron transfer from ubiquinol to the Rieske iron-sulfur protein (ISP) at the Q_o-site. Structures of the ISP from Rhodobacter sphaeroides show that serine 154 and tyrosine 156 form H-bonds to S-1 of the [2Fe-2S] cluster and to the sulfur atom of the cysteine liganding Fe-1 of the cluster, respectively. These are responsible in part for the high potential (E_m,7 ∼300 mV) and low pK_a (7.6) of the ISP, which determine the overall reaction rate of the bc₁ complex. We have made site-directed mutations at these residues, measured thermodynamic properties using protein film voltammetry to evaluate the E_m and pK_a values of ISPs, explored the local proton environment through two-dimensional electron spin echo envelope modulation, and characterized function in strains S154T, S154C, S154A, Y156F, and Y156W. Alterations in reaction rate were investigated under conditions in which concentration of one substrate (ubiquinol or ISP_ox) was saturating and the other was varied, allowing calculation of kinetic terms and relative affinities. These studies confirm that H-bonds to the cluster or its ligands are important determinants of the electrochemical characteristics of the ISP, likely through electron affinity of the interacting atom and the geometry of the H-bonding neighborhood. The calculated parameters were used in a detailed Marcus-Brønsted analysis of the dependence of rate on driving force and pH. The proton-first-then-electron model proposed accounts naturally for the effects of mutation on the overall reaction.     

Reversible inactivation of E. coli endonuclease III via modification of its [4Fe-4S] cluster by nitric oxide

Endonuclease III, a highly conserved enzyme initiating the base excision repair of oxidized DNA bases, hosts a [4Fe-4S] cluster. Unlike many other iron-sulfur clusters, the [4Fe-4S] cluster of endonuclease III is stable and resistant to both oxidation and reduction. Here we show that the [4Fe-4S] cluster of the E. coli endonuclease III can be readily modified by nitric oxide forming the protein-bound dinitrosyl iron complex in vitro and in vivo. Modification of the [4Fe-4S] cluster completely inhibits the DNA glycosylase activity of the endonuclease III. Remarkably, the enzymatic activity is restored when the [4Fe-4S] cluster is re-assembled in the endonuclease III dinitrosyl iron complex with L-cysteine, cysteine desulfurase (IscS) and ferrous iron in vitro. Furthermore, the nitric oxide-modified [4Fe-4S] cluster in endonuclease III is efficiently repaired in aerobically growing E. coli cells, and this repair does not require new protein synthesis. These results suggest that the E. coli endonuclease III can be reversibly inactivated by nitric oxide via modification of its [4Fe-4S] cluster.     

High and low reduction potential 4Fe-4S clusters in Azotobacter vinelandii (4Fe-4S) 2ferredoxin I. Influence of the polypeptide on the reduction potentials.

Azotobacter vinelandii (4Fe-4S)2 ferredoxin I (Fd I) is an electron transfer protein with Mr equals 14,500 and Eo equals -420 mv. It exhibits and EPR signal of g equals 2.01 in its isolated form. This resonance is almost identical with the signal that originates from a "super-oxidized" state of the 4Fe-4S cluster of potassium ferricyanide-treated Clostridium ferredoxin. A cluster that exhibits this EPR signal at g equals 2.01 is in the same formal oxidation state as the cluster in oxidized Chromatium High-Potential-Iron-Protein (HiPIP). On photoreduction of Fd I with spinach chloroplast fragments, the resonance at g equals 2.01 vanishes and no EPR signal is observed. This EPR behavior is analogous to that of reduced HiPIP, which also fails to exhibit an EPR spectrum. These characteristics suggest that a cluster in A. vinelandii Fd I functions between the same pair of states on reduction as does the cluster in HiPIP, but with a midpoint reduction potential of -420 mv in contrast to the value of +350 mv characteristic of HiPIP. Quantitative EPR and stoichoimetry studies showed that only one 4Fe-4S cluster in this (4Fe-4S)2 ferredoxin is reduced. Oxidation of Fd I with potassium ferricyanide results in the uptake of 1 electron/mol as determined by quantitative EPR spectroscopy. This indicates that a cluster in Fd I shows no electron paramagnetic resonance in the isolated form of the protein accepts an electron on oxidation, as indicated by the EPR spectrum, and becomes paramagnetic. The EPR behavior of this oxidizable cluster indicates that it also functions between the same pair of oxidation states as does the Fe-S cluster in HiPIP. The midpoint reduction potential of this cluster is approximately +340 mv. A. vinelandii Fd I is the first example of an iron-sulfur protein which contains both a high potential cluster (approximately +340 mv) and a low potential cluster (-420 mv). Both Fe-S clusters appear to function between the same pair of oxidation states as the single Fe-S cluster in Chromatium HiPIP, although the midpoint reduction potentials of the two clusters are approximately 760 mv different.     

A role for iron-sulfur clusters in DNA repair

The presence of 4Fe-4S clusters in enzymes involved in DNA repair has posed the question of the role of these intricate cofactors in damaged DNA recognition and repair. It is particularly intriguing that base excision repair glycosylases that remove a wide variety of damaged bases, and also have vastly different sequences and structures, have been found to contain this cofactor. The accumulating biochemical and structural evidence indicates that the region supported by the cluster is intimately involved in DNA binding, and that such binding interactions impact catalysis of base removal. Recent evidence has also established that binding of the glycosylases to DNA facilitates oxidation of the [4Fe-4S](2+) cluster to the [4Fe-4S](3+) form. Notably, the measured redox potentials for a variety of 4Fe-4S cluster-containing glycosylases are remarkably similar. Based on this DNA-mediated redox behavior, it has been suggested that this property may be used to enhance the activity of these enzymes by facilitating damaged DNA location.