Probing the N(5)-H bond of the isoalloxazine moiety of flavin radicals by X- and W-band pulsed electron-nuclear double resonance

Red Shift in the Absorption Spectrum of Phototropin LOV1 upon the Formation of a Semiquinone Radical: Reconstructing the Orbital Architecture

Flavin mononucleotide (FMN) is a ubiquitous blue-light pigment due to its ability to drive one- and two-electron transfer reactions. In both light-oxygen-voltage (LOV) domains of phototropin from the green algae Chlamydomonas reinhardtii, FMN is noncovalently bound. In the LOV1 cysteine-to-serine mutant (C57S), light-induced electron transfer from a nearby tryptophan occurs, and a transient spin-correlated radical pair (SCRP) is formed. Within this photocycle, nuclear hyperpolarization is created by the solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) effect. In a side reaction, a stable protonated semiquinone radical (FMNH·) forms undergoing a significant bathochromic shift of the first electronic transition from 445 to 591 nm. The incorporation of phototropin LOV1-C57S into an amorphous trehalose matrix, stabilizing the radical, allows for application of various magnetic resonance experiments at ambient temperatures, which are combined with quantum-chemical calculations. As a result, the bathochromic shift of the first absorption band is explained by lifting the degeneracy of the molecular orbital energy levels for electrons with alpha and beta spins in FMNH· due to the additional electron.

Completing the Picture: Determination of 13C Hyperfine Coupling Constants of Flavin Semiquinone Radicals by Photochemically Induced Dynamic Nuclear Polarization Spectroscopy

We investigate the electronic structure of flavin semiquinone radicals in terms of their 13C hyperfine coupling constants. Photochemically induced dynamic nuclear polarization (photo-CIDNP) spectroscopy was used to study both the neutral and anionic radical species of flavin mononucleotide (FMN) in bulk aqueous solution. Apart from universally 13C-labeled FMN, partially labeled isotopologues are used to increase sensitivity for nuclei exhibiting very small hyperfine couplings and to cope with spectral overlap. In addition, experimental findings are supported by quantum chemical calculations, and implications for the spin density distribution in free flavin radicals are discussed.

Selective 13C labelling reveals the electronic structure of flavocoenzyme radicals

Flavocoenzymes are nearly ubiquitous cofactors that are involved in the catalysis and regulation of a wide range of biological processes including some light-induced ones, such as the photolyase-mediated DNA repair, magnetoreception of migratory birds, and the blue-light driven phototropism in plants. One of the factors that enable versatile flavin-coenzyme biochemistry and biophysics is the fine-tuning of the cofactor's frontier orbital by interactions with the protein environment. Probing the singly-occupied molecular orbital (SOMO) of the intermediate radical state of flavins is therefore a prerequisite for a thorough understanding of the diverse functions of the flavoprotein family. This may be ultimately achieved by unravelling the hyperfine structure of a flavin by electron paramagnetic resonance. In this contribution we present a rigorous approach to obtaining a hyperfine map of the flavin's chromophoric 7,8-dimethyl isoalloxazine unit at an as yet unprecedented level of resolution and accuracy. We combine powerful high-microwave-frequency/high-magnetic-field electron-nuclear double resonance (ENDOR) with ¹³C isotopologue editing as well as spectral simulations and density functional theory calculations to measure and analyse ¹³C hyperfine couplings of the flavin cofactor in DNA photolyase. Our data will provide the basis for electronic structure considerations for a number of flavin radical intermediates occurring in blue-light photoreceptor proteins.

pH-dependence of signaling-state formation in Drosophila cryptochrome

Cryptochromes, FAD-dependent blue light photoreceptors, undergo a series of electron transfer reactions after light excitation. Time-resolved optical spectroscopy was employed to investigate the pH dependence of all light-dependent reactions in the cryptochrome from fruit flies. Signal state formation experiments on a time scale of seconds were found to be strongly pH dependent, and formation of both anionic and neutral FAD radicals could be detected, with reaction rates increasing by a factor of ~2.5 from basic to neutral pH values. Additionally, the influence of the amino acid His378 was investigated in further detail: Two protein variants, DmCry H378A and H378Q, showed significantly reduced rate constants for signal state formation, which again differed at neutral and alkaline pH values. Hence, His378 was identified as an amino acid responsible for the pronounced pH dependence; however, this amino acid can be excluded as a proton donor for the protonation of the anionic FAD radical. Other conserved amino acids appear to alter the overall polarity of the binding pocket and thus to be responsible for the pronounced pH dependence. Furthermore, the influence of pH and other experimental parameters, such as temperature, glycerol or ferricyanide concentrations, on the intermediately formed FAD-tryptophan radical pair was explored, which deprotonates on a microsecond time scale with a clear pH dependence, and subsequently recombines within milliseconds. Surprisingly, the latter reaction showed no pH dependence; potential reasons are discussed. All results are reviewed in terms of the photoreceptor and potential magnetoreceptor functions of Drosophila cryptochrome.

EPR-derived structures of flavin radical and iron-sulfur clusters from Methylosinus sporium 5 reductase

The electronic structures of two cofactors, the FAD radical and [2Fe–2S]⁺ of reduced MMOR from Methylosinus sporium strain 5 were investigated by advanced EPR spectroscopy. The findings provide long overdue detailed structural information of MMOR.

Coupled Methyl Group Rotation in FMN Radicals Revealed by Selective Deuterium Labeling

Flavin semiquinones are common intermediate redox states in flavoproteins, and thus, knowledge of their electronic structure is essential for fully understanding their chemistry and chemical versatility. In this contribution, we use a combination of high-field electron nuclear double resonance spectroscopy and selective deuterium labeling of flavin mononucleotide (FMN) with subsequent incorporation as cofactor into a variant Avena sativa LOV domain to extract missing traits of the electronic structure of a protein-bound FMN radical. From these experiments, precise values of small proton hyperfine and deuterium nuclear quadrupole couplings could be extracted. Specifically, isotropic hyperfine couplings of -3.34, -0.11, and +0.91 MHz were obtained for the protons H(6), H(9), and H(7α), respectively. These values are discussed in the light of specific protein-cofactor interactions. Furthermore, the temperature behavior of the H(7α) methyl-group rotation elicited by its energy landscape was analyzed in greater detail. Pronounced interplay between the two methyl groups at C(7) and C(8) of FMN could be revealed. Most strikingly, this rotational behavior could be modulated by selective deuterium editing.

Methyl groups matter: Photo-CIDNP characterizations of the semiquinone radicals of FMN and demethylated FMN analogs

In this contribution, the relative hyperfine couplings are determined for the ¹H nuclei of the flavin mononucleotide (FMN) radical in an aqueous environment. In addition, three structural analogs with different methylation patterns are characterized and the influence of the substituents at the isoalloxazine moiety on the electronic structure of the radicals is explored. By exploiting nuclear hyperpolarization generated via the photo-CIDNP (chemically induced dynamic nuclear polarization) effect, it is possible to study the short-lived radical species generated by in situ light excitation. Experimental data are extracted by least-squares fitting and supported by quantum chemical calculations and published values from electron paramagnetic resonance and electron-nuclear double resonance. Furthermore, mechanistic details of the photoreaction of the investigated flavin analogs with l-tryptophan are derived from the photo-CIDNP spectra recorded at different pH values. Thereby, the neutral and anionic radicals of FMN and three structural analogs are, for the first time, characterized in terms of their electronic structure in an aqueous environment.

EPR spectroscopy on flavin radicals in flavoproteins

Flavin semiquinone redox states are important intermediates in a broad variety of reactions catalyzed by flavoproteins. As paramagnetic states they can be favorably probed by EPR spectroscopy in all its flavors. This review summarizes recent results in the characterization of flavin radicals. On the one hand, flavin radical states, e.g., trapped as reaction intermediates, can be characterized using modern pulsed EPR methods to unravel their electronic structure and to gain information about the surrounding environment and its changes on protein action. On the other hand, short-lived intermediate flavin radical states generated, e.g., photochemically, can be followed by time-resolved EPR, which allows a direct tracking of flavin-dependent reactions with a temporal resolution reaching nanoseconds.

Spin Densities in Flavin Analogs within a Flavoprotein

Characterization by electron paramagnetic resonance techniques of several variants of Anabaena flavodoxin, where the naturally occurring FMN cofactor is substituted by different analogs, makes it possible to improve the details of the spin distribution map in the isoallosazine ring in its semiquinone state. The analyzed variants were selected to monitor the effects of intrinsic changes in the flavin ring electronic structure, as well as perturbations in the apoflavodoxin-flavin interaction, on the spin populations. When these effects were analyzed together with the functional properties of the different flavodoxin variants, a relationship between spin population and biochemical parameters, as the reduction potential, could be envisaged.

Inhomogeneous ensembles of radical pairs in chemical compasses

The biophysical basis for the ability of animals to detect the geomagnetic field and to use it for finding directions remains a mystery of sensory biology. One much debated hypothesis suggests that an ensemble of specialized light-induced radical pair reactions can provide the primary signal for a magnetic compass sensor. The question arises what features of such a radical pair ensemble could be optimized by evolution so as to improve the detection of the direction of weak magnetic fields. Here, we focus on the overlooked aspect of the noise arising from inhomogeneity of copies of biomolecules in a realistic biological environment. Such inhomogeneity leads to variations of the radical pair parameters, thereby deteriorating the signal arising from an ensemble and providing a source of noise. We investigate the effect of variations in hyperfine interactions between different copies of simple radical pairs on the directional response of a compass system. We find that the choice of radical pair parameters greatly influences how strongly the directional response of an ensemble is affected by inhomogeneity.

Methyl rotors in flavoproteins

ENDOR evidence shows that methyl groups in flavin behave as quantum locked rotors.

EPR on Flavoproteins

Flavoproteins often employ radical mechanisms in their enzymatic reactions. This involves paramagnetic species, which can ideally be investigated with electron paramagnetic resonance (EPR) spectroscopy. In this chapter we focus on the example of flavin-based photoreceptors and discuss, how different EPR methods have been used to extract information about the flavin radical's electronic state, its binding pocket, electron-transfer pathways, and about the protein's tertiary and quaternary structure.

Electronic structure of the lowest triplet state of flavin mononucleotide

The electronic structure of flavin mononucleotide (FMN), an organic cofactor that plays a role in many important enzymatic reactions, has been investigated by electron paramagnetic resonance (EPR) spectroscopy, optical spectroscopy, and quantum chemistry. In particular, the triplet state of FMN, which is paramagnetic (total spin S = 1), allows an investigation of the zero field splitting parameters D and E, which are directly related to the two singly occupied molecular orbitals. Triplet EPR spectra and optical absorption spectra at different pH values in combination with time dependent density functional theory (TDDFT) reveal that the highest occupied orbital (HOMO) and lowest unoccupied orbital (LUMO) of FMN are largely unaffected by changes in the protonation state of FMN. Rather, the orbital structure of the lower lying doubly occupied orbitals changes dramatically. Additional EPR experiments have been carried out in the presence of AgNO(3), which allows the formation of an Ag-FMN triplet state with different zero field splitting parameters and population and depopulation rates. Addition of AgNO(3) only induces small changes in the optical spectrum, indicating that the Ag(+) ion only contributes to the zero field splitting by second order spin-orbit coupling and leaves the orbital structure unaffected. By a combination of the three employed methods, the observed bands in the UV/vis spectra of FMN at different pH values are assigned to electronic transitions.

The electronic structure of the neutral isoalloxazine semiquinone within Anabaena flavodoxin: new insights from HYSCORE experiments

A complete study of Anabaena flavodoxin in the neutral semiquinone state by means of the EPR pulse technique HYSCORE is here presented. The results provide new information about the hyperfine interactions of the unpaired electronic spin and the nuclei in the isoalloxazine ring. This allows a better knowledge of the electronic structure of the neutral flavin radical within the protein. Combination of these results with other previously obtained by using other EPR related techniques allowed producing a very precise mapping of the flavin spin distribution in the neutral semiquinone state. This information can be very useful for determining the relationship between the electronic structure and mechanisms in flavoproteins. An experimental protocol for measuring the electronic structure details available to date is suggested.

The structure of formaldehyde-inhibited xanthine oxidase determined by 35 GHz 2H ENDOR spectroscopy

The formaldehyde-inhibited Mo(V) state of xanthine oxidase (I) has been studied for four decades, yet it has not proven possible to distinguish unequivocally among the several structures proposed for this form. The uniquely large isotropic hyperfine coupling for (13)C from CH(2)O led to the intriguing suggestion of a direct Mo-C bond for the active site of I. This suggestion was supported by the recent crystal structures of glycol- and glycerol-inhibited forms of aldehyde oxidoreductase, a member of the xanthine oxidase family. (1)H and (2)H ENDOR spectra of I(C(1,2)H(2)O) in H(2)O/D(2)O buffer now have unambiguously revealed that the active-site structure of I contains a CH(2)O adduct of Mo(V) in the form of a four-membered ring with S and O linking the C to Mo and have ruled out a direct Mo-C bond. Density functional theory computations are consistent with this conclusion. We interpret the large (13)C coupling as resulting from a "transannular hyperfine interaction".

Nitric oxide synthase stabilizes the tetrahydrobiopterin cofactor radical by controlling its protonation state

Nitric oxide synthase (NOS), a homodimeric enzyme with a flavin reductase domain and a P450-type heme-containing oxygenase domain, catalyzes the formation of NO from L-arginine, NADPH, and O(2) in a two-step reaction sequence. In the first step, a tetrahydrobiopterin (H(4)B) cofactor bound near one of the heme propionate groups acts as an electron donor to the P450-type heme active site, yielding a one-electron oxidized radical that is subsequently re-reduced. In solution, H(4)B undergoes two-electron oxidation, showing that the enzyme significantly alters the proton- and electron-transfer properties of the cofactor. Multifrequency EPR and ENDOR spectroscopy were used to determine magnetic parameters, and from them the (de)protonation state of the H(4)B radical in the oxygenase domain dimer of inducible NO synthase that was trapped by rapid freeze quench. From 9.5 and 330-416 GHz EPR and from 34 GHz (1)H ENDOR spectroscopy, the g tensor of the radical and the hyperfine tensors of several N and H nuclei in the radical were obtained. Density functional theory calculations at the PBE0/EPR-II level for H(4)B radical models predict different spin density distributions and g and hyperfine tensors for different protonation states. Comparison of the predicted and experimental values leads to the conclusion that the radical is cationic H(4)B(*+), suggesting that NOS stabilizes this protonated form to utilize the cofactor in a unique dual one-electron redox role, where it can deliver an electron to the active site for reductive oxygen activation and also remove an electron from the active site to generate NO and not NO(-). The protein environment also prevents further oxidation and subsequent loss of function of the cofactor, thus enabling the enzyme to perform the unusual catalytic one-electron chemistry.

Light-induced activation of class II cyclobutane pyrimidine dimer photolyases

Light-induced activation of class II cyclobutane pyrimidine dimer (CPD) photolyases of Arabidopsis thaliana and Oryza sativa has been examined by UV/Vis and pulsed Davies-type electron-nuclear double resonance (ENDOR) spectroscopy, and the results compared with structure-known class I enzymes, CPD photolyase and (6-4) photolyase. By ENDOR spectroscopy, the local environment of the flavin adenine dinucleotide (FAD) cofactor is probed by virtue of proton hyperfine couplings that report on the electron-spin density at the positions of magnetic nuclei. Despite the amino-acid sequence dissimilarity as compared to class I enzymes, the results indicate similar binding motifs for FAD in the class II photolyases. Furthermore, the photoreduction kinetics starting from the FAD cofactor in the fully oxidized redox state, FAD(ox), have been probed by UV/Vis spectroscopy. In Escherichia coli (class I) CPD photolyase, light-induced generation of FADH from FAD(ox), and subsequently FADH(-) from FADH, proceeds in a step-wise fashion via a chain of tryptophan residues. These tryptophans are well conserved among the sequences and within all known structures of class I photolyases, but completely lacking from the equivalent positions of class II photolyase sequences. Nevertheless, class II photolyases show photoreduction kinetics similar to those of the class I enzymes. We propose that a different, but also effective, electron-transfer cascade is conserved among the class II photolyases. The existence of such electron transfer pathways is supported by the observation that the catalytically active fully reduced flavin state obtained by photoreduction is maintained even under oxidative conditions in all three classes of enzymes studied in this contribution.

Formation of {[HIPTN(3)N]Mo(III)H}(-) by heterolytic cleavage of H(2) as established by EPR and ENDOR spectroscopy

MoN(2) (Mo = [(HIPTNCH(2)CH(2))(3)N]Mo, where HIPT = 3,5-(2,4,6-i-Pr(3)C(6)H(2))(2)C(6)H(3)) is the first stage in the reduction of N(2) to NH(3) by Mo. Its reaction with dihydrogen in fluid solution yields "MoH(2)", a molybdenum-dihydrogen compound. In this report, we describe a comprehensive electron paramagnetic resonance (EPR) and (1/2)H/(14)N electron nuclear double resonance (ENDOR) study of the product of the reaction between MoN(2) and H(2) that is trapped in frozen solution, 1. EPR spectra of 1 show that it has a near-axial g tensor, g = [2.086, 1.961, 1.947], with dramatically reduced g anisotropy relative to MoN(2). Analysis of the g values reveal that this anion has the Mo(III), [d(xz), d(yz)](3) orbital configuration, as proposed for the parent MoN(2) complex, and that it undergoes a strong pseudo-Jahn-Teller (PJT) distortion. Simulations of the 2D 35 GHz (1)H ENDOR pattern comprised of spectra taken at multiple fields across the EPR envelope (2 K) show that 1 is the [MoH](-) anion. The 35 GHz Mims pulsed (2)H ENDOR spectra of 1 prepared with (2)H(2) show the corresponding (2)H(-) signal, with a substantial deuterium isotope effect in a(iso). Radiolytic reduction of a structural analogue, Mo(IV)H, at 77 K, confirms the assignment of 1. Analysis of the 2D (14)N ENDOR pattern for the ligand amine nitrogen further reveals the presence of a linear N(ax)-Mo-H(-) molecular axis that is parallel to the unique magnetic direction (g(1)). The ENDOR pattern of the three equatorial nitrogens is well-reproduced by a model in which the Mo-N(eq) plane has undergone a static, not dynamic, PJT distortion, leading to a range of hyperfine couplings for the three N(eq)'s. The finding of a nearly axial hyperfine coupling tensor for the terminal hydride bound Mo supports the earlier proposal that the two exchangeable hydrogenic species bound to the FeMo cofactor of the nitrogense turnover intermediate, which has accumulated four electrons/protons (E(4)), are hydrides that bridge two metal ions, not terminal hydrides.

The Electronic State of Flavoproteins: Investigations with Proton Electron-Nuclear Double Resonance

Electron-nuclear double resonance (ENDOR) spectroscopy provides useful information on hyperfine interactions between nuclear magnetic moments and the magnetic moment of an unpaired electron spin. Because the hyperfine coupling constant reacts quite sensitively to polarity changes in the direct vicinity of the nucleus under consideration, ENDOR spectroscopy can be favorably used for the detection of subtle protein-cofactor interactions. A number of pulsed ENDOR studies on flavoproteins have been published during the past few years; most of them were designed to characterize the flavin cofactor by means of its protonation state, or to detect individual protein-cofactor interactions. The aim of this study is to compare the pulsed ENDOR spectra from different flavoproteins in terms of variations of characteristic proton hyperfine values. The general concept is to observe limits of possible influences on the cofactor's electronic state by surrounding amino acids. Furthermore, we compare ENDOR data obtained from in vivo experiments with in vitro data to emphasize the potential of the method for gaining molecular information in complex media.

Human and Drosophila cryptochromes are light activated by flavin photoreduction in living cells

Cryptochromes are a class of flavoprotein blue-light signaling receptors found in plants, animals, and humans that control plant development and the entrainment of circadian rhythms. In plant cryptochromes, light activation is proposed to result from photoreduction of a protein-bound flavin chromophore through intramolecular electron transfer. However, although similar in structure to plant cryptochromes, the light-response mechanism of animal cryptochromes remains entirely unknown. To complicate matters further, there is currently a debate on whether mammalian cryptochromes respond to light at all or are instead activated by non–light-dependent mechanisms. To resolve these questions, we have expressed both human and Drosophila cryptochrome proteins to high levels in living Sf21 insect cells using a baculovirus-derived expression system. Intact cells are irradiated with blue light, and the resulting cryptochrome photoconversion is monitored by fluorescence and electron paramagnetic resonance spectroscopic techniques. We demonstrate that light induces a change in the redox state of flavin bound to the receptor in both human and Drosophila cryptochromes. Photoreduction from oxidized flavin and subsequent accumulation of a semiquinone intermediate signaling state occurs by a conserved mechanism that has been previously identified for plant cryptochromes. These results provide the first evidence of how animal-type cryptochromes are activated by light in living cells. Furthermore, human cryptochrome is also shown to undergo this light response. Therefore, human cryptochromes in exposed peripheral and/or visual tissues may have novel light-sensing roles that remain to be elucidated.

Vision in animals is generally associated with light-sensitive rhodopsin pigments located in the eyes. However, animals ranging from flies to humans also possess ancient visual receptors known as cryptochromes in multiple cell types. In this work, we study the mechanism of light sensing in two representative animal cryptochromes: a light-sensitive Drosophila cryptochrome (Dmcry) and a presumed light-insensitive mammalian cryptochrome from humans (Hscry1). We expressed recombinant cryptochromes to high levels in living cells, irradiated the cells with blue light, and analyzed the proteins' response to irradiation with electron paramagnetic resonance and fluorescence spectroscopic techniques. Photoreduction of protein-bound oxidized FAD cofactor to its radical form emerged as the primary cryptochrome photoreaction in living cells, and was correlated with a light-sensitive biological response in whole organisms. These results indicate that both Dmcry and Hscry1 are capable of undergoing similar light-driven reactions and suggest the possibility of an as-yet unknown photo-perception role for human cryptochromes in tissues exposed to light.

Cryptochromes are blue-light-absorbing receptors found in plants, animals, and humans. In mammals, they are not thought to respond to light, but this study demonstrates contrary evidence that indeed, human cryptochromes undergo a photochemical transformation in response to light.

Active site of Escherichia coli DNA photolyase: Asn378 is crucial both for stabilizing the neutral flavin radical cofactor and for DNA repair

Escherichia coli DNA photolyase repairs cyclobutane pyrimidine dimer (CPD) in UV-damaged DNA through a photoinduced electron transfer mechanism. The catalytic activity of the enzyme requires fully reduced FAD (FADH (-)). After purification in vitro, the cofactor FADH (-) in photolyase is oxidized into the neutral radical form FADH (*) under aerobic conditions and the enzyme loses its repair function. We have constructed a mutant photolyase in which asparagine 378 (N378) is replaced with serine (S). In comparison with wild-type photolyase, we found N378S mutant photolyase containing oxidized FAD (FAD ox) but not FADH (*) after routine purification procedures, but evidence shows that the mutant protein contains FADH (-) in vivo as the wild type. Although N378S mutant photolyase is photoreducable and capable of binding CPD in DNA, the activity assays indicate the mutant protein is catalytically inert. We conclude that the Asn378 residue of E. coli photolyase is crucial both for stabilizing the neutral flavin radical cofactor and for catalysis.

G-tensors of the flavin adenine dinucleotide radicals in glucose oxidase: a comparative multifrequency electron paramagnetic resonance and electron-nuclear double resonance study

The flavin adenine dinucleotide (FAD) cofactor of Aspergillus niger glucose oxidase (GO) in its anionic (FAD*-) and neutral (FADH*) radical form was investigated by electron paramagnetic resonance (EPR) at high microwave frequencies (93.9 and 360 GHz) and correspondingly high magnetic fields and by pulsed electron-nuclear double resonance (ENDOR) spectroscopy at 9.7 GHz. Because of the high spectral resolution of the frozen-solution continuous-wave EPR spectrum recorded at 360 GHz, the anisotropy of the g-tensor of FAD*- could be fully resolved. By least-squares fittings of spectral simulations to experimental data, the principal values of g have been established with high precision: gX=2.00429(3), gY=2.00389(3), gZ=2.00216(3) (X, Y, and Z are the principal axes of g) yielding giso=2.00345(3). The gY-component of FAD*- from GO is moderately shifted upon deprotonation of FADH*, rendering the g-tensor of FAD*- slightly more axially symmetric as compared to that of FADH*. In contrast, significantly altered proton hyperfine couplings were observed by ENDOR upon transforming the neutral FADH* radical into the anionic FAD*- radical by pH titration of GO. That the g-principal values of both protonation forms remain largely identical demonstrates the robustness of g against local changes in the electron-spin density distribution of flavins. Thus, in flavins, the g-tensor reflects more global changes in the electronic structure and, therefore, appears to be ideally suited to identify chemically different flavin radicals.

Electron nuclear double resonance differentiates complementary roles for active site histidines in (6-4) photolyase

(6-4) photolyase catalyzes the light-dependent repair of UV-damaged DNA containing (6-4) photoproducts. Blue light excitation of the enzyme generates the neutral FAD radical, FADH., which is believed to be transiently formed during the enzymatic DNA repair. Here (6-4) photolyase has been examined by optical spectroscopy, electron paramagnetic resonance, and pulsed electron nuclear double resonance spectroscopy. Characterization of selected proton hyperfine couplings of FADH., namely those of H(8alpha) and H(1'), yields information on the micropolarity at the site where the DNA substrate is expected to bind. Shifts in the hyperfine couplings as a function of structural modifications induced by point mutations and pH changes distinguish the protonation states of two highly conserved histidines, His(354) and His(358), in Xenopus laevis (6-4) photolyase. These are proposed to catalyze formation of the oxetane intermediate that precedes light-initiated DNA repair. The results show that at pH 9.5, where the enzymatic repair activity is highest, His(358) is deprotonated, whereas His(354) is protonated. Hence, the latter is likely the proton donor that initiates oxetane formation from the (6-4) photoproduct.

Determination of the g-matrix orientation in flavin radicals by high-field/high-frequency electron-nuclear double resonance

A high-microwave-frequency/high-magnetic-field pulsed electron-nuclear double resonance (ENDOR) study performed at 94 GHz on the flavin semiquinone cofactor of Xenopus laevis (6-4) photolyase in its neutral radical state is presented. Although the principal values of the flavin radical's g-matrix are not fully resolved in the 94-GHz EPR spectrum in a nonoriented sample, the orientation of the principal axes of g is obtained by exploiting the orientation selection of the proton ENDOR signals from the methyl protons at C-8alpha and the deuteron ENDOR signals from D-5 in an enzyme sample in deuterated buffer. This procedure for assigning the orientation of g relative to the molecular frame makes use of commercially available ENDOR instrumentation without the necessity to perform single-crystal studies.

Unambiguous Determination of the g-Matrix Orientation in a Neutral Flavin Radical by Pulsed Electron−Nuclear Double Resonance at 94 GHz

The recent observation of photoinduced radical pairs comprising a flavin radical and an oxidized amino acid residue in various blue-light-sensitive proteins has highlighted the need to gain a more complete understanding of the electronic structure of flavin radicals. In particular, precise knowledge of the anisotropy of the Zeeman interaction quantified by the g-tensor is necessary for attaining an unambiguous identification of flavin radicals by electron paramagnetic resonance (EPR). In a recent study of a protein-bound neutral flavin radical, we have determined the principal values of the g-tensor using high-frequency/high magnetic field EPR performed at 360 GHz/12.8 T. However, in those experiments, the orientation of the principal axes of g could not be unambiguously established with respect to the molecular frame of the isoalloxazine moiety. In this contribution we resolve this ambiguity by pulsed electron-nuclear double resonance (ENDOR) at 95 GHz/3.5 T (W-band). At such high values of the microwave frequency and the magnetic field, the g anisotropy provides improved spectral resolution compared to an ENDOR experiment performed at conventional 9.5 GHz/0.35 mT (X-band). This enables one to utilize Zeeman magnetoselection to obtain single-crystal-like data from disordered samples in frozen solution. Experiments exploiting this orientation selection have allowed us to use the hyperfine coupling of the methyl protons at C(8alpha) of the isoalloxazine ring to determine the angle between the molecular frame and the principal axes of g. Quite surprisingly, the g-tensor in FADH* is not oriented as one would have expected for a 1,3-semibenzoquinone radical. For the latter, the X-axis of g commonly bisects the smaller angle between the two axes along the C=O bonds. In FADH*, the large spin density on N(5) and C(4a) apparently contributes to a significant (44 degrees ) reorientation of the g-tensor axes.