Light-generated paramagnetic intermediates in BLUF domains

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.

How can EPR spectroscopy help to unravel molecular mechanisms of flavin-dependent photoreceptors?

Electron paramagnetic resonance (EPR) spectroscopy is a well-established spectroscopic method for the examination of paramagnetic molecules. Proteins can contain paramagnetic moieties in form of stable cofactors, transiently formed intermediates, or spin labels artificially introduced to cysteine sites. The focus of this review is to evaluate potential scopes of application of EPR to the emerging field of optogenetics. The main objective for EPR spectroscopy in this context is to unravel the complex mechanisms of light-active proteins, from their primary photoreaction to downstream signal transduction. An overview of recent results from the family of flavin-containing, blue-light dependent photoreceptors is given. In detail, mechanistic similarities and differences are condensed from the three classes of flavoproteins, the cryptochromes, LOV (Light-oxygen-voltage), and BLUF (blue-light using FAD) domains. Additionally, a concept that includes spin-labeled proteins and examination using modern pulsed EPR is introduced, which allows for a precise mapping of light-induced conformational changes.

Detecting a new source for photochemically induced dynamic nuclear polarization in the LOV2 domain of phototropin by magnetic-field dependent (13)C NMR spectroscopy

Phototropin is a flavin mononucleotide (FMN) containing blue-light receptor, which regulates, governed by its two LOV domains, the phototropic response of higher plants. Upon photoexcitation, the FMN cofactor triplet state, (3)F, reacts with a nearby cysteine to form a covalent adduct. Cysteine-to-alanine mutants of LOV domains instead generate a flavin radical upon illumination. Here, we explore the formation of photochemically induced dynamic nuclear polarization (CIDNP) in LOV2-C450A of Avena sativa phototropin and demonstrate that photo-CIDNP observed in solution (13)C NMR spectra can reliably be interpreted in terms of solid-state mechanisms including a novel triplet mechanism. To minimize cross-polarization, which transfers light-induced magnetization to adjacent (13)C nuclei, our experiments were performed on proteins reconstituted with specifically (13)C-labeled flavins. Two potential sources for photo-CIDNP can be identified: The photogenerated triplet state, (3)F, and the triplet radical pair (3)(F(-•)W(+•)), formed by electron abstraction of (3)F from tryptophan W491. To separate the two contributions, photo-CIDNP studies were performed at four different magnetic fields ranging from 4.7 to 11.8 T. Analysis revealed that, at fields <9 T, both (3)(F(-•)W(+•)) and (3)F contribute to photo-CIDNP, whereas at high magnetic fields, the calculated enhancement factors of (3)F agree favorably with their experimental counterparts. Thus, we have for the first time detected that a triplet state is the major source for photo-CIDNP in a photoactive protein. Since triplet states are frequently encountered upon photoexcitation of flavoproteins, the novel triplet mechanism opens up new means of studying electronic structures of the active cofactors in these proteins at atomic resolution.

Molecular eyes: proteins that transform light into biological information

Most biological photoreceptors are protein/cofactor complexes that induce a physiological reaction upon absorption of a photon. Therefore, these proteins represent signal converters that translate light into biological information. Researchers use this property to stimulate and study various biochemical processes conveniently and non-invasively by the application of light, an approach known as optogenetics. Here, we summarize the recent experimental progress on the family of blue light receptors using FAD (BLUF) receptors. Several BLUF photoreceptors modulate second messenger levels and thus represent highly interesting tools for optogenetic application. In order to activate a coupled effector protein, the flavin-binding pocket of the BLUF domain undergoes a subtle rearrangement of the hydrogen network upon blue light absorption. The hydrogen bond switch is facilitated by the ultrafast light-induced proton-coupled electron transfer (PCET) between a tyrosine and the flavin in less than a nanosecond and remains stable on a long enough timescale for biochemical reactions to take place. The cyclic nature of the photoinduced reaction makes BLUF domains powerful model systems to study protein/cofactor interaction, protein-modulated PCET and novel mechanisms of biological signalling. The ultrafast nature of the photoconversion as well as the subtle structural rearrangement requires sophisticated spectroscopic and molecular biological methods to study and understand this highly intriguing signalling process.

Variable electron transfer pathways in an amphibian cryptochrome: tryptophan versus tyrosine-based radical pairs

Electron transfer reactions play vital roles in many biological processes. Very often the transfer of charge(s) proceeds stepwise over large distances involving several amino acid residues. By using time-resolved electron paramagnetic resonance and optical spectroscopy, we have studied the mechanism of light-induced reduction of the FAD cofactor of cryptochrome/photolyase family proteins. In this study, we demonstrate that electron abstraction from a nearby amino acid by the excited FAD triggers further electron transfer steps even if the conserved chain of three tryptophans, known to be an effective electron transfer pathway in these proteins, is blocked. Furthermore, we were able to characterize this secondary electron transfer pathway and identify the amino acid partner of the resulting flavin-amino acid radical pair as a tyrosine located at the protein surface. This alternative electron transfer pathway could explain why interrupting the conserved tryptophan triad does not necessarily alter photoreactions of cryptochromes in vivo. Taken together, our results demonstrate that light-induced electron transfer is a robust property of cryptochromes and more intricate than commonly anticipated.

Photoinduced electron transfer facilitates tautomerization of the conserved signaling glutamine side chain in BLUF protein light sensors

The BLUF domain (sensor of blue light using flavin adenine dinucleotide) from a bacterial photoreceptor protein AppA undergoes a cascade of chemical transformations, including hydrogen bond rearrangements around the flavin adenine dinucleotide (FAD) chromophore, in response to light illumination. These transformations are initiated by photoinduced electron and proton transfer from a tyrosine residue to the photoexcited flavin which is assisted by a glutamine residue. According to the recent studies, the proton-coupled electron transfer leads to formation of a radical-pair intermediate Tyr•···FADH• and a tautomeric EE form of glutamine in the ground electronic state. This intermediate is a precursor of the light-induced state of the BLUF photoreceptor implicated in biological signaling. In order to describe evolution of the radical pair, we computed reaction pathways on the ground state potential energy surface employing quantum-chemical calculations in the DFT PBE0/cc-pVDZ approximation for a molecular cluster mimicking the chromophore containing pocket of the AppA BLUF protein. We found a minimum-energy pathway comprised of the following consecutive reaction steps: (1) rotation of the imidic group of the EE glutamine side chain around the Cγ-Cδ bond; (2) flip of the OεH group and formation of the ZE form of the glutamine side chain; and (3) biradical recombination via coupled proton and electron transfer, leading to the ZZ form of the glutamine side chain. The potential-energy barriers for stages 1-3 do not exceed 9 kcal/mol. Energy barrier 3 describing the ZE to ZZ glutamine tautomerization is significantly smaller in the BLUF model than in isolated glutamine, since tautomerization in BLUF is facilitated by electron transfer and radical recombination. Thus, our study shows that tautomerization of the conserved glutamine is coupled to the light-induced electron transfer process in BLUF and, thus, is a viable candidate for the photoactivation mechanism which at present is very much debated.

Redox modulation of flavin and tyrosine determines photoinduced proton-coupled electron transfer and photoactivation of BLUF photoreceptors

Photoinduced electron transfer in biological systems, especially in proteins, is a highly intriguing matter. Its mechanistic details cannot be addressed by structural data obtained by crystallography alone because this provides only static information on a given redox system. In combination with transient spectroscopy and site-directed manipulation of the protein, however, a dynamic molecular picture of the ET process may be obtained. In BLUF (blue light sensors using FAD) photoreceptors, proton-coupled electron transfer between a tyrosine and the flavin cofactor is the key reaction to switch from a dark-adapted to a light-adapted state, which corresponds to the biological signaling state. Particularly puzzling is the fact that, although the various naturally occurring BLUF domains show little difference in the amino acid composition of the flavin binding pocket, the reaction rates of the forward reaction differ quite largely from a few ps up to several hundred ps. In this study, we modified the redox potential of the flavin/tyrosine redox pair by site-directed mutagenesis close to the flavin C2 carbonyl and fluorination of the tyrosine, respectively. We provide information on how changes in the redox potential of either reaction partner significantly influence photoinduced proton-coupled electron transfer. The altered redox potentials allowed us furthermore to experimentally describe an excited state charge transfer intermediately prior to electron transfer in the BLUF photocycle. Additionally, we show that the electron transfer rate directly correlates with the quantum yield of signaling state formation.