The primary photophysics of the Avena sativa phototropin 1 LOV2 domain observed with time-resolved emission spectroscopy

Two distinct mechanisms of flavoprotein spectral tuning revealed by low-temperature and time-dependent spectroscopy

Flavins such as flavin mononucleotide or flavin adenine dinucleotide are bound by diverse proteins, yet have very similar spectra when in the oxidized state. Recently, we developed new variants of flavin-binding protein CagFbFP exhibiting notable blue (Q148V) or red (I52V A85Q) shifts of fluorescence emission maxima. Here, we use time-resolved and low-temperature spectroscopy to show that whereas the chromophore environment is static in Q148V, an additional protein-flavin hydrogen bond is formed upon photoexcitation in the I52V A85Q variant. Consequently, in Q148V, excitation, emission, and phosphorescence spectra are shifted, whereas in I52V A85Q, excitation and low-temperature phosphorescence spectra are relatively unchanged, while emission spectrum is altered. We also determine the x-ray structures of the two variants to reveal the flavin environment and complement the spectroscopy data. Our findings illustrate two distinct color-tuning mechanisms of flavin-binding proteins and could be helpful for the engineering of new variants with improved optical properties.

Sub-Millisecond Photoinduced Dynamics of Free and EL222-Bound FMN by Stimulated Raman and Visible Absorption Spectroscopies

Time-resolved femtosecond-stimulated Raman spectroscopy (FSRS) provides valuable information on the structural dynamics of biomolecules. However, FSRS has been applied mainly up to the nanoseconds regime and above 700 cm^-1, which covers only part of the spectrum of biologically relevant time scales and Raman shifts. Here we report on a broadband (~200-2200 cm^-1) dual transient visible absorption (visTA)/FSRS set-up that can accommodate time delays from a few femtoseconds to several hundreds of microseconds after illumination with an actinic pump. The extended time scale and wavenumber range allowed us to monitor the complete excited-state dynamics of the biological chromophore flavin mononucleotide (FMN), both free in solution and embedded in two variants of the bacterial light-oxygen-voltage (LOV) photoreceptor EL222. The observed lifetimes and intermediate states (singlet, triplet, and adduct) are in agreement with previous time-resolved infrared spectroscopy experiments. Importantly, we found evidence for additional dynamical events, particularly upon analysis of the low-frequency Raman region below 1000 cm^-1. We show that fs-to-sub-ms visTA/FSRS with a broad wavenumber range is a useful tool to characterize short-lived conformationally excited states in flavoproteins and potentially other light-responsive proteins.

QM calculations predict the energetics and infrared spectra of transient glutamine isomers in LOV photoreceptors

Photosensory receptors containing the flavin-binding light-oxygen-voltage (LOV) domain are modular proteins that fulfil a variety of biological functions ranging from gene expression to phototropism. The LOV photocycle is initiated by blue-light and involves a cascade of intermediate species, including an electronically excited triplet state, that leads to covalent bond formation between the flavin mononucleotide (FMN) chromophore and a nearby cysteine residue. Subsequent conformational changes in the polypeptide chain arise due to the remodelling of the hydrogen bond network in the cofactor binding pocket, whereby a conserved glutamine residue plays a key role in coupling FMN photochemistry with LOV photobiology. Although the dark-to-light transition of LOV photosensors has been previously addressed by spectroscopy and computational approaches, the mechanistic basis of the underlying reactions is still not well understood. Here we present a detailed computational study of three distinct LOV domains: EL222 from Erythrobacter litoralis, AsLOV2 from the second LOV domain of Avena sativa phototropin 1, and RsLOV from Rhodobacter sphaeroides LOV protein. Extended protein-chromophore models containing all known crucial residues involved in the initial steps (femtosecond-to-microsecond) of the photocycle were employed. Energies and rotational barriers were calculated for possible rotamers and tautomers of the critical glutamine side chain, which allowed us to postulate the most energetically favoured glutamine orientation for each LOV domain along the assumed reaction path. In turn, for each evolving species, infrared difference spectra were constructed and compared to experimental EL222 and AsLOV2 transient infrared spectra, the former from original work presented here and the latter from the literature. The good agreement between theory and experiment permitted the assignment of the majority of observed bands, notably the ∼1635 cm-1 transient of the adduct state to the carbonyl of the glutamine side chain after rotation. Moreover, both the energetic and spectroscopic approaches converge in suggesting a facile glutamine flip at the adduct intermediate for EL222 and more so for AsLOV2, while for RsLOV the glutamine keeps its initial configuration. Additionally, the computed infrared shifts of the glutamine and interacting residues could guide experimental research addressing early events of signal transduction in LOV proteins.

Resonance energy transfer sensitises and monitors in situ switching of LOV2-based optogenetic actuators

Engineered light-dependent switches provide uniquely powerful opportunities to investigate and control cell regulatory mechanisms. Existing tools offer high spatiotemporal resolution, reversibility and repeatability. Cellular optogenetics applications remain limited with diffusible targets as the response of the actuator is difficult to independently validate. Blue light levels commonly needed for actuation can be cytotoxic, precluding long-term experiments. We describe a simple approach overcoming these obstacles. Resonance energy transfer can be used to constitutively or dynamically modulate actuation sensitivity. This simultaneously offers on-line monitoring of light-dependent switching and precise quantification of activation-relaxation properties in intact living cells. Applying this approach to different LOV2-based switches reveals that flanking sequences can lead to relaxation times up to 11-fold faster than anticipated. In situ–measured parameter values guide the design of target-inhibiting actuation trains with minimal blue-light exposure, and context-based optimisation can increase sensitivity and experimental throughput a further 10-fold without loss of temporal precision.

Cellular optogenetics applications are limited by difficulties in quantification and blue light toxicity. Here the authors design LOV2-based switches that use resonance energy transfer to overcome these concerns.

Femtosecond-to-nanosecond dynamics of flavin mononucleotide monitored by stimulated Raman spectroscopy and simulations

Flavin mononucleotide (FMN) belongs to the large family of flavins, ubiquitous yellow-coloured biological chromophores that contain an isoalloxazine ring system. As a cofactor in flavoproteins, it is found in various enzymes and photosensory receptors, like those featuring the light-oxygen-voltage (LOV) domain. The photocycle of FMN is triggered by blue light and proceeds via a cascade of intermediate states. In this work, we have studied isolated FMN in an aqueous solution in order to elucidate the intrinsic electronic and vibrational changes of the chromophore upon excitation. The ultrafast transitions of excited FMN were monitored through the joint use of femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption spectroscopy encompassing a time window between 0 ps and 6 ns with 50 fs time resolution. Global analysis of the obtained transient visible absorption and transient Raman spectra in combination with extensive quantum chemistry calculations identified unambiguously the singlet and triplet FMN populations and addressed solvent dynamics effects. The good agreement between the experimental and theoretical spectra facilitated the assignment of electronic transitions and vibrations. Our results represent the first steps towards more complex experiments aimed at tracking structural changes of FMN embedded in light-inducible proteins upon photoexcitation.

Photoreaction Dynamics of Full-Length Phototropin from Chlamydomonas reinhardtii

Phototropin (phot) is a blue light sensor involved in the light responses of several species from green algae to higher plants. Phot consists of two photoreceptive domains (LOV1 and LOV2) and a Ser/Thr kinase domain. These domains are connected by a hinge and a linker domain. So far, studies on the photochemical reaction dynamics of phot have been limited to short fragments, and the reactions of intact phot have not been well elucidated. Here, the photoreactions of full-length phot and of several mutants from Chlamydomonas reinhardtii (Cr) were investigated by the transient grating and circular dichroism (CD) methods. Full-length Cr phot is in monomeric form in both dark and light states and shows conformational changes upon photoexcitation. When LOV1 is excited, the hinge helix unfolds with a time constant of 77 ms. Upon excitation of LOV2, the linker helix unfolds initially followed by a tertiary structural change of the kinase domain with a time constant of 91 ms. The quantum yield of conformational change after adduct formation of LOV2 is much smaller than that of LOV1, indicating that reactive and nonreactive forms exist. The conformational changes associated with the excitations of LOV1 and LOV2 occur independently and additively, even when they are excited simultaneously. Hence, the role of LOV1 is not to enhance the kinase activity in addition to LOV2 function; we suggest LOV1 has different functions such as regulation of intermolecular interactions.

Dual Photochemical Reaction Pathway in Flavin-Based Photoreceptor LOV Domain: A Combined Quantum-Mechanics/Molecular-Mechanics Investigation

The primary photochemical reaction of the light, oxygen, and voltage (LOV) domain of the blue-light photosensor YtvA of Bacillus subtilis were investigated using high-level QM(DFT/MRCI)/MM methods. After blue-light excitation, the S_γ atom of the reactive cysteine forms a covalent bond with the C_4a of the flavin mononucleotide (FMN) ring. Two conformations for the side chain of reactive cysteine with occupancies of 70% (conf A) and 30% (conf B) are observed in the X-ray crystallographic structures of the YtvA-LOV ( Möglich , A. ; Moffat , K. J. Mol. Biol. 2007 , 373 , 112 - 126 ). In conf A, the thiol group is directed toward the dimethylbenzene moiety of the FMN ring whereas it is placed directly above the N₅ atom of the FMN ring in conf B. Starting from both conformations, the singlet and triplet excited pathways were evaluated. The singlet states excited from conf A decay nonradiatively to the triplet states by intersystem crossing (ISC). After the formation of a neutral biradical, the triplet states cross over to the electronic ground state by a second ISC and the adducts are efficiently formed. The singlet states excited from conf B are located near the S₁/S₀ conical intersection (CIn). A major fraction returns to the initial states through the CIn. The rest may directly reach the adduct state. Thus, the photoexcitation has a dual reaction pathway. In YtvA-LOV, it is inferred that the efficient triplet excitation from conf A was chosen by bypassing the less efficient singlet excitation from conf B.

Quantum Mechanics/Molecular Mechanics Study on the Photoreactions of Dark- and Light-Adapted States of a Blue-Light YtvA LOV Photoreceptor

The dark- and light-adapted states of YtvA LOV domains exhibit distinct excited-state behavior. We have employed high-level QM(MS-CASPT2)/MM calculations to study the photochemical reactions of the dark- and light-adapted states. The photoreaction from the dark-adapted state starts with an S₁ →T₁ intersystem crossing followed by a triplet-state hydrogen transfer from the thiol to the flavin moiety that produces a diradical intermediate, and a subsequent internal conversion that triggers a barrierless C-S bond formation in the S₀ state. The energy profiles for these transformations are different for the four conformers of the dark-adapted state considered. The photochemistry of the light-adapted state does not involve the triplet state: photoexcitation to the S₁ state triggers C-S bond cleavage followed by recombination in the S₀ state; both these processes are essentially barrierless and thus ultrafast. The present work offers new mechanistic insights into the photoresponse of flavin-containing blue-light photoreceptors.

Bright blue-shifted fluorescent proteins with Cys in the GAF domain engineered from bacterial phytochromes: fluorescence mechanisms and excited-state dynamics

Near-infrared fluorescent proteins (NIR FPs) engineered from bacterial phytochromes (BphPs) are of great interest for in vivo imaging. They utilize biliverdin (BV) as a chromophore, which is a heme degradation product, and therefore they are straightforward to use in mammalian tissues. Here, we report on fluorescence properties of NIR FPs with key alterations in their BV binding sites. BphP1-FP, iRFP670 and iRFP682 have Cys residues in both PAS and GAF domains, rather than in the PAS domain alone as in wild-type BphPs. We found that NIR FP variants with Cys in the GAF or with Cys in both PAS and GAF show blue-shifted emission with long fluorescence lifetimes. In contrast, mutants with Cys in the PAS only or no Cys residues at all exhibit red-shifted emission with shorter lifetimes. Combining these results with previous biochemical and BphP1-FP structural data, we conclude that BV adducts bound to Cys in the GAF are the origin of bright blue-shifted fluorescence. We propose that the long fluorescence lifetime follows from (i) a sterically more constrained thioether linkage, leaving less mobility for ring A than in canonical BphPs, and (ii) that π-electron conjugation does not extend on ring A, making excited-state deactivation less sensitive to ring A mobility.

Photoadduct Formation from the FMN Singlet Excited State in the LOV2 Domain of Chlamydomonas reinhardtii Phototropin

The two light, oxygen, and voltage domains of phototropin are blue-light photoreceptor domains that control various functions in plants and green algae. The key step of the light-driven reaction is the formation of a photoadduct between its FMN chromophore and a conserved cysteine, where the canonical reaction proceeds through the FMN triplet state. Here, complete photoreaction mapping of CrLOV2 from Chlamydomonas reinhardtii phototropin and AsLOV2 from Avena sativa phototropin-1 was realized by ultrafast broadband spectroscopy from femtoseconds to microseconds. We demonstrate that in CrLOV2, a direct photoadduct formation channel originates from the initially excited singlet state, in addition to the canonical reaction through the triplet state. This direct photoadduct reaction is coupled by a proton or hydrogen transfer process, as indicated by a significant kinetic isotope effect of 1.4 on the fluorescence lifetime. Kinetic model analyses showed that 38% of the photoadducts are generated from the singlet excited state.

Structure and Function of the ZTL/FKF1/LKP2 Group Proteins in Arabidopsis

The ZTL/FKF1/LKP2 group proteins are LOV-domain-based blue-light photoreceptors that control protein degradation by ubiquitination. These proteins were identified relatively recently and are known to be involved in the regulation of the circadian clock and photoperiodic flowering in Arabidopsis. In this review, we focus on two topics. First, we summarize the molecular mechanisms by which ZTL and FKF1 regulate these biological phenomena based on genetic and biochemical analyses. Next, we discuss the chemical properties of the ZTL family LOV domains obtained from structural, biophysical, and photochemical characterizations of the LOV domains. These two different levels of approach unveiled the molecular mechanisms by which plants utilize ZTL and FKF1 proteins to monitor light for daily and seasonal adaptation.

Photoactivation mechanisms of flavin-binding photoreceptors revealed through ultrafast spectroscopy and global analysis methods

Flavin-binding photoreceptor proteins use the isoalloxazine moiety of flavin cofactors to absorb light in the blue/UV-A wavelength region and subsequently translate it into biological information. The underlying photochemical reactions and protein structural dynamics are delicately tuned by the protein environment and represent fundamental reactions in biology and chemistry. Due to their photo-switchable nature, these proteins can be studied efficiently with laser-flash induced transient absorption and emission spectroscopy with temporal precision down to the femtosecond time domain. Here, we describe the application of both visible and mid-IR ultrafast transient absorption and time-resolved fluorescence methods in combination with sophisticated global analysis procedures to elucidate the photochemistry and signal transduction of BLUF (Blue light receptors using FAD) and LOV (Light oxygen voltage) photoreceptor domains.

Why flavins are not competitors of chlorophyll in the evolution of biological converters of solar energy

Excited flavin molecules can photocatalyze reactions, leading to the accumulation of free energy in the products, and the data accumulated through biochemical experiments and by modeling prebiological processes suggest that flavins were available in the earliest stages of evolution. Furthermore, model experiments have shown that abiogenic flavin conjugated with a polyamino acid matrix, a pigment that photocatalyzes the phosphorylation of ADP to form ATP, could have been present in the prebiotic environment. Indeed, excited flavin molecules play key roles in many photoenzymes and regulatory photoreceptors, and the substantial structural differences between photoreceptor families indicate that evolution has repeatedly used flavins as chromophores for photoreceptor proteins. Some of these photoreceptors are equipped with a light-harvesting antenna, which transfers excitation energy to chemically reactive flavins in the reaction center. The sum of the available data suggests that evolution could have led to the formation of a flavin-based biological converter to convert light energy into energy in the form of ATP.