The dynamics of the flavin, NADPH, and active site loops determine the mechanism of activation of class B flavin-dependent monooxygenases.

Flavin-dependent monooxygenases (FMOs) constitute a diverse enzyme family that catalyzes crucial hydroxylation, epoxidation, and Baeyer-Villiger reactions across various metabolic pathways in all domains of life. Due to the intricate nature of this enzyme family's mechanisms, some aspects of their functioning remain unknown. Here, we present the results of molecular dynamics computations, supplemented by a bioinformatics analysis, that clarify the early stages of their catalytic cycle. We have elucidated the intricate binding mechanism of NADPH and L-Orn to a class B monooxygenase, the ornithine hydroxylase from Aspergillus $$ Aspergillus $$ fumigatus $$ fumigatus $$ known as SidA. Our investigation involved a comprehensive characterization of the conformational changes associated with the FAD (Flavin Adenine Dinucleotide) cofactor, transitioning from the out to the in position. Furthermore, we explored the rotational dynamics of the nicotinamide ring of NADPH, shedding light on its role in facilitating FAD reduction, supported by experimental evidence. Finally, we also analyzed the extent of conservation of two Tyr-loops that play critical roles in the process.

Redox Properties of Flavin in BLUF and LOV Photoreceptor Proteins from Hybrid QM/MM Molecular Dynamics Simulation.

Flavins play an important role in many oxidation and reduction processes in biological systems. For example, flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) are common cofactors found in enzymatic proteins that use the special redox properties of these flavin molecules for their catalytic or photoactive functions. The redox potential of the flavin is strongly affected by its (protein) environment; however, the underlying molecular interactions of this effect are still unknown. Using hybrid quantum mechanics/molecular mechanics (QM/MM) simulation techniques, we have studied the redox properties of flavin in the gas phase, aqueous solution, and two different protein environments, in particular, a BLUF and a LOV photoreceptor domain. By mapping the changes in electrostatic potential and solvent structure, we gain insight into how specific polarization of the flavin by its environment tunes the reduction potential. We find also that accurate calculation of the reduction potentials of these systems by using the hybrid QM/MM approach is hampered by a too limited sampling of the counterion configurations and by artifacts at the QM/MM boundary. We make suggestions for how these issues can be overcome.

Light-induced Trpin/Metout Switching During BLUF Domain Activation in ATP-bound Photoactivatable Adenylate Cyclase OaPAC.

The understanding of signal transduction mechanisms in photoreceptor proteins is essential for elucidating how living organisms respond to light as environmental stimuli. In this study, we investigated the ATP binding, photoactivation and signal transduction process in the photoactivatable adenylate cyclase from Oscillatoria acuminata (OaPAC) upon blue light excitation. Structural models with ATP bound in the active site of native OaPAC at cryogenic as well as room temperature are presented. ATP is found in one conformation at cryogenic- and in two conformations at ambient-temperature, and is bound in an energetically unfavorable conformation for the conversion to cAMP. However, FTIR spectroscopic experiments confirm that this conformation is the native binding mode in dark state OaPAC and that transition to a productive conformation for ATP turnover only occurs after light activation. A combination of time-resolved crystallography experiments at synchrotron and X-ray Free Electron Lasers sheds light on the early events around the Flavin Adenine Dinucleotide (FAD) chromophore in the light-sensitive BLUF domain of OaPAC. Early changes involve the highly conserved amino acids Tyr6, Gln48 and Met92. Crucially, the Gln48 side chain performs a 180° rotation during activation, leading to the stabilization of the FAD chromophore. Cryo-trapping experiments allowed us to investigate a late light-activated state of the reaction and revealed significant conformational changes in the BLUF domain around the FAD chromophore. In particular, a Trpin/Metout transition upon illumination is observed for the first time in the BLUF domain and its role in signal transmission via α-helix 3 and 4 in the linker region between sensor and effector domain is discussed.

Structural insights into the semiquinone form of human Cytochrome P450 reductase by DEER distance measurements between a native flavin and a spin labelled non-canonical amino acid.

The flavoprotein Cytochrome P450 reductase (CPR) is the unique electron pathway from NADPH to Cytochrome P450 (CYPs). The conformational dynamics of human CPR in solution, which involves transitions from a "locked/closed" to an "unlocked/open" state, is crucial for electron transfer. To date, however, the factors guiding these changes remain unknown. By Site-Directed Spin Labelling coupled to Electron Paramagnetic Resonance spectroscopy, we have incorporated a non-canonical amino acid onto the flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) domains of soluble human CPR, and labelled it with a specific nitroxide spin probe. Taking advantage of the endogenous FMN cofactor, we successfully measured for the first time, the distance distribution by DEER between the semiquinone state FMNH• and the nitroxide. The DEER data revealed a salt concentration-dependent distance distribution, evidence of an "open" CPR conformation at high salt concentrations exceeding previous reports. We also conducted molecular dynamics simulations which unveiled a diverse ensemble of conformations for the "open" semiquinone state of the CPR at high salt concentration. This study unravels the conformational landscape of the one electron reduced state of CPR, which had never been studied before.

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.

Ligand bound structure of a 6-hydroxynicotinic acid 3-monooxygenase provides mechanistic insights.

6-Hydroxynicotinic acid 3-monooxygenase (NicC) is a bacterial enzyme involved in the degradation of nicotinic acid. This enzyme is a Class A flavin-dependent monooxygenase that catalyzes a unique decarboxylative hydroxylation. The unliganded structure of this enzyme has previously been reported and studied using steady- and transient-state kinetics to support a comprehensive kinetic mechanism. Here we report the crystal structure of the H47Q NicC variant in both a ligand-bound (solved to 2.17 Å resolution) and unliganded (1.51 Å resolution) form. Interestingly, in the liganded form, H47Q NicC is bound to 2-mercaptopyridine (2-MP), a contaminant present in the commercial stock of 6-mercaptopyridine-3-carboxylic acid(6-MNA), a substrate analogue. 2-MP binds weakly to H47Q NicC and is not a substrate for the enzyme. Based on kinetic and thermodynamic characterization, we have fortuitously captured a catalytically inactive H47Q NicC•2-MP complex in our crystal structure. This complex reveals interesting mechanistic details about the reaction catalyzed by 6-hydroxynicotinic acid 3-monooxygenase.

Characterization of the Membrane-Associated Electron-Bifurcating Flavoenzyme EtfABCX from the Hyperthermophilic Bacterium Thermotoga maritima.

Electron bifurcation is an energy-conservation mechanism in which a single enzyme couples an exergonic reaction with an endergonic one. Heterotetrameric EtfABCX drives the reduction of low-potential ferredoxin (E°' ∼ -450 mV) by oxidation of the midpotential NADH (E°' = -320 mV) by simultaneously coupling the reaction to reduction of the high-potential menaquinone (E°' = -74 mV). Electron bifurcation occurs at the NADH-oxidizing bifurcating-flavin adenine dinucleotide (BF-FAD) in EtfA, which has extremely crossed half-potentials and passes the first, high-potential electron to an electron-transferring FAD and via two iron-sulfur clusters eventually to menaquinone. The low-potential electron on the BF-FAD semiquinone simultaneously reduces ferredoxin. We have expressed the genes encodingThermotoga maritimaEtfABCX in E. coli and purified the EtfABCX holoenzyme and the EtfAB subcomplex. The bifurcation activity of EtfABCX was demonstrated by using electron paramagnetic resonance (EPR) to follow accumulation of reduced ferredoxin. To elucidate structural factors that impart the bifurcating ability, EPR and NADH titrations monitored by visible spectroscopy and dye-linked enzyme assays have been employed to characterize four conserved residues, R38, P239, and V242 in EtfA and R140 in EtfB, in the immediate vicinity of the BF-FAD. The R38, P239, and V242 variants showed diminished but still significant bifurcation activity. Despite still being partially reduced by NADH, the R140 variant had no bifurcation activity, and electron transfer to its two [4Fe-4S] clusters was prevented. The role of R140 is discussed in terms of the bifurcation mechanism in EtfABCX and in the other three families of bifurcating enzymes.

Dihydropyrmidine dehydrogenase from Escherichia coli: Transient state analysis reveals both reductive activation prior to turnover and diminished substrate effector roles relative to the mammalian form.

Dihydropyrimidine dehydrogenase (DPD) is an enzyme that uses an elaborate architecture to catalyze a simple net reaction: the reduction of the vinylic bond of uracil and thymine. Known DPDs have two active sites separated by approximately 60 Å. One active site has an FAD cofactor and binds NAD(P) and the other has an FMN cofactor and binds pyrimidines. The intervening distance is spanned by four Fe4S4 centers that act as an electron conduit. Recent advancements with porcine DPD have revealed unexpected chemical sequences where the enzyme undergoes reductive activation by transferring two electrons from NADPH to the FMN via the FAD such that the active form has the cofactor set FAD•4(Fe4S4)•FMNH2. Here we describe the first comprehensive kinetic investigation of a bacterial form of DPD. Using primarily transient state methods, DPD from E. coli (EcDPD) was shown to have a similar mechanism to that observed with the mammalian form in that EcDPD is observed to undergo reductive activation before pyrimidine reduction and displays half-of-sites activity. However, two distinct aspects of the EcDPD reaction relative to the mammalian enzyme were observed that relate to the effector roles for substrates: (i) the enzyme will rapidly take up electrons from NADH, reducing a flavin in the absence of pyrimidine substrate, and (ii) the activated form of the enzyme can become fully oxidized by transferring electrons to pyrimidine substrates in the absence of NADH.

Riboflavin interactions with the chicken isolated carrier protein.

Riboflavin, a member of the B vitamin family, is a water-soluble vitamin that participates in energy metabolism processes via two coenzymes, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), in oxidized and reduced forms. Low levels of riboflavin have been associated with growth and developmental problems. In an effort to investigate the role of hydrogen bonding in the interactions between riboflavin and chicken riboflavin binding protein, the solid state geometry characteristics of a riboflavin derivative stripped of hydroxyl groups except the primary one, N-(6'-hydroxyhexyl)isoalloxazine, were investigated and found that π-stacking and hydrogen bonding involving the isoalloxazine rings are the primary intermolecular interactions. Subsequent comparative fluorescence studies showed that at neutral pH, in presence of the protein, quenching of N-(6'-hydroxyhexyl)isoalloxazine and riboflavin occurred similarly suggesting that the hydroxyl groups were not a key component of the vitamin protein interactions in the binding pocket.

Orientation-Controllable Enzyme Cascade on Electrode for Bioelectrocatalytic Chain Reaction.

The accomplishment of concurrent interenzyme chain reaction and direct electric communication in a multienzyme-electrode is challenging since the required condition of multienzymatic binding conformation is quite complex. In this study, an enzyme cascade-induced bioelectrocatalytic system has been constructed using solid binding peptide (SBP) as a molecular binder that coimmobilizes the invertase (INV) and flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase gamma-alpha complex (GDHγα) cascade system on a single electrode surface. The SBP-fused enzyme cascade was strategically designed to induce diverse relative orientations of coupling enzymes while enabling efficient direct electron transfer (DET) at the FAD cofactor of GDHγα and the electrode interface. The interenzyme relative orientation was found to determine the intermediate delivery route and affect overall chain reaction efficiency. Moreover, interfacial DET between the fusion GDHγα and the electrode was altered by the binding conformation of the coimmobilized enzyme and fusion INVs. Collectively, this work emphasizes the importance of interenzyme orientation when incorporating enzymatic cascade in an electrocatalytic system and demonstrates the efficacy of SBP fusion technology as a generic tool for developing cascade-induced direct bioelectrocatalytic systems. The proposed approach is applicable to enzyme cascade-based bioelectronics such as biofuel cells, biosensors, and bioeletrosynthetic systems utilizing or producing complex biomolecules.

Mechanism of the Multistep Catalytic Cycle of 6-Hydroxynicotinate 3-Monooxygenase Revealed by Global Kinetic Analysis.

The class A flavoenzyme 6-hydroxynicotinate 3-monooxygenase (NicC) catalyzes a rare decarboxylative hydroxylation reaction in the degradation of nicotinate by aerobic bacteria. While the structure and critical residues involved in catalysis have been reported, the mechanism of this multistep enzyme has yet to be determined. A kinetic understanding of the NicC mechanism would enable comparison to other phenolic hydroxylases and illuminate its bioengineering potential for remediation of N-heterocyclic aromatic compounds. Toward these goals, transient state kinetic analyses by stopped-flow spectrophotometry were utilized to follow rapid changes in flavoenzyme absorbance spectra during all three stages of NicC catalysis: (1) 6-HNA binding; (2) NADH binding and FAD reduction; and (3) O2 binding with C4a-adduct formation, substrate hydroxylation, and FAD regeneration. Global kinetic simulations by numeric integration were used to supplement analytical fitting of time-resolved data and establish a kinetic mechanism. Results indicate that 6-HNA binding is a two-step process that substantially increases the affinity of NicC for NADH and enables the formation of a charge-transfer-complex intermediate to enhance the rate of flavin reduction. Singular value decomposition of the time-resolved spectra during the reaction of the substrate-bound, reduced enzyme with dioxygen provides evidence for the involvement of C4a-hydroperoxy-flavin and C4a-hydroxy-flavin intermediates in NicC catalysis. Global analysis of the full kinetic mechanism suggests that steady-state catalytic turnover is partially limited by substrate hydroxylation and C4a-hydroxy-flavin dehydration to regenerate the flavoenzyme. Insights gleaned from the kinetic model and determined microscopic rate constants provide a fundamental basis for understanding NicC's substrate specificity and reactivity.

SolS-catalyzed sulfoxidation of labionin to solabionin drives antibacterial activity of solabiomycins.

Ribosomally synthesized and posttranslationally modified peptides (RiPPs) with polar-functionalized fatty acyl groups are newly found lipopeptide-class natural products. We recently employed a combined approach of genome mining and stable isotope labeling and discovered solabiomycins as one of the polar-functionalized fatty-acylated RiPPs (PFARs) from Streptomyces lydicus NBRC13058. The solabiomycins contained a characteristic sulfoxide group in the labionin moiety referred to as the 'solabionin' structure for the RiPP moiety. A previous gene knockout experiment indicated that solS, which encodes a putative flavin adenine dinucleotide (FAD)-nicotinamide adenine dinucleotide (phosphate) (NAD(P))-binding protein, is involved in the sulfoxidation of an alkyl sulfide in the solabionin. In this study, we isolated deoxysolabiomycins A and B from ΔsolS mutant and fully determined the chemical structures using a series of NMR experiments. We also tested the bioactivity of deoxysolabiomycins against Gram-positive bacteria, including Mycolicibacterium smegmatis, and notably found that the sulfoxide is critical for the antibacterial activity. To characterize the catalytic activity of SolS, the recombinant protein was incubated with a putative substrate, deoxysolabiomycins, and the cofactors FAD and NADPH. In vitro reactions demonstrated that SolS catalyzes the sulfoxidation, converting deoxysolabiomycins to solabiomycins.

Reduced Flavin in Aqueous Solution Is Nonfluorescent.

Flavins are blue-light-absorbing chromophores with rich redox activity. Biologically, the most important are riboflavin (vitamin B2), flavin mononucleotide, and flavin adenine dinucleotide, the latter two of which are catalytic cofactors in enzymes. Flavins pivot between oxidized, one electron-, and two electron-reduced forms in different protonation states, depending on enzymatic requirements. Some flavoenzymes use light as a reagent for chemical bond formation, photoinduced electron transfer, or conformational changes required for light-sensitive signaling. Therefore, the photochemistry and photophysics of flavins have received wide attention. Fluorescence from oxidized flavin is often used to detect and track changes in flavin oxidation states. However, there have been conflicting reports over the past 45 years as to whether reduced flavin in solution has detectable fluorescence. Here, using single photon counting emission spectroscopy with rigorous sample preparation, we show definitively that reduced flavins are essentially nonfluorescent, having a quantum yield more than three orders of magnitude lower than oxidized flavin. This result will force a re-evaluation of experiments and models that assumed otherwise.

Mammalian dihydropyrimidine dehydrogenase: Added mechanistic details from transient-state analysis of charge transfer complexes.

Dihydropyrimidine dehydrogenase (DPD) is a flavin dependent enzyme that catalyzes the reduction of the 5,6-vinylic bond of pyrimidines uracil and thymine with electrons from NADPH. DPD has two active sites that are separated by ∼60 Å. At one site NADPH binds adjacent to an FAD cofactor and at the other pyrimidine binds proximal to an FMN. Four Fe4S4 centers span the distance between these active sites. It has recently been established that the enzyme undergoes reductive activation prior to reducing the pyrimidine. In this initial process NADPH is oxidized at the FAD site and electrons are transmitted to the FMN via the Fe4S4 centers to yield the active state with a cofactor set of FAD•4(Fe4S4)•FMNH2. The catalytic chemistry of DPD can be studied in transient-state by observation of either NADPH consumption or charge transfer absorption associated with complexation of NADPH adjacent to the FAD. Here we have utilized both sets of absorption transitions to find evidence for specific additional aspects of the DPD mechanism. Competition for binding with NADP+ indicates that the two charge transfer species observed in activation/single turnover reactions arise from NADPH populating the FAD site before and after reductive activation. An additional charge transfer species is observed to accumulate at longer times when high NADPH concentrations are mixed with the enzyme•pyrimidine complex and this data can be modelled based on asymmetry in the homodimer. It was also shown that, like pyrimidines, dihydropyrimidines induce rapid reductive activation indicating that the reduced pyrimidine formed in turnover can stimulate the reinstatement of the active state of the enzyme. Investigation of the reverse reaction revealed that dihydropyrimidines alone can reductively activate the enzyme, albeit inefficiently. In the presence of dihydropyrimidine and NADP+ DPD will form NADPH but apparently without measurable reductive activation. Pyrimidines that have 5-substituent halogens were utilized to probe both reductive activation and turnover. The linearity of the Hammett plot based on the rate of hydride transfer to the pyrimidine establishes that, at least to the radius of an iodo-group, the 5-substituent volume does not have influence on the observed kinetics of pyrimidine reduction.

Spectroscopic and Computational Observation of Glutamine Tautomerization in the Blue Light Sensing Using Flavin Domain Photoreaction.

Blue light sensing using flavin (BLUF) domains constitute a family of flavin-binding photoreceptors of bacteria and eukaryotic algae. BLUF photoactivation proceeds via a light-driven hydrogen-bond switch among flavin adenine dinucleotide (FAD) and glutamine and tyrosine side chains, whereby FAD undergoes electron and proton transfer with tyrosine and is subsequently re-oxidized by a hydrogen back-shuttle in picoseconds, constituting an important model system to understand proton-coupled electron transfer in biology. The specific structure of the hydrogen-bond patterns and the prevalence of glutamine tautomeric states in dark-adapted (DA) and light-activated (LA) states have remained controversial. Here, we present a combined femtosecond stimulated Raman spectroscopy (FSRS), computational chemistry, and site-selective isotope labeling Fourier-transform infrared spectroscopy (FTIR) study of the Slr1694 BLUF domain. FSRS showed distinct vibrational bands from the FADS1 singlet excited state. We observed small but significant shifts in the excited-state vibrational frequency patterns of the DA and LA states, indicating that these frequencies constitute a sensitive probe for the hydrogen-bond arrangement around FAD. Excited-state model calculations utilizing four different realizations of hydrogen bond patterns and glutamine tautomeric states were consistent with a BLUF reaction model that involved glutamine tautomerization to imidic acid, accompanied by a rotation of its side chain. A combined FTIR and double-isotope labeling study, with 13C labeling of FAD and 15N labeling of glutamine, identified the glutamine imidic acid C═N stretch vibration in the LA state and the Gln C═O in the DA state. Hence, our study provides support for glutamine tautomerization and side-chain rotation in the BLUF photoreaction.

Structural and Functional Landscape of FAD-Dependent Histone Lysine Demethylases for New Drug Discovery.

Small molecules targeting the flavin adenine dinucleotide (FAD)-dependent histone lysine demethylase LSD family have displayed therapeutic promise against various diseases. Nine clinical candidates targeting the classic demethylase-dependent functions of the LSD family are currently being investigated for treating cancers, neurodegenerative diseases, etc. Moreover, targeting noncatalytic functions of LSDs also represents an emerging strategy for treating human diseases. In this Perspective, we provide full structural and functional landscape of the LSD family and action modes of different types of LSD inhibitors including natural products, peptides, and synthetic compounds, aiming to reveal new druggable space for the design of new LSD inhibitors. Particularly, we first classify these inhibitors into three types based on their unique binding modes. Additionally, the strategies targeting the demethylase-independent functions of LSDs are also briefly discussed. This Perspective may benefit the discovery of new LSD inhibitors for probing LSD biology and/or treating human diseases.

Merits of Diazirine Photo-Immobilization for Target Profiling of Natural Products and Cofactors.

Finding the targets of natural products is of key importance in both chemical biology and drug discovery, and deconvolution of cofactor interactomes contributes to the functional annotation of the proteome. Identifying the proteins that underlie natural compound activity in phenotypic screens helps to validate the respective targets and, potentially, expand the druggable proteome. Here, we present a generally applicable protocol for the photoactivated immobilization of unmodified and microgram quantities of natural products on diazirine-decorated beads and their use for systematic affinity-based proteome profiling. We show that among 31 molecules of very diverse reported activity and biosynthetic origin, 25 could indeed be immobilized. Dose-response competition binding experiments using lysates of human or bacterial cells followed by quantitative mass spectrometry recapitulated targets of 9 molecules with <100 µM affinity. Among them, immobilization of coenzyme A produced a tool to interrogate proteins containing a HotDog domain. Surprisingly, immobilization of the cofactor flavin adenine dinucleotide (FAD) led to the identification of nanomolar interactions with dozens of RNA-binding proteins.

A higher plant FAD synthetase is fused to an inactivated FAD pyrophosphatase.

The riboflavin derivatives FMN and flavin adenine dinucleotide (FAD) are critical cofactors for wide-ranging biological processes across all kingdoms of life. Although it is well established that these flavins can be readily interconverted, in plants, the responsible catalysts and regulatory mechanisms remain poorly understood. Here, we report the cloning and biochemical characterization of an FAD synthetase encoded by the gene At5g03430, which we have designated AtFADS1 (A. thaliana FADS1). The catalytic properties of the FAD synthetase activity are similar to those reported for other FAD synthetases, except that we observed maximum activity with Zn2+ as the associated divalent metal cation. Like human FAD synthetase, AtFADS1 exists as an apparent fusion with an ancestral FAD pyrophosphatase, a feature that is conserved across plants. However, we detected no pyrophosphatase activity with AtFADS1, consistent with an observed loss of a key catalytic residue in higher plant evolutionary history. In contrast, we determined that algal FADS1 retains both FAD synthetase and pyrophosphatase activity. We discuss the implications, including the potential for yet-unstudied biologically relevant noncatalytic functions, and possible evolutionary pressures that have led to the loss of FAD pyrophosphatase activity, yet universal retention of an apparently nonfunctional domain in FADS of land plants.

Different phenotypic outcome due to site-specific phosphorylation in the cancer-associated NQO1 enzyme studied by phosphomimetic mutations.

Protein phosphorylation is a common phenomenon in human flavoproteins although the functional consequences of this site-specific modification are largely unknown. Here, we evaluated the effects of site-specific phosphorylation (using phosphomimetic mutations at sites S40, S82 and T128) on multiple functional aspects as well as in the structural stability of the antioxidant and disease-associated human flavoprotein NQO1 using biophysical and biochemical methods. In vitro biophysical studies revealed effects of phosphorylation at different sites such as decreased binding affinity for FAD and structural stability of its binding site (S82), conformational stability (S40 and S82) and reduced catalytic efficiency and functional cooperativity (T128). Local stability measurements by H/D exchange in different ligation states provided structural insight into these effects. Transfection of eukaryotic cells showed that phosphorylation at sites S40 and S82 may reduce steady-levels of NQO1 protein by enhanced proteasome-induced degradation. We show that site-specific phosphorylation of human NQO1 may cause pleiotropic and counterintuitive effects on this multifunctional protein with potential implications for its relationships with human disease. Our approach allows to establish relationships between site-specific phosphorylation, functional and structural stability effects in vitro and inside cells paving the way for more detailed analyses of phosphorylation at the flavoproteome scale.

Unraveling the Photoactivation Mechanism of a Light-Activated Adenylyl Cyclase Using Ultrafast Spectroscopy Coupled with Unnatural Amino Acid Mutagenesis.

The hydrogen bonding network that surrounds the flavin in blue light using flavin adenine dinucleotide (BLUF) photoreceptors plays a crucial role in sensing and communicating the changes in the electronic structure of the flavin to the protein matrix upon light absorption. Using time-resolved infrared spectroscopy (TRIR) and unnatural amino acid incorporation, we investigated the photoactivation mechanism and the role of the conserved tyrosine (Y6) in the forward reaction of the photoactivated adenylyl cyclase from Oscillatoria acuminata (OaPAC). Our work elucidates the direct connection between BLUF photoactivation and the structural and functional implications on the partner protein for the first time. The TRIR results demonstrate the formation of the neutral flavin radical as an intermediate species on the photoactivation pathway which decays to form the signaling state. Using fluorotyrosine analogues to modulate the physical properties of Y6, the TRIR data reveal that a change in the pKa and/or reduction potential of Y6 has a profound effect on the forward reaction, consistent with a mechanism involving proton transfer or proton-coupled electron transfer from Y6 to the electronically excited FAD. Decreasing the pKa from 9.9 to <7.2 and/or increasing the reduction potential by 200 mV of Y6 prevents proton transfer to the flavin and halts the photocycle at FAD•-. The lack of protonation of the anionic flavin radical can be directly linked to photoactivation of the adenylyl cyclase (AC) domain. While the 3F-Y6 and 2,3-F2Y6 variants undergo the complete photocycle and catalyze the conversion of ATP into cAMP, enzyme activity is abolished in the 3,5-F2Y6 and 2,3,5-F3Y6 variants where the photocycle is halted at FAD•-. Our results thus show that proton transfer plays an essential role in initiating the structural reorganization of the AC domain that results in AC activity.