The functions of the flavin contact residues, alphaArg249 and betaTyr16, in human electron transfer flavoprotein

A single hydrogen bond that tunes flavin redox reactivity and activates it for modification

Electron bifurcation produces high-energy products based on less energetic reagents. This feat enables biological systems to exploit abundant mediocre fuel to drive vital but demanding reactions, including nitrogen fixation and CO2 capture. Thus, there is great interest in understanding principles that can be portable to man-made devices. Bifurcating electron transfer flavoproteins (Bf ETFs) employ two flavins with contrasting reactivities to acquire pairs of electrons from a modest reductant, NADH. The bifurcating flavin then dispatches the electrons individually to a high and a low reduction midpoint potential (E°) acceptor, the latter of which captures most of the energy. Maximum efficiency requires that only one electron accesses the exergonic path that will 'pay for' the production of the low-E° product. It is therefore critical that one of the flavins, the 'electron transfer' (ET) flavin, is tuned to execute single-electron (1e-) chemistry only. To learn how, and extract fundamental principles, we systematically altered interactions with the ET-flavin O2 position. Removal of a single hydrogen bond (H-bond) disfavored the formation of the flavin anionic semiquinone (ASQ) relative to the oxidized (OX) state, lowering by 150 mV and retuning the flavin's tendency for 1e-vs. 2e- reactivity. This was achieved by replacing conserved His 290 with Phe, while also replacing the supporting Tyr 279 with Ile. Although this variant binds oxidized FADs at 90% the WT level, the ASQ state of the ET-flavin is not stable in the absence of H290's H-bond, and dissociates, in contrast to the WT. Removal of this H-bond also altered the ET-flavin's covalent chemistry. While the WT ETF accumulates modified flavins whose formation is believed to rely on an anionic paraquinone methide intermediate, the FADs of the H-bond lacking variant remain unchanged over weeks. Hence the variant that destabilizes the anionic semiquinone also suppresses the anionic intermediate in flavin modification, verifying electronic similarities between these two species. These correlations suggest that the H-bond that stabilizes the crucial flavin ASQ also promotes flavin modification. The two effects may indeed be inseparable, as a Jekyll and Hydrogen bond.

Non-covalent interactions that tune the reactivities of the flavins in bifurcating electron transferring flavoprotein

Bifurcating electron transferring flavoproteins (Bf-ETFs) tune chemically identical flavins to two contrasting roles. To understand how, we used hybrid quantum mechanical molecular mechanical calculations to characterize non-covalent interactions applied to each flavin by the protein. Our computations replicated the differences between the reactivities of the flavins: the electron transferring flavin (^ETflavin) was calculated to stabilize anionic semiquinone (ASQ) as needed to execute its single-electron transfers, whereas the Bf flavin (^Bfflavin) was found to disfavor the ASQ state more than does free flavin and to be less susceptible to reduction. The stability of ^ETflavin ASQ was attributed in part to H-bond donation to the flavin O2 from a nearby His side chain, via comparison of models employing different tautomers of His. This H-bond between O2 and the ET site was uniquely strong in the ASQ state, whereas reduction of ^ETflavin to the anionic hydroquinone (AHQ) was associated with side chain reorientation, backbone displacement and reorganization of its H-bond network including a Tyr from the other domain and subunit of the ETF. The Bf site was less responsive overall, but formation of the ^Bfflavin AHQ allowed a nearby Arg side chain to adopt an alternative rotamer that can H-bond to the ^Bfflavin O4. This would stabilize the anionic ^Bfflavin and rationalize effects of mutation at this position. Thus, our computations provide insights on states and conformations that have not been possible to characterize experimentally, offering explanations for observed residue conservation and raising possibilities that can now be tested.

Unusual reactivity of a flavin in a bifurcating electron-transferring flavoprotein leads to flavin modification and a charge-transfer complex

From the outset, canonical electron transferring flavoproteins (ETFs) earned a reputation for containing modified flavin. We now show that modification occurs in the recently recognized bifurcating (Bf) ETFs as well. In Bf ETFs, the 'electron transfer' (ET) flavin mediates single electron transfer via a stable anionic semiquinone state, akin to the FAD of canonical ETFs, whereas a second flavin mediates bifurcation (the Bf FAD). We demonstrate that the ET FAD undergoes transformation to two different modified flavins by a sequence of protein-catalyzed reactions that occurs specifically in the ET site, when the enzyme is maintained at pH 9 in an amine-based buffer. Our optical and mass spectrometric characterizations identify 8-formyl flavin early in the process and 8-amino flavins (8AFs) at later times. The latter have not previously been documented in an ETF to our knowledge. Mass spectrometry of flavin products formed in Tris or bis-tris-aminopropane solutions demonstrates that the source of the amine adduct is the buffer. Stepwise reduction of the 8AF demonstrates that it can explain a charge transfer band observed near 726 nm in Bf ETF, as a complex involving the hydroquinone state of the 8AF in the ET site with the oxidized state of unmodified flavin in the Bf site. This supports the possibility that Bf ETF can populate a conformation enabling direct electron transfer between its two flavins, as has been proposed for cofactors brought together in complexes between ETF and its partner proteins.

Contrasting roles for two conserved arginines: stabilizing flavin semiquinone or quaternary structure, in bifurcating electron transfer flavoproteins

Bifurcating electron transfer flavoproteins (Bf ETFs) are important redox enzymes that contain two flavin adenine dinucleotide (FAD) cofactors, with contrasting reactivities and complementary roles in electron bifurcation. However, for both the “electron transfer” (ET) and the “bifurcating” (Bf) FADs, the only charged amino acid within 5 Å of the flavin is a conserved arginine (Arg) residue. To understand how the two sites produce different reactivities utilizing the same residue, we investigated the consequences of replacing each of the Arg residues with lysine, glutamine, histidine, or alanine. We show that absence of a positive charge in the ET site diminishes accumulation of the anionic semiquinone (ASQ) that enables the ET flavin to act as a single electron carrier, due to depression of the oxidized versus. ASQ reduction midpoint potential, E°_OX/ASQ. Perturbation of the ET site also affected the remote Bf site, whereas abrogation of Bf FAD binding accelerated chemical modification of the ET flavin. In the Bf site, removal of the positive charge impaired binding of FAD or AMP, resulting in unstable protein. Based on pH dependence, we propose that the Bf site Arg interacts with the phosphate(s) of Bf FAD or AMP, bridging the domain interface via a conserved peptide loop (“zipper”) and favoring nucleotide binding. We further propose a model that rationalizes conservation of the Bf site Arg even in non-Bf ETFs, as well as AMP's stabilizing role in the latter, and provides a mechanism for coupling Bf flavin redox changes to domain-scale motion.

Flavins in the electron bifurcation process

The discovery of a new energy-coupling mechanism termed flavin-based electron bifurcation (FBEB) in 2008 revealed a novel field of application for flavins in biology. The key component is the bifurcating flavin endowed with strongly inverted one-electron reduction potentials (FAD/FAD•^- ≪ FAD•^-/FADH^-) that cooperatively transfers in its reduced state one low and one high-energy electron into different directions and thereby drives an endergonic with an exergonic reduction reaction. As energy splitting at the bifurcating flavin apparently implicates one-electron chemistry, the FBEB machinery has to incorporate prior to and behind the central bifurcating flavin 2e-to-1e and 1e-to-2e switches, frequently also flavins, for oxidizing variable medium-potential two-electron donating substrates and for reducing high-potential two-electron accepting substrates. The one-electron carriers ferredoxin or flavodoxin serve as low-potential (high-energy) electron acceptors, which power endergonic processes almost exclusively in obligate anaerobic microorganisms to increase the efficiency of their energy metabolism. In this review, we outline the global organization of FBEB enzymes, the functions of the flavins therein and the surrounding of the isoalloxazine rings by which their reduction potentials are specifically adjusted in a finely tuned energy landscape.

Closing the gap: yeast electron-transferring flavoprotein links the oxidation of d-lactate and d-α-hydroxyglutarate to energy production via the respiratory chain

Electron‐transferring flavoproteins (ETFs) have been found in all kingdoms of life, mostly assisting in shuttling electrons to the respiratory chain for ATP production. While the human (h) ETF has been studied in great detail, very little is known about the biochemical properties of the homologous protein in the model organism Saccharomyces cerevisiae (yETF). In view of the absence of client dehydrogenases, for example, the acyl‐CoA dehydrogenases involved in the β‐oxidation of fatty acids, d‐lactate dehydrogenase 2 (Dld2) appeared to be the only relevant enzyme that is serviced by yETF for electron transfer to the mitochondrial electron transport chain. However, this hypothesis was never tested experimentally. Here, we report the biochemical properties of yETF and Dld2 as well as the electron transfer reaction between the two proteins. Our study revealed that Dld2 oxidizes d‐α‐hydroxyglutarate more efficiently than d‐lactate exhibiting k_catapp/K_Mapp values of 1200 ± 300 m⁻¹·s⁻¹ and 11 ± 2 m⁻¹·s⁻¹, respectively. As expected, substrate‐reduced Dld2 very slowly reacted with oxygen or the artificial electron acceptor 2,6‐dichlorophenol indophenol. However, photoreduced Dld2 was rapidly reoxidized by oxygen, suggesting that the reaction products, that is, α‐ketoglutarate and pyruvate, ‘lock’ the reduced enzyme in an unreactive state. Interestingly, however, we could demonstrate that substrate‐reduced Dld2 rapidly transfers electrons to yETF. Therefore, we conclude that the formation of a product‐reduced Dld2 complex suppresses electron transfer to dioxygen but favors the rapid reduction in yETF, thus preventing the loss of electrons and the generation of reactive oxygen species.

Oxidation of the FAD cofactor to the 8-formyl-derivative in human electron-transferring flavoprotein

The heterodimeric human (h) electron-transferring flavoprotein (ETF) transfers electrons from at least 13 different flavin dehydrogenases to the mitochondrial respiratory chain through a non-covalently bound FAD cofactor. Here, we describe the discovery of an irreversible and pH-dependent oxidation of the 8α-methyl group to 8-formyl-FAD (8f-FAD), which represents a unique chemical modification of a flavin cofactor in the human flavoproteome. Furthermore, a set of hETF variants revealed that several conserved amino acid residues in the FAD-binding pocket of electron-transferring flavoproteins are required for the conversion to the formyl group. Two of the variants generated in our study, namely αR249C and αT266M, cause glutaric aciduria type II, a severe inherited disease. Both of the variants showed impaired formation of 8f-FAD shedding new light on the potential molecular cause of disease development. Interestingly, the conversion of FAD to 8f-FAD yields a very stable flavin semiquinone that exhibited slightly lower rates of electron transfer in an artificial assay system than hETF containing FAD. In contrast, the formation of 8f-FAD enhanced the affinity to human dimethylglycine dehydrogenase 5-fold, indicating that formation of 8f-FAD modulates the interaction of hETF with client enzymes in the mitochondrial matrix. Thus, we hypothesize that the FAD cofactor bound to hETF is subject to oxidation in the alkaline (pH 8) environment of the mitochondrial matrix, which may modulate electron transport between client dehydrogenases and the respiratory chain. This discovery challenges the current concepts of electron transfer processes in mitochondria.

Distinct properties underlie flavin-based electron bifurcation in a novel electron transfer flavoprotein FixAB from Rhodopseudomonas palustris

A newly recognized third fundamental mechanism of energy conservation in biology, electron bifurcation, uses free energy from exergonic redox reactions to drive endergonic redox reactions. Flavin-based electron bifurcation furnishes low-potential electrons to demanding chemical reactions, such as reduction of dinitrogen to ammonia. We employed the heterodimeric flavoenzyme FixAB from the diazotrophic bacterium Rhodopseudomonas palustris to elucidate unique properties that underpin flavin-based electron bifurcation. FixAB is distinguished from canonical electron transfer flavoproteins (ETFs) by a second FAD that replaces the AMP of canonical ETF. We exploited near-UV-visible CD spectroscopy to resolve signals from the different flavin sites in FixAB and to interrogate the putative bifurcating FAD. CD aided in assigning the measured reduction midpoint potentials (E° values) to individual flavins, and the E° values tested the accepted model regarding the redox properties required for bifurcation. We found that the higher-E° flavin displays sequential one-electron (1-e^-) reductions to anionic semiquinone and then to hydroquinone, consistent with the reactivity seen in canonical ETFs. In contrast, the lower-E° flavin displayed a single two-electron (2-e^-) reduction without detectable accumulation of semiquinone, consistent with unstable semiquinone states, as required for bifurcation. This is the first demonstration that a FixAB protein possesses the thermodynamic prerequisites for bifurcating activity, and the separation of distinct optical signatures for the two flavins lays a foundation for mechanistic studies to learn how electron flow can be directed in a protein environment. We propose that a novel optical signal observed at long wavelength may reflect electron delocalization between the two flavins.

Glutaryl-coenzyme A dehydrogenase from Geobacter metallireducens - interaction with electron transferring flavoprotein and kinetic basis of unidirectional catalysis

Glutaryl-CoA dehydrogenases (GDHs) are FAD containing acyl-CoA dehydrogenases that usually catalyze the dehydrogenation and decarboxylation of glutaryl-CoA to crotonyl-CoA with an electron transferring flavoprotein (ETF) acting as natural electron acceptor. In anaerobic bacteria, GDHs play an important role in the benzoyl-CoA degradation pathway of monocyclic aromatic compounds. In the present study, we identified, purified and characterized the benzoate-induced BamOP as the electron accepting ETF of GDH (BamM) from the Fe(III)-respiring Geobacter metallireducens. The BamOP heterodimer contained FAD and AMP as cofactors. In the absence of an artificial electron acceptor, at pH values above 8, the BamMOP-components catalyzed the expected glutaryl-CoA oxidation to crotonyl-CoA and CO2 ; however, at pH values below 7, the redox-neutral glutaryl-CoA conversion to butyryl-CoA and CO2 became the dominant reaction. This previously unknown, strictly ETF-dependent coupled glutaryl-CoA oxidation/crotonyl-CoA reduction activity was facilitated by an unexpected two-electron transfer between FAD(BamM) and FAD(BamOP) , as well as by the similar redox potentials of the two FAD cofactors in the substrate-bound state. The strict order of electron/proton transfer and C-C-cleavage events including transient charge-transfer complexes did not allow an energetic coupling of electron transfer and decarboxylation. This explains why it was difficult to release the glutaconyl-CoA intermediate from reduced GDH. Moreover, it provides a kinetic rational for the apparent inability of BamM to catalyze the reverse reductive crotonyl-CoA carboxylation, even under thermodynamically favourable conditions. For this reason reductive crotonyl-CoA carboxylation, a key reaction in C2-assimilation via the ethylmalonyl-CoA pathway, is accomplished by a different crotonyl-CoA carboxylase/reductase via a covalent NADPH/ene-adduct.

Decomposition of the fluorescence spectra of two FAD molecules in electron-transferring flavoprotein from Megasphaera elsdenii

Electron-transferring flavoprotein (ETF) from Megasphaera elsdenii contains two FAD molecules, FAD-1 and FAD-2. FAD-2 shows an unusual absorption spectrum with a 400-nm peak. In contrast, ETFs from other sources such as pig contain one FAD and one AMP with the FAD showing a typical flavin absorption spectrum with 380- and 440-nm peaks. It is presumed that FAD-2 is the counterpart of the FAD in other ETFs. In this study, the FAD-1 and FAD-2 fluorescence spectra were determined by titration of FAD-1-bound ETF with FAD using excitation-emission matrix (EEM) fluorescence spectroscopy. The EEM data were globally analysed, and the FAD fluorescence spectra were calculated from the principal components using their respective absorption spectra. The FAD-2 fluorescence spectrum was different from that of pig ETF, which is more intense and blue-shifted. AMP-free pig ETF in acidic solution, which has a comparable absorption spectrum to FAD-2, also had a similar fluorescence spectrum. This result suggests that FAD-2 in M. elsdenii ETF and the FAD in acidic AMP-free pig ETF share a common microenvironment. A review of published ETF fluorescence spectra led to the speculation that the majority of ETF molecules in solution are in the conformation depicted by the crystal structure.

Interaction between NADH and electron-transferring flavoprotein from Megasphaera elsdenii

Electron-transferring flavoprotein (ETF) from the anaerobic bacterium Megasphaera elsdenii is a heterodimer containing two FAD cofactors. Isolated ETF contains only one FAD molecule, FAD-1, because the other, FAD-2, is lost during purification. FAD-2 is recovered by adding FAD to the isolated ETF. The two FAD molecules in holoETF were characterized using NADH. Spectrophotometric titration of isolated ETF with NADH showed a two-electron reduction of FAD-1 according to a monophasic profile indicating that FAD-1 receives electrons from NADH without involvement of FAD-2. When holoETF was titrated with NADH, FAD-2 was reduced to an anionic semiquinone and then was fully reduced before the reduction of FAD-1. The midpoint potential values at pH 7 were +81, -136 and -279 mV for the reduction of oxidized FAD-2 to semiquinone, semiquinone to the fully reduced FAD-2 and the two-electron reduction of FAD-1, respectively. Both FAD-1 and FAD-2 in holoETF were reduced by excess NADH very rapidly. The reduction of FAD-2 was slowed by replacement of FAD-1 with 8-cyano-FAD indicating that FAD-2 receives electrons from FAD-1 but not from NADH directly. The present results suggest that FAD-2 is the counterpart of the FAD in human ETF, which contains one FAD and one AMP.

Mechanism of superoxide and hydrogen peroxide generation by human electron-transfer flavoprotein and pathological variants

Reactive oxygen species production by mitochondrial enzymes plays a fundamental role both in cellular signaling and in the progression of dysfunctional states. However, sources of reactive oxygen species and the mechanisms by which enzymes produce these reactive species still remain elusive. We characterized the generation of reactive oxygen species by purified human electron-transfer flavoprotein (ETF), a mitochondrial enzyme that has a central role in the metabolism of lipids, amino acids, and choline. The results showed that ETF produces significant amounts of both superoxide and hydrogen peroxide in the presence of its partner enzyme medium-chain acyl-CoA dehydrogenase (MCAD). ETF-mediated production of reactive oxygen species is partially inhibited at high MCAD/ETF ratios, whereas it is enhanced at high ionic strength. Determination of the reduction potentials of ETF showed that thermodynamic properties of the FAD cofactor are changed upon formation of a complex between ETF and MCAD, supporting the notion that protein:protein interactions modulate the reactivity of the protein with dioxygen. Two pathogenic ETF variants were also studied to determine which factors modulate the reactivity toward molecular oxygen and promote reactive oxygen species production. The results obtained show that destabilized conformations and defective protein:protein interactions increase the ability of ETF to generate reactive oxygen species. A possible role for these processes in mitochondrial dysfunction in metabolic disorders of fatty acid β-oxidation is discussed.

A second FMN binding site in yeast NADPH-cytochrome P450 reductase suggests a mechanism of electron transfer by diflavin reductases

NADPH-cytochrome P450 reductase transfers two reducing equivalents derived from a hydride ion of NADPH via FAD and FMN to the large family of microsomal cytochrome P450 monooxygenases in one-electron transfer steps. The mechanism of electron transfer by diflavin reductases remains elusive and controversial. Here, we determined the crystal structure of truncated yeast NADPH-cytochrome P450 reductase, which is functionally active toward its physiological substrate cytochrome P450, and discovered a second FMN binding site at the interface of the connecting and FMN binding domains. The two FMN binding sites have different accessibilities to the bulk solvent and different amino acid environments, suggesting stabilization of different electronic structures of the reduced flavin. Since only one FMN cofactor is required for function, a hypothetical mechanism of electron transfer is discussed that proposes shuttling of a single FMN between these two sites coupled with the transition between two semiquinone forms, neutral (blue) and anionic (red).

Extensive Domain Motion and Electron Transfer in the Human Electron Transferring Flavoprotein·Medium Chain Acyl-CoA Dehydrogenase Complex

The crystal structure of the human electron transferring flavoprotein (ETF).medium chain acyl-CoA dehydrogenase (MCAD) complex reveals a dual mode of protein-protein interaction, imparting both specificity and promiscuity in the interaction of ETF with a range of structurally distinct primary dehydrogenases. ETF partitions the functions of partner binding and electron transfer between (i) the recognition loop, which acts as a static anchor at the ETF.MCAD interface, and (ii) the highly mobile redox active FAD domain. Together, these enable the FAD domain of ETF to sample a range of conformations, some compatible with fast interprotein electron transfer. Disorders in amino acid or fatty acid catabolism can be attributed to mutations at the protein-protein interface. Crucially, complex formation triggers mobility of the FAD domain, an induced disorder that contrasts with general models of protein-protein interaction by induced fit mechanisms. The subsequent interfacial motion in the MCAD.ETF complex is the basis for the interaction of ETF with structurally diverse protein partners. Solution studies using ETF and MCAD with mutations at the protein-protein interface support this dynamic model and indicate ionic interactions between MCAD Glu(212) and ETF Arg alpha(249) are likely to transiently stabilize productive conformations of the FAD domain leading to enhanced electron transfer rates between both partners.

Hydrogen-bonding dynamics of free flavins in benzene and FAD in electron-transferring flavoprotein upon excitation

The dynamic natures of two hydrogen-bonding model systems, riboflavin tetrabutylate (RFTB)-trichloroacetic acid (TCA) and RFTB-phenol in benzene, and of electron-transferring flavoprotein (ETF) from pig kidney upon excitation of flavins was investigated by means of steady state and time-resolved fluorescence spectroscopy. In both model systems fluorescence intensities of RFTB decreased as TCA or phenol was added. The spectral characteristics of ETF under steady state excitation were quite similar to those of the RFTB-TCA system, but not to those of the RFTB-phenol system. The observed fluorescence decay curves of ETF fit well with the calculated decay curves with two lifetime components, as in the model systems. Averaged lifetime was 0.9 ns. The time-resolved fluorescence spectrum of ETF shifted toward longer wavelength with time after pulsed excitation, which was also observed in the RFTB-TCA system. In the RFTB-phenol system the emission spectrum did not shift at all with time. These results reveal that the dynamic nature of ETF can be ascribed to aliphatic hydrogen-bonding(s) of the isoalloxazine ring with surrounding amino acid(s). From the fluorescence characteristics of ETF in comparison with the model systems, human ETF and other flavoproteins, it was suggested that ETF from pig kidney does not contain Tyr-16 in the beta subunit, unlike human ETF.

Molecular study of electron transfer flavoprotein alpha-subunit deficiency in two Japanese children with different phenotypes of glutaric acidemia type II

BACKGROUND

Electron transfer flavoprotein is a mitochondrial matrix protein composed of alpha- and beta-subunits (ETF alpha and ETF beta, respectively). This protein transfers electrons between several mitochondrial dehydrogenases and the main respiratory chain via ETF dehydrogenase (ETF-DH). Defects in ETF or ETF-DH cause glutaric acidemias type II (GAII).

MATERIALS AND METHODS

We investigated the molecular basis of ETF alpha deficiency in two Japanese children with different clinical phenotypes using expression study.

RESULTS

Patient 1 had the severe form of GAII, a compound heterozygote of two mutations: 799G to A (alpha G267R) and nonsense 7C to T (alpha R3X). Patient 2 had the mild form and carried two heterozygous mutations: 764G to T (alpha G255V) and 478delG (frameshift). Both patients had one each of missense mutations in one allele; the others were either nonsense or truncated. Restriction enzyme digestion assay using genomic DNAs from 100 healthy Japanese revealed that these mutations were all novel. No signal for ETF alpha was detected by immunoblotting in cases of missense mutants, while wild-type cDNA resulted in expression of ETF alpha protein. Transfection with wild-type ETF alpha cDNA into cultured cells from both patients elevated incorporation of radioisotope-labelled fatty acids.

CONCLUSION

These four mutations were pathogenic for GAII and missense mutations, alpha G255V and alpha G267R were considered anecdotal for mild and severe forms, respectively.

Protein dynamics enhance electronic coupling in electron transfer complexes

Electron-transferring flavoproteins (ETFs) from human and Paracoccus denitrificans have been analyzed by small angle x-ray scattering, showing that neither molecule exists in a rigid conformation in solution. Both ETFs sample a range of conformations corresponding to a large rotation of domain II with respect to domains I and III. A model of the human ETF.medium chain acyl-CoA dehydrogenase complex, consistent with x-ray scattering data, indicates that optimal electron transfer requires domain II of ETF to rotate by approximately 30 to 50 degrees toward domain I relative to its position in the x-ray structure. Domain motion establishes a new "robust engineering principle" for electron transfer complexes, tolerating multiple configurations of the complex while retaining efficient electron transfer.

alpha Arg-237 in Methylophilus methylotrophus (sp. W3A1) electron-transferring flavoprotein affords approximately 200-millivolt stabilization of the FAD anionic semiquinone and a kinetic block on full reduction to the dihydroquinone

The midpoint reduction potentials of the FAD cofactor in wild-type Methylophilus methylotrophus (sp. W3A1) electron-transferring flavoprotein (ETF) and the alphaR237A mutant were determined by anaerobic redox titration. The FAD reduction potential of the oxidized-semiquinone couple in wild-type ETF (E'(1)) is +153 +/- 2 mV, indicating exceptional stabilization of the flavin anionic semiquinone species. Conversion to the dihydroquinone is incomplete (E'(2) < -250 mV), because of the presence of both kinetic and thermodynamic blocks on full reduction of the FAD. A structural model of ETF (Chohan, K. K., Scrutton, N. S., and Sutcliffe, M. J. (1998) Protein Pept. Lett. 5, 231-236) suggests that the guanidinium group of Arg-237, which is located over the si face of the flavin isoalloxazine ring, plays a key role in the exceptional stabilization of the anionic semiquinone in wild-type ETF. The major effect of exchanging alphaArg-237 for Ala in M. methylotrophus ETF is to engineer a remarkable approximately 200-mV destabilization of the flavin anionic semiquinone (E'(2) = -31 +/- 2 mV, and E'(1) = -43 +/- 2 mV). In addition, reduction to the FAD dihydroquinone in alphaR237A ETF is relatively facile, indicating that the kinetic block seen in wild-type ETF is substantially removed in the alphaR237A ETF. Thus, kinetic (as well as thermodynamic) considerations are important in populating the redox forms of the protein-bound flavin. Additionally, we show that electron transfer from trimethylamine dehydrogenase to alphaR237A ETF is severely compromised, because of impaired assembly of the electron transfer complex.