SciCombinator

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Concept: Riboflavin

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ABSTRACT Shewanella oneidensis strain MR-1 is widely studied for its ability to respire a diverse array of soluble and insoluble electron acceptors. The ability to breathe insoluble substrates is defined as extracellular electron transfer and can occur via direct contact or by electron shuttling in S. oneidensis. To determine the contribution of flavin electron shuttles in extracellular electron transfer, a transposon mutagenesis screen was performed with S. oneidensis to identify mutants unable to secrete flavins. A multidrug and toxin efflux transporter encoded by SO_0702 was identified and renamed bfe (bacterial flavin adenine dinucleotide [FAD] exporter) based on phenotypic characterization. Deletion of bfe resulted in a severe decrease in extracellular flavins, while overexpression of bfe increased the concentration of extracellular flavins. Strains lacking bfe had no defect in reduction of soluble Fe(III), but these strains were deficient in the rate of insoluble Fe(III) oxide reduction, which was alleviated by the addition of exogenous flavins. To test a different insoluble electron acceptor, graphite electrode bioreactors were set up to measure current produced by wild-type S. oneidensis and the Δbfe mutant. With the same concentration of supplemented flavins, the two strains produced similar amounts of current. However, when exogenous flavins were not supplemented to bioreactors, bfe mutant strains produced significantly less current than the wild type. We have demonstrated that flavin electron shuttling accounts for ~75% of extracellular electron transfer to insoluble substrates by S. oneidensis and have identified the first FAD transporter in bacteria. IMPORTANCE Extracellular electron transfer by microbes is critical for the geochemical cycling of metals, bioremediation, and biocatalysis using electrodes. A controversy in the field was addressed by demonstrating that flavin electron shuttling, not direct electron transfer or nanowires, is the primary mechanism of extracellular electron transfer employed by the bacterium Shewanella oneidensis. We have identified a flavin adenine dinucleotide transporter conserved in all sequenced Shewanella species that facilitates export of flavin electron shuttles in S. oneidensis. Analysis of a strain that is unable to secrete flavins demonstrated that electron shuttling accounts for ~75% of the insoluble extracellular electron transfer capacity in S. oneidensis.

Concepts: Bacteria, Adenosine triphosphate, Redox, Electrochemistry, Adenine, Electron acceptor, FAD, Riboflavin

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The cryptochrome (CRY) flavoproteins act as blue-light receptors in plants and insects, but perform light-independent functions at the core of the mammalian circadian clock. To drive clock oscillations, mammalian CRYs associate with the Period proteins (PERs) and together inhibit the transcription of their own genes. The SCF(FBXL3) ubiquitin ligase complex controls this negative feedback loop by promoting CRY ubiquitination and degradation. However, the molecular mechanisms of their interactions and the functional role of flavin adenine dinucleotide (FAD) binding in CRYs remain poorly understood. Here we report crystal structures of mammalian CRY2 in its apo, FAD-bound and FBXL3-SKP1-complexed forms. Distinct from other cryptochromes of known structures, mammalian CRY2 binds FAD dynamically with an open cofactor pocket. Notably, the F-box protein FBXL3 captures CRY2 by simultaneously occupying its FAD-binding pocket with a conserved carboxy-terminal tail and burying its PER-binding interface. This novel F-box-protein-substrate bipartite interaction is susceptible to disruption by both FAD and PERs, suggesting a new avenue for pharmacological targeting of the complex and a multifaceted regulatory mechanism of CRY ubiquitination.

Concepts: Protein, Adenine, Feedback, Negative feedback, Circadian rhythm, FAD, Riboflavin, Cryptochrome

27

The preparation of flavin mononucleotide (FMN) and FMN analogs from their corresponding riboflavin precursors is traditionally performed in a two-step procedure. After initial enzymatic conversion of riboflavin to flavin adenine dinucleotide (FAD) by a bifunctional FAD synthetase, the adenyl moiety of FAD is hydrolyzed with snake venom phosphodiesterase to yield FMN. To simplify the protocol, we have engineered the FAD synthetase from Corynebacterium ammoniagenes by deleting its N-terminal adenylation domain. The newly created biocatalyst is stable and efficient for direct and quantitative phosphorylation of riboflavin and riboflavin analogs to their corresponding FMN cofactors at preparative-scale.

Concepts: Adenosine triphosphate, Enzyme, Oxidative phosphorylation, Adenine, FAD, Riboflavin, Flavin mononucleotide, Cofactors

27

Riboflavin, or vitamin B2, as a precursor of the coenzymes FAD and FMN, has an indirect influence on many metabolic processes and determines the proper functioning of several systems, including the immune system. In the human population, plasma riboflavin concentration varies from 3·1 nm (in a moderate deficiency, e.g. in pregnant women) to 10·4 nm (in healthy adults) and 300 nm (in cases of riboflavin supplementation). The purpose of the present study was to investigate the effects of riboflavin concentration on the activity and viability of macrophages, i.e. on one of the immunocompetent cell populations. The study was performed on the murine monocyte/macrophage RAW 264.7 cell line cultured in medium with various riboflavin concentrations (3·1, 10·4, 300 and 531 nm). The results show that riboflavin deprivation has negative effects on both the activity and viability of macrophages and reduces their ability to generate an immune response. Signs of riboflavin deficiency developed in RAW 264.7 cells within 4 d of culture in the medium with a low riboflavin concentration (3·1 nm). In particular, the low riboflavin content reduced the proliferation rate and enhanced apoptotic cell death connected with the release of lactate dehydrogenase. The riboflavin deprivation impaired cell adhesion, completely inhibited the respiratory burst and slightly impaired phagocytosis of the zymosan particles. In conclusion, macrophages are sensitive to riboflavin deficiency; thus, a low riboflavin intake in the diet may affect the immune system and may consequently decrease proper host immune defence.

Concepts: Immune system, White blood cell, Antibody, Phagocytosis, Macrophage, Apoptosis, Riboflavin

26

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, 3F, 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 13C-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 13C nuclei, our experiments were performed on proteins reconstituted with specifically 13C-labeled flavins. Two potential sources for photo-CIDNP can be identified: The photo-generated triplet state, 3F, and the triplet radical pair 3(F-•W+•), formed by electron abstraction of 3F 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 3F contribute to photo-CIDNP, whereas at high magnetic fields, the calculated enhancement factors of 3F 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.

Concepts: Protein, Electron, Magnetic field, Riboflavin, Diradical, Flavin mononucleotide, Cofactors, Flavoprotein

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Glucose dehydrogenases have been highly promoted to high-accuracy blood glucose (BG) monitors. The flavin adenine dinucleotide glucose dehydrogenase (FAD-GDH) and mutant variant of quinoprotein glucose dehydorgenase (Mut. Q-GDH) are widely used in high-performance BG monitors for multi-patient use. Therefore we conducted accuracy evaluation of the GDH monitors, FAD-GDH-based GM700 and Mut. Q-GDH-based Performa.

Concepts: Adenosine triphosphate, Blood sugar, Adenine, Blood glucose monitoring, Alcohol dehydrogenase, Dehydrogenase, FAD, Riboflavin

24

The pileus of Mycena chlorophos actively, spontaneously, and continuously emits green light. Molecular mechanisms underlying this bioluminescence remain unclear. We investigated light emitters in the pileus of M. chlorophos to determine the underlying mechanisms. High-performance liquid chromatography-fluorescence-photodiode array-mass detection analyses showed that actively luminescent gills in the pileus exclusively and abundantly possessed riboflavin, riboflavin 5'-monophosphate, and flavin adenine dinucleotide as green-fluorescent components. These components were localized in the bioluminescent region of the gills at the microscopic level. Fluorescence spectra of these green-fluorescent components and the gills were identical with the spectrum of gill bioluminescence (maximum emission wavelength, 525 nm). Thus, our results indicated that the possible light emitters in the pileus of M. chlorophos were riboflavin, riboflavin 5'-monophosphate, and/or flavin adenine dinucleotide. Copyright © 2016 John Wiley & Sons, Ltd.

Concepts: Fluorescence, Adenosine triphosphate, Light, Adenine, Luminescence, Bioluminescence, FAD, Riboflavin

2

Ubiquinone (also known as coenzyme Q) is a ubiquitous lipid-soluble redox cofactor that is an essential component of electron transfer chains. Eleven genes have been implicated in bacterial ubiquinone biosynthesis, including ubiX and ubiD, which are responsible for decarboxylation of the 3-octaprenyl-4-hydroxybenzoate precursor. Despite structural and biochemical characterization of UbiX as a flavin mononucleotide (FMN)-binding protein, no decarboxylase activity has been detected. Here we report that UbiX produces a novel flavin-derived cofactor required for the decarboxylase activity of UbiD. UbiX acts as a flavin prenyltransferase, linking a dimethylallyl moiety to the flavin N5 and C6 atoms. This adds a fourth non-aromatic ring to the flavin isoalloxazine group. In contrast to other prenyltransferases, UbiX is metal-independent and requires dimethylallyl-monophosphate as substrate. Kinetic crystallography reveals that the prenyltransferase mechanism of UbiX resembles that of the terpene synthases. The active site environment is dominated by π systems, which assist phosphate-C1' bond breakage following FMN reduction, leading to formation of the N5-C1' bond. UbiX then acts as a chaperone for adduct reorientation, via transient carbocation species, leading ultimately to formation of the dimethylallyl C3'-C6 bond. Our findings establish the mechanism for formation of a new flavin-derived cofactor, extending both flavin and terpenoid biochemical repertoires.

Concepts: Archaea, Amino acid, Metabolism, Hydrogen, Glycolysis, Coenzyme Q10, FAD, Riboflavin

1

Heme cytotoxicity is minimized by a two-step catabolic reaction that generates biliverdin (BV) and bilirubin (BR) tetrapyrroles. The second step is regulated by two non-redundant biliverdin reductases (IXα[BLVRA] and IXβ [BLVRB]), which retain isomeric specificity and NAD(P)H-dependent redox coupling linked to BR’s antioxidant function.  Defective BLVRB enzymatic activity with antioxidant mishandling has been implicated in metabolic consequences of hematopoietic lineage fate and enhanced platelet counts in humans.  We now outline an integrated platform of in silico and crystallographic studies for the identification of an initial class of compounds inhibiting BLVRB with potencies in the nanomolar range.  We found that the most potent BLVRB inhibitors contain a tricyclic hydrocarbon core structure similar to the isoalloxazine ring of flavin mononucleotide and that both xanthene- and acridine-based compounds inhibit BLVRB’s flavin and dichlorophenolindophenol (DCPIP) reductase functions.  Crystallographic studies of ternary complexes with BLVRB/NADP+/xanthene-based compounds confirmed inhibitor binding adjacent to the cofactor nicotinamide and interactions with the Ser-111 side chain. This residue previously has been identified as critical for maintaining the enzymatic active site and cellular reductase functions in hematopoietic cells.  Both acridine- and xanthene-based compounds caused selective and concentration-dependent loss of redox coupling in BLVRB-overexpressing promyelocytic HL-60 cells.  These results provide promising chemical scaffolds for the development of enhanced BLVRB inhibitors and identify chemical probes to better dissect the role of biliverdins, alternative substrates, and BLVRB function in physiologically relevant cellular contexts.

Concepts: Metabolism, Enzyme, Enzyme inhibitor, Inhibitor, Vitamin C, Xanthine oxidase inhibitor, Riboflavin, Flavin mononucleotide

1

Recent advances in bioorthogonal catalysis promise to deliver new chemical tools for performing chemoselective transformations in complex biological environments. Herein we report how FAD (flavin adenine dinucleotide), FMN (flavin mononucleotide) and four flavoproteins behave as unconventional photocatalysts capable of converting PtIV and RuII complexes into potentially toxic PtII or RuII-OH2 species. Using electron donors and low doses of visible light, the flavoproteins mini Singlet Oxygen Generator (miniSOG) and NADH oxidase (NOX) catalytically activate PtIV prodrugs with bioorthogonal selectivity. In the presence of NADH, NOX catalyzes PtIV activation in the dark as well, indicating for the first time that flavoenzymes may contribute to initiate the activity of PtIV chemotherapeutic agents.

Concepts: Oxygen, Adenosine triphosphate, Light, Oxidative phosphorylation, Adenine, FAD, Riboflavin, Flavin mononucleotide