Mitochondrial localization of human FAD synthetase isoform 1

Supplying LSD1 with FAD in pancreatic cancer: A matter of protein-protein interaction?

Lysine-specific demethylase 1 (LSD1) is a key regulator in cancer epigenetic, and its activity is reliant on flavin adenine dinucleotide (FAD) as a cofactor. In this study, we investigated the correlation between LSD1 and FAD synthase isoform 2 (FADS2) protein levels in pancreatic ductal adenocarcinoma (PDAC) cell lines. We first assessed LSD1 protein and mRNA levels in mutant p53-expressing PANC-1 and MiaPaCa2 cells and p53-null AsPc-1 cells, compared to human pancreatic ductal epithelial (HPDE) controls. Our results confirmed elevated LSD1 protein levels in PANC-1 and MiaPaCa2, but not in AsPc-1, despite mRNA overexpression across all cell lines. Similarly, FADS2 levels were significantly upregulated in PANC-1 and MiaPaCa2, but not in AsPc-1, highlighting a possible link between FADS2 expression and p53 gain-of-function mutations. These results prompted us to better investigate the functional relationship between FADS2 and LSD1 by performing in cellulo protein-protein interaction analyses. Our results indicate a direct interaction between LSD1 and FADS2, while no significant interaction was observed between LSD1 and FADS1. These findings reinforce the role of FAD synthesis and its delivery to LSD1 as critical events in cancer progression and shed light on potential implications of FADS2-LSD1 dynamics as targeted therapies in cancer.

Exploring the impact of flavin homeostasis on cancer cell metabolism

Flavins and their associated proteins have recently emerged as compelling players in the landscape of cancer biology. Flavins, encompassing flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), serve as coenzymes in a multitude of cellular processes, such as metabolism, apoptosis, and cell proliferation. Their involvement in oxidative phosphorylation, redox homeostasis, and enzymatic reactions has long been recognized. However, recent research has unveiled an extended role for flavins in the context of cancer. In parallel, riboflavin transporters (RFVTs), FAD synthase (FADS), and riboflavin kinase (RFK) have gained prominence in cancer research. These proteins, responsible for riboflavin uptake, FAD biosynthesis, and FMN generation, are integral components of the cellular machinery that governs flavin homeostasis. Dysregulation in the expression/function of these proteins has been associated with various cancers, underscoring their potential as diagnostic markers, therapeutic targets, and key determinants of cancer cell behavior. This review embarks on a comprehensive exploration of the multifaceted role of flavins and of the flavoproteins involved in nucleus-mitochondria crosstalk in cancer. We journey through the influence of flavins on cancer cell energetics, the modulation of RFVTs in malignant transformation, the diagnostic and prognostic significance of FADS, and the implications of RFK in drug resistance and apoptosis. This review also underscores the potential of these molecules and processes as targets for novel diagnostic and therapeutic strategies, offering new avenues for the battle against this relentless disease.

Revitalising Riboflavin: Unveiling Its Timeless Significance in Human Physiology and Health

Since the early twentieth century, research on vitamins has revealed their therapeutic potential beyond their role as essential micronutrients. Riboflavin, known as vitamin B2, stands out for its unique characteristics. Despite numerous studies, riboflavin remains vital, with implications for human health. Abundantly present in various foods, riboflavin acts as a coenzyme in numerous enzymatic reactions crucial for human metabolism. Its role in energy production, erythrocyte synthesis, and vitamin metabolism underscores its importance in maintaining homeostasis. The impact of riboflavin extends to neurological function, skin health, and cardiovascular well-being, with adequate levels linked to reduced risks of various ailments. However, inadequate intake or physiological stress can lead to deficiency, a condition that poses serious health risks, including severe complications. This underscores the importance of maintaining sufficient levels of riboflavin for general wellness. The essential role of riboflavin in immune function further emphasises its significance for human health and vitality. This paper examines the diverse effects of riboflavin on health and stresses the importance of maintaining sufficient levels for overall well-being.

Structural insights into the bifunctional enzyme human FAD synthase

Human flavin adenine dinucleotide synthase (hFADS) is a bifunctional, multi-domain enzyme that exhibits both flavin mononucleotide adenylyltransferase and pyrophosphatase activities. Here we report the crystal structure of full-length hFADS2 and its C-terminal PAPS domain in complex with flavin adenine dinucleotide (FAD), and dissect the structural determinants underlying the contribution of each individual domain, within isoforms 1 and 2, to each of the two enzymatic activities. Structural and functional characterization performed on complete or truncated constructs confirmed that the C-terminal domain tightly binds FAD and catalyzes its synthesis, while the combination of the N-terminal molybdopterin-binding and KH domains is the minimal essential substructure required for the hydrolysis of FAD and other ADP-containing dinucleotides. hFADS2 associates in a stable C2-symmetric dimer, in which the packing of the KH domain of one protomer against the N-terminal domain of the other creates the adenosine-specific active site responsible for the hydrolytic activity.

Transcriptomic and genomic effects of gamma-radiation exposure on strains of the black yeast Exophiala dermatitidis evolved to display increased ionizing radiation resistance

IMPORTANCE

Ionizing radiation poses a significant threat to living organisms and human health, given its destructive nature and widespread use in fields such as medicine and the potential for nuclear disasters. Melanized fungi exhibit remarkable survival capabilities, enduring doses up to 1,000-fold higher than mammals. Through adaptive laboratory evolution, we validated the protective role of constitutive upregulation of DNA repair genes in the black yeast Exophiala dermatitidis, enhancing survival after radiation exposure. Surprisingly, we found that evolved strains lacking melanin still achieved high levels of radioresistance. Our study unveiled the significance of robust activation and enhancement of redox homeostasis, as evidenced by the profound transcriptional changes and increased accumulation of mutations, in substantially improving ionizing radiation resistance in the absence of melanin. These findings underscore the delicate balance between DNA repair and redox homeostasis for an organism's ability to endure and recover from radiation exposure.

Increased demand for FAD synthesis in differentiated and stem pancreatic cancer cells is accomplished by modulating FLAD1 gene expression: the inhibitory effect of Chicago Sky Blue

FLAD1, along with its FAD synthase (FADS, EC 2.7.7.2) product, is crucial for flavin homeostasis and, due to its role in the mitochondrial respiratory chain and nuclear epigenetics, is closely related to cellular metabolism. Therefore, it is not surprising that it could be correlated with cancer. To our knowledge, no previous study has investigated FLAD1 prognostic significance in pancreatic ductal adenocarcinoma (PDAC). Thus, in the present work, the FAD synthesis process was evaluated in two PDAC cell lines: (a) PANC-1- and PANC-1-derived cancer stem cells (CSCs), presenting the R273H mutation in the oncosuppressor p53, and (b) MiaPaca2 and MiaPaca2-derived CSCs, presenting the R248W mutation in p53. As a control, HPDE cells expressing wt-p53 were used. FADS expression/activity increase was found with malignancy and even more with stemness. An increased FAD synthesis rate in cancer cell lines is presumably demanded by the increase in the FAD-dependent lysine demethylase 1 protein amount as well as by the increased expression levels of the flavoprotein subunit of complex II of the mitochondrial respiratory chain, namely succinate dehydrogenase. With the aim of proposing FADS as a novel target for cancer therapy, the inhibitory effect of Chicago Sky Blue on FADS enzymatic activity was tested on the recombinant 6His-hFADS2 (IC50  = 1.2 μm) and PANC-1-derived CSCs' lysate (IC50  = 2-10 μm). This molecule was found effective in inhibiting the growth of PANC-1 and even more of its derived CSC line, thus assessing its role as a potential chemotherapeutic drug.

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.

Mitochondrial transport and metabolism of the vitamin B-derived cofactors thiamine pyrophosphate, coenzyme A, FAD and NAD+ , and related diseases: A review

Multiple mitochondrial matrix enzymes playing key roles in metabolism require cofactors for their action. Due to the high impermeability of the mitochondrial inner membrane, these cofactors need to be synthesized within the mitochondria or be imported, themselves or one of their precursors, into the organelles. Transporters belonging to the protein family of mitochondrial carriers have been identified to transport the coenzymes: thiamine pyrophosphate, coenzyme A, FAD and NAD+ , which are all structurally similar to nucleotides and derived from different B-vitamins. These mitochondrial cofactors bind more or less tightly to their enzymes and, after having been involved in a specific reaction step, are regenerated, spontaneously or by other enzymes, to return to their active form, ready for the next catalysis round. Disease-causing mutations in the mitochondrial cofactor carrier genes compromise not only the transport reaction but also the activity of all mitochondrial enzymes using that particular cofactor and the metabolic pathways in which the cofactor-dependent enzymes are involved. The mitochondrial transport, metabolism and diseases of the cofactors thiamine pyrophosphate, coenzyme A, FAD and NAD+ are the focus of this review.

Retrograde response to mitochondrial dysfunctions associated to LOF variations in FLAD1 exon 2: unraveling the importance of RFVT2

Flavin adenine dinucleotide (FAD) synthase (EC 2.7.7.2), encoded by human flavin adenine dinucleotide synthetase 1 (FLAD1), catalyzes the last step of the pathway converting riboflavin (Rf) into FAD. FLAD1 variations were identified as a cause of LSMFLAD (lipid storage myopathy due to FAD synthase deficiency, OMIM #255100), resembling Multiple Acyl-CoA Dehydrogenase Deficiency, sometimes treatable with high doses of Rf; no alternative therapeutic strategies are available. We describe here cell morphological and mitochondrial alterations in dermal fibroblasts derived from a LSMFLAD patient carrying a homozygous truncating FLAD1 variant (c.745C > T) in exon 2. Despite a severe decrease in FAD synthesis rate, the patient had decreased cellular levels of Rf and flavin mononucleotide and responded to Rf treatment. We hypothesized that disturbed flavin homeostasis and Rf-responsiveness could be due to a secondary impairment in the expression of the Rf transporter 2 (RFVT2), encoded by SLC52A2, in the frame of an adaptive retrograde signaling to mitochondrial dysfunction. Interestingly, an antioxidant response element (ARE) is found in the region upstream of the transcriptional start site of SLC52A2. Accordingly, we found that abnormal mitochondrial morphology and impairments in bioenergetics were accompanied by increased cellular reactive oxygen species content and mtDNA oxidative damage. Concomitantly, an active response to mitochondrial stress is suggested by increased levels of PPARγ-co-activator-1α and Peroxiredoxin III. In this scenario, the treatment with high doses of Rf might compensate for the secondary RFVT2 molecular defect, providing a molecular rationale for the Rf responsiveness in patients with loss of function variants in FLAD1 exon 2.HIGHLIGHTSFAD synthase deficiency alters mitochondrial morphology and bioenergetics;FAD synthase deficiency triggers a mitochondrial retrograde response;FAD synthase deficiency evokes nuclear signals that adapt the expression of RFVT2.

Molecular characteristics of the multi-functional FAO enzyme ACAD9 illustrate the importance of FADH2 /NADH ratios for mitochondrial ROS formation

A decade ago I postulated that ROS formation in mitochondria was influenced by different FADH₂ /NADH (F/N) ratios of catabolic substrates. Thus, fatty acid oxidation (FAO) would give higher ROS formation than glucose oxidation. Both the emergence of peroxisomes and neurons not using FAO, could be explained thus. ROS formation in NADH:ubiquinone oxidoreductase (Complex I) comes about by reverse electron transport (RET) due to high QH₂ levels, and scarcity of its electron-acceptor (Q) during FAO. The then new, unexpected, finding of an FAO enzyme, ACAD9, being involved in complex I biogenesis, hinted at connections in line with the hypothesis. Recent findings about ACAD9's role in regulation of respiration fit with predictions the model makes: cementing connections between ROS production and F/N ratios. I describe how ACAD9 might be central to reversing the oxidative damage in complex I resulting from FAO. This seems to involve two distinct, but intimately connected, ACAD9 characteristics: (i) its upregulation of complex I biogenesis, and (ii) releasing FADH₂ , with possible conversion into FMN, the crucial prosthetic group of complex I.

The ins and outs of the flavin mononucleotide cofactor of respiratory complex I

The flavin mononucleotide (FMN) cofactor of respiratory complex I occupies a key position in the electron transport chain. Here, the electrons coming from NADH start the sequence of oxidoreduction reactions, which drives the generation of the proton-motive force necessary for ATP synthesis. The overall architecture and the general catalytic proprieties of the FMN site are mostly well established. However, several aspects regarding the complex I flavin cofactor are still unknown. For example, the flavin binding to the N-module, the NADH-oxidizing portion of complex I, lacks a molecular description. The dissociation of FMN from the enzyme is beginning to emerge as an important regulatory mechanism of complex I activity and ROS production. Finally, how mitochondria import and metabolize FMN is still uncertain. This review summarizes the current knowledge on complex I flavin cofactor and discusses the open questions for future research.

Cofactors and pathogens: Flavin mononucleotide and flavin adenine dinucleotide (FAD) biosynthesis by the FAD synthase from Brucella ovis

The biosynthesis of the flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), cofactors used by 2% of proteins, occurs through the sequential action of two ubiquitous activities: a riboflavinkinase (RFK) that phosphorylates the riboflavin (RF) precursor to FMN, and a FMN:adenylyltransferase (FMNAT) that transforms FMN into FAD. In most mammals two different monofunctional enzymes have each of these activities, but in prokaryotes a single bifunctional enzyme, FAD synthase (FADS), holds them. Differential structural and functional traits for RFK and FMNAT catalysis between bacteria and mammals, as well as within the few bacterial FADSs so far characterized, has envisaged the potentiality of FADSs from pathogens as targets for the development of species‐specific inhibitors. Here, we particularly characterize the FADS from the ovine pathogen Brucella ovis (BoFADS), causative agent of brucellosis. We show that BoFADS has RFK activity independently of the media redox status, but its FMNAT activity (in both forward and reverse senses) only occurs under strong reducing conditions. Moreover, kinetics for flavin and adenine nucleotides binding to the RFK site show that BoFADS binds preferentially the substrates of the RFK reaction over the products and that the adenine nucleotide must bind prior to flavin entrapment. These results, together with multiple sequence alignments and phylogenetic analysis, point to variability in the less conserved regions as contributing to the species‐specific features in prokaryotic FADSs, including those from pathogens, that allow them to adopt alternative strategies in FMN and FAD biosynthesis and overall flavin homeostasis.

Purification and characterization of recombinant FAD synthetase from Neurospora crassa

FAD Synthetase (FADS) [EC 2.7.7.2], the second enzyme in flavin cofactor biosynthetic pathway converts FMN to FAD, plays an important role in many redox reactions. Neurospora crassa FADS (NcFADS) was cloned and overexpressed in E. coli cells. Recombinant NcFADS was purified in high yields of ∼8 mg per liter of bacterial culture using a single step glutathione sepharose affinity chromatography. SDS-PAGE and MALDI-MS revealed that NcFADS has a molecular mass of ∼31 kDa. Enzyme kinetic analysis monitored by reverse phase HPLC demonstrate a specific activity and k_cat of 1356 nmol/min/mg and 0.69sec⁻¹ respectively. Steady state kinetic analysis of NcFADS exhibited a K_m of NcFADS for FMN is 2.7 μM and for MgATP⁻² is 88.7 μM. Isothermal titration calorimetry experiments showed that the recombinant protein binds to the substrates with apparent K_d of 20.8 μM for FMN and 16.6 μM for MgATP⁻². Biophysical characterization using intrinsic fluorescence suggests that the enzyme is in folded conformation. Far-UV CD data suggest that the backbone of the enzyme is predominantly in a helical conformation. Differential scanning calorimetry data shows that the T_m is 53 °C ± 1. This is the first report on cloning, purification and characterization of FADS from N. crassa. The specific activity of NcFADS is the highest than any of the reported FADS from any other source. The results obtained in this study is expected to pave way for intensive research aimed to understand the molecular basis for the extraordinarily high turnover rate of NcFADS.

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rNcFADS active at pH7.5.

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High turnover, more thermal stability.

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2^ostructure-helical.

Flavin-adenine-dinucleotide gold complex nanoparticles: chemical modeling design, physico-chemical assessment and perspectives in nanomedicine

Flavoproteins play an important role in the regulatory process of cell life, and they are involved in several redox reactions that regulate the metabolism of carbohydrates, amino acids, and lipids. The development of effective drug delivery systems is one of the major challenges in the fight against cancer. This study involves a nanomedicine pathway to encapsulate the cofactor flavin adenine dinucleotide (FAD) using polymeric gold nanoparticles (PEG-AuNPs) through two chemical methods of functionalization (chelation (IN); carbodiimide chemistry (ON)). These hybrid gold nanoparticles and their precursors were characterized by analytical techniques (Raman, UV-Vis, and H1-NMR spectroscopy and transmission electron microscopy (TEM)) which confirmed the grafting of the cofactor agent. The results of the computational studies (Density Functional Theory (DFT)) were in agreement with the experimental observations. We also monitored the interaction of our hybrid nanoparticle systems with small aptamers (APT) in order to validate the hypotheses on the biomolecular mechanisms and also investigate their biological efficiency on pancreatic cancer cells (MIAPaCa-2 cells).

Subcellular Localization of Fad1p in Saccharomyces cerevisiae: A Choice at Post-Transcriptional Level?

FAD synthase is the last enzyme in the pathway that converts riboflavin into FAD. In Saccharomyces cerevisiae, the gene encoding for FAD synthase is FAD1, from which a sole protein product (Fad1p) is expected to be generated. In this work, we showed that a natural Fad1p exists in yeast mitochondria and that, in its recombinant form, the protein is able, per se, to both enter mitochondria and to be destined to cytosol. Thus, we propose that FAD1 generates two echoforms-that is, two identical proteins addressed to different subcellular compartments. To shed light on the mechanism underlying the subcellular destination of Fad1p, the 3' region of FAD1 mRNA was analyzed by 3'RACE experiments, which revealed the existence of (at least) two FAD1 transcripts with different 3'UTRs, the short one being 128 bp and the long one being 759 bp. Bioinformatic analysis on these 3'UTRs allowed us to predict the existence of a cis-acting mitochondrial localization motif, present in both the transcripts and, presumably, involved in protein targeting based on the 3'UTR context. Here, we propose that the long FAD1 transcript might be responsible for the generation of mitochondrial Fad1p echoform.

Alteration of Flavin Cofactor Homeostasis in Human Neuromuscular Pathologies

The aim of this short review chapter is to provide a brief summary of the relevance of riboflavin (Rf or vitamin B2) and its derived cofactors flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) for human neuromuscular bioenergetics.Therefore, as a completion of this book we would like to summarize what kind of human pathologies could derive from genetic disturbances of Rf transport, flavin cofactor synthesis and delivery to nascent apoflavoproteins, as well as by alteration of vitamin recycling during protein turnover.

Heterologous Overexpression of Human FAD Synthase Isoforms 1 and 2

The study of human FAD synthase enzymes requires a recombinant strategy to produce large amount of purified proteins in a soluble form. E. coli was exploited to this aim. To achieve the production of FAD synthase in a large scale, E. coli strains, plasmids (promoter, tags), growth temperature, inducer concentration, medium composition, and osmotic pressure were optimized. To date there is no universal protocol for protein expression, but for each protein a specific combination of "expression parameters" can be selected in order to maximize the results. An experimental protocol for the expression of two isoforms of the human FAD synthase was set up. The final procedures are based on the use of E. coli Rosetta(DE3) strain. Two different plasmids were used to obtain optimal amount of the two protein isoforms. In both cases, following the addition of the IPTG inducer, the growth temperature was lowered to increase the solubility of the recombinant protein. The detailed procedures for FAD synthase isoform 1 and isoform 2 overproduction are described in this protocol.

Development of Novel Experimental Models to Study Flavoproteome Alterations in Human Neuromuscular Diseases: The Effect of Rf Therapy

Inborn errors of Riboflavin (Rf) transport and metabolism have been recently related to severe human neuromuscular disorders, as resulting in profound alteration of human flavoproteome and, therefore, of cellular bioenergetics. This explains why the interest in studying the “flavin world”, a topic which has not been intensively investigated before, has increased much over the last few years. This also prompts basic questions concerning how Rf transporters and FAD (flavin adenine dinucleotide) -forming enzymes work in humans, and how they can create a coordinated network ensuring the maintenance of intracellular flavoproteome. The concept of a coordinated cellular “flavin network”, introduced long ago studying humans suffering for Multiple Acyl-CoA Dehydrogenase Deficiency (MADD), has been, later on, addressed in model organisms and more recently in cell models. In the frame of the underlying relevance of a correct supply of Rf in humans and of a better understanding of the molecular rationale of Rf therapy in patients, this review wants to deal with theories and existing experimental models in the aim to potentiate possible therapeutic interventions in Rf-related neuromuscular diseases.

Riboflavin Deficiency-Implications for General Human Health and Inborn Errors of Metabolism

As an essential vitamin, the role of riboflavin in human diet and health is increasingly being highlighted. Insufficient dietary intake of riboflavin is often reported in nutritional surveys and population studies, even in non-developing countries with abundant sources of riboflavin-rich dietary products. A latent subclinical riboflavin deficiency can result in a significant clinical phenotype when combined with inborn genetic disturbances or environmental and physiological factors like infections, exercise, diet, aging and pregnancy. Riboflavin, and more importantly its derivatives, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), play a crucial role in essential cellular processes including mitochondrial energy metabolism, stress responses, vitamin and cofactor biogenesis, where they function as cofactors to ensure the catalytic activity and folding/stability of flavoenzymes. Numerous inborn errors of flavin metabolism and flavoenzyme function have been described, and supplementation with riboflavin has in many cases been shown to be lifesaving or to mitigate symptoms. This review discusses the environmental, physiological and genetic factors that affect cellular riboflavin status. We describe the crucial role of riboflavin for general human health, and the clear benefits of riboflavin treatment in patients with inborn errors of metabolism.

Insights into the FMNAT Active Site of FAD Synthase: Aromaticity is Essential for Flavin Binding and Catalysis

The last step in the biosynthesis of flavin adenine dinucleotide (FAD) is considered a target for the design of antimicrobial drugs because it is carried out by two non-homologous proteins in eukaryotic and prokaryotic organisms. Monofunctional FMN: adenylyltransferases (FMNAT) in Eukarya and FMNAT modules of bifunctional FAD synthases (FADS) in Prokarya belong to different structural families with dissimilar chemistry and binding modes for the substrates. In this study, we analyzed the relevance of the hydrophobic environment of the flavin isoalloxazine in the FMNAT active site of Corynebacterium ammoniagenes FADS (CaFADS) through the mutational analysis of its F62, Y106, and F128 residues. They form the isoalloxazine binding cavity and are highly conserved in the prokaryotic FADS family. The spectroscopic, steady-state kinetics and thermodynamic data presented indicate that distortion of aromaticity at the FMNAT isoalloxazine binding cavity prevents FMN and FAD from correct accommodation in their binding cavity and, as a consequence, decreases the efficiency of the FMNAT activity. Therefore, the side-chains of F62, Y106 and F128 are relevant in the formation of the catalytic competent complex during FMNAT catalysis in CaFADS. The introduced mutations also modulate the activity occurring at the riboflavin kinase (RFK) module of CaFADS, further evidencing the formation of quaternary assemblies during catalysis.

Mutation of Aspartate 238 in FAD Synthase Isoform 6 Increases the Specific Activity by Weakening the FAD Binding

FAD synthase (FADS, or FMN:ATP adenylyl transferase) coded by the FLAD1 gene is the last enzyme in the pathway of FAD synthesis. The mitochondrial isoform 1 and the cytosolic isoform 2 are characterized by the following two domains: the C-terminal PAPS domain (FADSy) performing FAD synthesis and pyrophosphorolysis; the N-terminal molybdopterin-binding domain (FADHy) performing a Co⁺⁺/K⁺-dependent FAD hydrolysis. Mutations in FLAD1 gene are responsible for riboflavin responsive and non-responsive multiple acyl-CoA dehydrogenases and combined respiratory chain deficiency. In patients harboring frameshift mutations, a shorter isoform (hFADS6) containing the sole FADSy domain is produced representing an emergency protein. With the aim to ameliorate its function we planned to obtain an engineered more efficient hFADS6. Thus, the D238A mutant, resembling the D181A FMNAT “supermutant” of C. glabrata, was overproduced and purified. Kinetic analysis of this enzyme highlighted a general increase of K_m, while the k_cat was two-fold higher than that of WT. The data suggest that the FAD synthesis rate can be increased. Additional modifications could be performed to further improve the synthesis of FAD. These results correlate with previous data produced in our laboratory, and point towards the following proposals (i) FAD release is the rate limiting step of the catalytic cycle and (ii) ATP and FMN binding sites are synergistically connected.

The assembly of succinate dehydrogenase: a key enzyme in bioenergetics

Succinate dehydrogenase (SDH) also known as complex II or succinate:quinone oxidoreductase is an enzyme involved in both oxidative phosphorylation and tricarboxylic acid cycle; the processes that generate energy. SDH is a multi-subunit enzyme which requires a series of proteins for its proper assembly at several steps. This enzyme has medical significance as there is a broad range of human diseases from cancers to neurodegeneration related to SDH malfunction. Some of these disorders have recently been linked to defective assembly factors, reinvigorating further research in this area. Apart from that this enzyme has agricultural importance as many fungicides have been/will be designed targeting specifically this enzyme in plant fungal pathogens. In addition, we speculate it might be possible to design novel fungicides specifically targeting fungal assembly factors. Considering the medical and agricultural implications of SDH, the aim of this review is an overview of the SDH assembly factors and critical analysis of controversial issues around them.

The hidden side of the human FAD synthase 2

FAD synthase, the last enzyme of the pathway converting riboflavin to FAD, exists in humans in different isoforms, with isoforms 1, 2 and 6 being characterized at the functional and molecular levels. Isoform 2, the cytosolic and most abundant FADS, consists of two domains: a PAPS reductase C-terminus domain (here named FADSy) responsible for FAD synthesis, and an N-terminus molybdopterin-binding resembling domain (MPTb - here named FADHy), whose FAD hydrolytic activity is hidden unless both Co²⁺ and chemical mercurial reagents are added to the enzyme. To investigate the hFADS2 hydrolytic function under conditions closer to the physiological context, the hydrolytic activity was further characterized. Co²⁺ induced FAD hydrolysis was strongly stimulated in the presence of K⁺, reaching a Vmax higher than that of FAD synthesis. The pH dependence together with the inhibition of the hydrolysis by NaF and KI allow excluding that the reaction occurs via a NUDIX type catalysis. The K_0.5 for K⁺ or Co²⁺ was 7.2 or 0.035 mM, respectively. Other monovalent or divalent cations can partially substitute K⁺ or Co²⁺. Reduced glutathione stimulated whereas NADH inhibited the hydrolytic activity. The latter aspects correlate with an interconnection of the homeostasis of NAD and FAD.

Disorders of riboflavin metabolism

Riboflavin (vitamin B2), a water-soluble vitamin, is an essential nutrient in higher organisms as it is not endogenously synthesised, with requirements being met principally by dietary intake. Tissue-specific transporter proteins direct riboflavin to the intracellular machinery responsible for the biosynthesis of the flavocoenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). These flavocoenzymes play a vital role in ensuring the functionality of a multitude of flavoproteins involved in bioenergetics, redox homeostasis, DNA repair, chromatin remodelling, protein folding, apoptosis, and other physiologically relevant processes. Hence, it is not surprising that the impairment of flavin homeostasis in humans may lead to multisystem dysfunction including neuromuscular disorders, anaemia, abnormal fetal development, and cardiovascular disease. In this review, we provide an overview of riboflavin absorption, transport, and metabolism. We then focus on the clinical and biochemical features associated with biallelic FLAD1 mutations leading to FAD synthase deficiency, the only known primary defect in flavocoenzyme synthesis, in addition to providing an overview of clinical disorders associated with nutritional deficiency of riboflavin and primary defects of riboflavin transport. Finally, we give a brief overview of disorders of the cellular flavoproteome. Because riboflavin therapy may be beneficial in a number of primary or secondary disorders of the cellular flavoproteome, early recognition and prompt management of these disorders is imperative.

d-amino acid oxidase promotes cellular senescence via the production of reactive oxygen species

This study reveals a novel role of d-amino acid oxidase in promoting cellular senescence induced by genotoxic stresses via enzymatic generation of reactive oxygen species.

d-amino acid oxidase (DAO) is a flavin adenine dinucleotide (FAD)–dependent oxidase metabolizing neutral and polar d-amino acids. Unlike l-amino acids, the amounts of d-amino acids in mammalian tissues are extremely low, and therefore, little has been investigated regarding the physiological role of DAO. We have recently identified DAO to be up-regulated in cellular senescence, a permanent cell cycle arrest induced by various stresses, such as persistent DNA damage and oxidative stress. Because DAO produces reactive oxygen species (ROS) as byproducts of substrate oxidation and the accumulation of ROS mediates the senescence induction, we explored the relationship between DAO and senescence. We found that inhibition of DAO impaired senescence induced by DNA damage, and ectopic expression of wild-type DAO, but not enzymatically inactive mutant, enhanced it in an ROS-dependent manner. Furthermore, addition of d-amino acids and riboflavin, a metabolic precursor of FAD, to the medium potentiated the senescence-promoting effect of DAO. These results indicate that DAO promotes senescence through the enzymatic ROS generation, and its activity is regulated by the availability of its substrate and coenzyme.

Discovery of Potent Inhibitors for the Large Neutral Amino Acid Transporter 1 (LAT1) by Structure-Based Methods

The large neutral amino acid transporter 1 (LAT1) is a promising anticancer target that is required for the cellular uptake of essential amino acids that serve as building blocks for cancer growth and proliferation. Here, we report a structure-based approach to identify chemically diverse and potent inhibitors of LAT1. First, a homology model of LAT1 that is based on the atomic structures of the prokaryotic homologs was constructed. Molecular docking of nitrogen mustards (NMs) with a wide range of affinity allowed for deriving a common binding mode that could explain the structure−activity relationship pattern in NMs. Subsequently, validated binding hypotheses were subjected to molecular dynamics simulation, which allowed for extracting a set of dynamic pharmacophores. Finally, a library of ~1.1 million molecules was virtually screened against these pharmacophores, followed by docking. Biological testing of the 30 top-ranked hits revealed 13 actives, with the best compound showing an IC₅₀ value in the sub-μM range.

Cys Site-Directed Mutagenesis of the Human SLC1A5 (ASCT2) Transporter: Structure/Function Relationships and Crucial Role of Cys467 for Redox Sensing and Glutamine Transport

The human plasma membrane transporter ASCT2 is responsible for mediating Na- dependent antiport of neutral amino acids. New insights into structure/function relationships were unveiled by a combined approach of recombinant over-expression, site-directed mutagenesis, transport assays in proteoliposomes and bioinformatics. WT and Cys mutants of hASCT2 were produced in P. pastoris and purified for functional assay. The reactivity towards SH reducing and oxidizing agents of WT protein was investigated and opposite effects were revealed; transport activity increased upon treatment with the Cys reducing agent DTE, i.e., when Cys residues were in thiol (reduced) state. Methyl-Hg, which binds to SH groups, was able to inhibit WT and seven out of eight Cys to Ala mutants. On the contrary, C467A loses the sensitivity to both DTE activation and Methyl-Hg inhibition. The C467A mutant showed a Km for Gln one order of magnitude higher than that of WT. Moreover, the C467 residue is localized in the substrate binding region of the protein, as suggested by bioinformatics on the basis of the EAAT1 structure comparison. Taken together, the experimental data allowed identifying C467 residue as crucial for substrate binding and for transport activity modulation of hASCT2.

Bacterial Production, Characterization and Protein Modeling of a Novel Monofuctional Isoform of FAD Synthase in Humans: An Emergency Protein?

FAD synthase (FADS, EC 2.7.7.2) is the last essential enzyme involved in the pathway of biosynthesis of Flavin cofactors starting from Riboflavin (Rf). Alternative splicing of the human FLAD1 gene generates different isoforms of the enzyme FAD synthase. Besides the well characterized isoform 1 and 2, other FADS isoforms with different catalytic domains have been detected, which are splice variants. We report the characterization of one of these novel isoforms, a 320 amino acid protein, consisting of the sole C-terminal 3′-phosphoadenosine 5′-phosphosulfate (PAPS) reductase domain (named FADS6). This isoform has been previously detected in Riboflavin-Responsive (RR-MADD) and Non-responsive Multiple Acyl-CoA Dehydrogenase Deficiency (MADD) patients with frameshift mutations of FLAD1 gene. To functionally characterize the hFADS6, it has been over-expressed in Escherichia coli and purified with a yield of 25 mg·L⁻¹ of cell culture. The protein has a monomeric form, it binds FAD and is able to catalyze FAD synthesis (k_cat about 2.8 min⁻¹), as well as FAD pyrophosphorolysis in a strictly Mg²⁺-dependent manner. The synthesis of FAD is inhibited by HgCl₂. The enzyme lacks the ability to hydrolyze FAD. It behaves similarly to PAPS. Combining threading and ab-initio strategy a 3D structural model for such isoform has been built. The relevance to human physio-pathology of this FADS isoform is discussed.

Kinetics and thermodynamics of the protein-ligand interactions in the riboflavin kinase activity of the FAD synthetase from Corynebacterium ammoniagenes

Enzymes known as bifunctional and bimodular prokaryotic type-I FAD synthetase (FADS) exhibit ATP:riboflavin kinase (RFK) and FMN:ATP adenylyltransferase (FMNAT) activities in their C-terminal and N-terminal modules, respectively, and produce flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). These act as cofactors of a plethora of flavoproteins in all organisms. Therefore, regulation of their production maintains the cellular flavoproteome homeostasis. Here, we focus on regulation of the FMN synthesis in Corynebacterium ammoniagenes (Ca) by the inhibition of its RFK activity by substrates and products of the reaction. We use a truncated CaFADS variant consisting in the isolated C-terminal RFK module, whose RFK activity is similar to that of the full-length enzyme. Inhibition of the RFK activity by the RF substrate is independent of the FMNAT module, and FMN production, in addition to being inhibited by an excess of RF, is also inhibited by both of the reaction products. Pre-steady-state kinetic and thermodynamic studies reveal key aspects to the substrates induced fit to produce the catalytically competent complex. Among them, the role of Mg²⁺ in the concerted allocation of substrates for catalysis and the ensemble of non-competent complexes that contribute to the regulated inhibition of the RFK activity are particularly relevant.

Atrazine exposure elicits copy number alterations in the zebrafish genome

Atrazine is an agricultural herbicide used throughout the Midwestern United States that frequently contaminates potable water supplies resulting in human exposure. Using the zebrafish model system, an embryonic atrazine exposure was previously reported to decrease spawning rates with an increase in progesterone and ovarian follicular atresia in adult females. In addition, alterations in genes associated with distinct molecular pathways of the endocrine system were observed in brain and gonad tissue of the adult females and males. Current hypotheses for mechanistic changes in the developmental origins of health and disease include genetic (e.g., copy number alterations) or epigenetic (e.g., DNA methylation) mechanisms. As such, in the current study we investigated whether an atrazine exposure would generate copy number alterations (CNAs) in the zebrafish genome. A zebrafish fibroblast cell line was used to limit detection to CNAs caused by the chemical exposure. First, cells were exposed to a range of atrazine concentrations and a crystal violet assay was completed, showing confluency decreased by ~60% at 46.3μM. Cells were then exposed to 0, 0.463, 4.63, or 46.3μM atrazine and array comparative genomic hybridization completed. Results showed 34, 21, and 44 CNAs in the 0.463, 4.63, and 46.3μM treatments, respectively. Furthermore, CNAs were associated with previously reported gene expression alterations in adult male and female zebrafish. This study demonstrates that atrazine exposure can generate CNAs that are linked to gene expression alterations observed in adult zebrafish exposed to atrazine during embryogenesis providing a mechanism of the developmental origins of atrazine endocrine disruption.

Riboflavin transport and metabolism in humans

Recent studies elucidated how riboflavin transporters and FAD forming enzymes work in humans and create a coordinated flavin network ensuring the maintenance of cellular flavoproteome. Alteration of this network may be causative of severe metabolic disorders such as multiple acyl-CoA dehydrogenase deficiency (MADD) or Brown-Vialetto-van Laere syndrome. A crucial step in the maintenance of FAD homeostasis is riboflavin uptake by plasma and mitochondrial membranes. Therefore, studies on recently identified human plasma membrane riboflavin transporters are presented, together with those in which still unidentified mitochondrial riboflavin transporter(s) have been described. A main goal of future research is to fill the gaps still existing as for some transcriptional, functional and structural details of human FAD synthases (FADS) encoded by FLAD1 gene, a novel "redox sensing" enzyme. In the frame of the hypothesis that FADS, acting as a "FAD chaperone", could play a crucial role in the biogenesis of mitochondrial flavo-proteome, several basic functional aspects of flavin cofactor delivery to cognate apo-flavoenzyme are also briefly dealt with. The establishment of model organisms performing altered FAD homeostasis will improve the molecular description of human pathologies. The molecular and functional studies of transporters and enzymes herereported, provide guidelines for improving therapies which may have beneficial effects on the altered metabolism.

Riboflavin-Responsive and -Non-responsive Mutations in FAD Synthase Cause Multiple Acyl-CoA Dehydrogenase and Combined Respiratory-Chain Deficiency

Multiple acyl-CoA dehydrogenase deficiencies (MADDs) are a heterogeneous group of metabolic disorders with combined respiratory-chain deficiency and a neuromuscular phenotype. Despite recent advances in understanding the genetic basis of MADD, a number of cases remain unexplained. Here, we report clinically relevant variants in FLAD1, which encodes FAD synthase (FADS), as the cause of MADD and respiratory-chain dysfunction in nine individuals recruited from metabolic centers in six countries. In most individuals, we identified biallelic frameshift variants in the molybdopterin binding (MPTb) domain, located upstream of the FADS domain. Inasmuch as FADS is essential for cellular supply of FAD cofactors, the finding of biallelic frameshift variants was unexpected. Using RNA sequencing analysis combined with protein mass spectrometry, we discovered FLAD1 isoforms, which only encode the FADS domain. The existence of these isoforms might explain why affected individuals with biallelic FLAD1 frameshift variants still harbor substantial FADS activity. Another group of individuals with a milder phenotype responsive to riboflavin were shown to have single amino acid changes in the FADS domain. When produced in E. coli, these mutant FADS proteins resulted in impaired but detectable FADS activity; for one of the variant proteins, the addition of FAD significantly improved protein stability, arguing for a chaperone-like action similar to what has been reported in other riboflavin-responsive inborn errors of metabolism. In conclusion, our studies identify FLAD1 variants as a cause of potentially treatable inborn errors of metabolism manifesting with MADD and shed light on the mechanisms by which FADS ensures cellular FAD homeostasis.

Human FAD synthase is a bi-functional enzyme with a FAD hydrolase activity in the molybdopterin binding domain

FAD synthase (FMN:ATP adenylyl transferase, FMNAT or FADS, EC 2.7.7.2) is involved in the biochemical pathway for converting riboflavin into FAD. Human FADS exists in different isoforms. Two of these have been characterized and are localized in different subcellular compartments. hFADS2 containing 490 amino acids shows a two domain organization: the 3'-phosphoadenosine-5'-phosphosulfate (PAPS) reductase domain, that is the FAD-forming catalytic domain, and a resembling molybdopterin-binding (MPTb) domain. By a multialignment of hFADS2 with other MPTb containing proteins of various organisms from bacteria to plants, the critical residues for hydrolytic function were identified. A homology model of the MPTb domain of hFADS2 was built, using as template the solved structure of a T. acidophilum enzyme. The capacity of hFADS2 to catalyse FAD hydrolysis was revealed. The recombinant hFADS2 was able to hydrolyse added FAD in a Co(2+) and mersalyl dependent reaction. The recombinant PAPS reductase domain is not able to perform the same function. The mutant C440A catalyses the same hydrolytic function of WT with no essential requirement for mersalyl, thus indicating the involvement of C440 in the control of hydrolysis switch. The enzyme C440A is also able to catalyse hydrolysis of FAD bound to the PAPS reductase domain, which is quantitatively converted into FMN.

Remaining challenges in cellular flavin cofactor homeostasis and flavoprotein biogenesis

The primary role of the water-soluble vitamin B₂ (riboflavin) in cell biology is connected with its conversion into FMN and FAD, the cofactors of a large number of dehydrogenases, oxidases and reductases involved in a broad spectrum of biological activities, among which energetic metabolism and chromatin remodeling. Subcellular localisation of FAD synthase (EC 2.7.7.2, FADS), the second enzyme in the FAD forming pathway, is addressed here in HepG2 cells by confocal microscopy, in the frame of its relationships with kinetics of FAD synthesis and delivery to client apo-flavoproteins. FAD synthesis catalyzed by recombinant isoform 2 of FADS occurs via an ordered bi-bi mechanism in which ATP binds prior to FMN, and pyrophosphate is released before FAD. Spectrophotometric continuous assays of the reconstitution rate of apo-D-aminoacid oxidase with its cofactor, allowed us to propose that besides its FAD synthesizing activity, hFADS is able to operate as a FAD “chaperone.” The physical interaction between FAD forming enzyme and its clients was further confirmed by dot blot and immunoprecipitation experiments carried out testing as a client either a nuclear lysine-specific demethylase 1 (LSD1) or a mitochondrial dimethylglycine dehydrogenase (Me₂GlyDH, EC 1.5.8.4). Both enzymes carry out similar reactions of oxidative demethylation, in which tetrahydrofolate is converted into 5,10-methylene-tetrahydrofolate. A direct transfer of the cofactor from hFADS2 to apo-dimethyl glycine dehydrogenase was also demonstrated. Thus, FAD synthesis and delivery to these enzymes are crucial processes for bioenergetics and nutri-epigenetics of liver cells.

Protein-mediated assembly of succinate dehydrogenase and its cofactors

Succinate dehydrogenase (or complex II; SDH) is a heterotetrameric protein complex that links the tribarboxylic acid cycle with the electron transport chain. SDH is composed of four nuclear-encoded subunits that must translocate independently to the mitochondria and assemble into a mature protein complex embedded in the inner mitochondrial membrane. Recently, it has become clear that failure to assemble functional SDH complexes can result in cancer and neurodegenerative syndromes. The effort to thoroughly elucidate the SDH assembly pathway has resulted in the discovery of four subunit-specific assembly factors that aid in the maturation of individual subunits and support the assembly of the intact complex. This review will focus on these assembly factors and assess the contribution of each factor to the assembly of SDH. Finally, we propose a model of the SDH assembly pathway that incorporates all extant data.

The flavoproteome of the yeast Saccharomyces cerevisiae

Genome analysis of the yeast Saccharomyces cerevisiae identified 68 genes encoding flavin-dependent proteins (1.1% of protein encoding genes) to which 47 distinct biochemical functions were assigned. The majority of flavoproteins operate in mitochondria where they participate in redox processes revolving around the transfer of electrons to the electron transport chain. In addition, we found that flavoenzymes play a central role in various aspects of iron metabolism, such as iron uptake, the biogenesis of iron-sulfur clusters and insertion of the heme cofactor into apocytochromes. Another important group of flavoenzymes is directly (Dus1-4p and Mto1p) or indirectly (Tyw1p) involved in reactions leading to tRNA-modifications. Despite the wealth of genetic information available for S. cerevisiae, we were surprised that many flavoproteins are poorly characterized biochemically. For example, the role of the yeast flavodoxins Pst2p, Rfs1p and Ycp4p with regard to their electron donor and acceptor is presently unknown. Similarly, the function of the heterodimeric Aim45p/Cir1p, which is homologous to the electron-transferring flavoproteins of higher eukaryotes, in electron transfer processes occurring in the mitochondrial matrix remains to be elucidated. This lack of information extends to the five membrane proteins involved in riboflavin or FAD transport as well as FMN and FAD homeostasis within the yeast cell. Nevertheless, several yeast flavoproteins, were identified as convenient model systems both in terms of their mechanism of action as well as structurally to improve our understanding of diseases caused by dysfunctional human flavoprotein orthologs.

From cholesterogenesis to steroidogenesis: role of riboflavin and flavoenzymes in the biosynthesis of vitamin D

Flavin-dependent monooxygenases and oxidoreductases are located at critical branch points in the biosynthesis and metabolism of cholesterol and vitamin D. These flavoproteins function as obligatory intermediates that accept 2 electrons from NAD(P)H with subsequent 1-electron transfers to a variety of cytochrome P450 (CYP) heme proteins within the mitochondria matrix (type I) and the (microsomal) endoplasmic reticulum (type II). The mode of electron transfer in these systems differs slightly in the number and form of the flavin prosthetic moiety. In the type I mitochondrial system, FAD-adrenodoxin reductase interfaces with adrenodoxin before electron transfer to CYP heme proteins. In the microsomal type II system, a diflavin (FAD/FMN)-dependent cytochrome P450 oxidoreductase [NAD(P)H-cytochrome P450 reductase (CPR)] donates electrons to a multitude of heme oxygenases. Both flavoenzyme complexes exhibit a commonality of function with all CYP enzymes and are crucial for maintaining a balance of cholesterol and vitamin D metabolites. Deficits in riboflavin availability, imbalances in the intracellular ratio of FAD to FMN, and mutations that affect flavin binding domains and/or interactions with client proteins result in marked structural alterations within the skeletal and central nervous systems similar to those of disorders (inborn errors) in the biosynthetic pathways that lead to cholesterol, steroid hormones, and vitamin D and their metabolites. Studies of riboflavin deficiency during embryonic development demonstrate congenital malformations similar to those associated with genetic alterations of the flavoenzymes in these pathways. Overall, a deeper understanding of the role of riboflavin in these pathways may prove essential to targeted therapeutic designs aimed at cholesterol and vitamin D metabolism.

Localization of mitochondrial carnitine/acylcarnitine translocase in sensory neurons from rat dorsal root ganglia

The carnitine/acylcarnitine transporter is a transport system whose function is essential for the mitochondrial β-oxidation of fatty acids. Here, the presence of carnitine/acylcarnitine carrier (CACT) in nervous tissue and its sub-cellular localization in dorsal root ganglia (DRG) neurons have been investigated. Western blot analysis using a polyclonal anti-CACT antibody produced in our laboratory revealed the presence of CACT in all the nervous tissue extracts analyzed. Confocal microscopy experiments performed on fixed and permeabilized DRG neurons co-stained with the anti-CACT antibody and the mitochondrial marker MitoTracker Red clearly showed a mitochondrial localization for the carnitine/acylcarnitine transporter. The transport activity of CACT from DRG extracts reconstituted into liposomes was about 50 % in respect to liver extracts. The experimental data here reported represent the first direct evidence of the expression of the carnitine/acylcarnitine transporter in sensory neurons, thus supporting the existence of the β-oxidation pathway in these cells.

Key residues at the riboflavin kinase catalytic site of the bifunctional riboflavin kinase/FMN adenylyltransferase from Corynebacterium ammoniagenes

Many known prokaryotic organisms depend on a single bifunctional enzyme, encoded by the RibC of RibF gene and named FAD synthetase (FADS), to convert Riboflavin (RF), first into FMN and then into FAD. The reaction occurs through the sequential action of two activities present on a single polypeptide chain where the N-terminus is responsible for the ATP:FMN adenylyltransferase (FMNAT) activity and the C-terminus for the ATP: riboflavin kinase (RFK) activity. Sequence and structural analysis suggest that T208, N210 and E268 at the C-terminus RFK module of Corynebacterium ammoniagenes FADS (CaFADS) might be key during RF phosphorylation. The effect of site-directed mutagenesis on the RFK activity, as well as on substrates and products binding, indicates that T208 and N210 provide the RFK active-site geometry for binding and catalysis, while E268 might be involved in the catalytic step as catalytic base. These data additionally suggest concerted conformational changes at the RFK module of CaFADS during its activity. Mutations at the RFK site also modulate the binding parameters at the FMNAT active site of CaFADS, altering the catalytic efficiency in the transformation of FMN into FAD. This observation supports the hypothesis that the hexameric assembly previously revealed by the crystal structure of CaFADS might play a functional role during catalysis.

A regulatory role of NAD redox status on flavin cofactor homeostasis in S. cerevisiae mitochondria

Flavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide (NAD) are two redox cofactors of pivotal importance for mitochondrial functionality and cellular redox balance. Despite their relevance, the mechanism by which intramitochondrial NAD(H) and FAD levels are maintained remains quite unclear in Saccharomyces cerevisiae. We investigated here the ability of isolated mitochondria to degrade externally added FAD and NAD (in both its reduced and oxidized forms). A set of kinetic experiments demonstrated that mitochondrial FAD and NAD(H) destroying enzymes are different from each other and from the already characterized NUDIX hydrolases. We studied here, in some detail, FAD pyrophosphatase (EC 3.6.1.18), which is inhibited by NAD(+) and NADH according to a noncompetitive inhibition, with Ki values that differ from each other by an order of magnitude. These findings, together with the ability of mitochondrial FAD pyrophosphatase to metabolize endogenous FAD, presumably deriving from mitochondrial holoflavoproteins destined to degradation, allow for proposing a novel possible role of mitochondrial NAD redox status in regulating FAD homeostasis and/or flavoprotein degradation in S. cerevisiae.

FAD synthesis and degradation in the nucleus create a local flavin cofactor pool

FAD is a redox cofactor ensuring the activity of many flavoenzymes mainly located in mitochondria but also relevant for nuclear redox activities. The last enzyme in the metabolic pathway producing FAD is FAD synthase (EC 2.7.7.2), a protein known to be localized both in cytosol and in mitochondria. FAD degradation to riboflavin occurs via still poorly characterized enzymes, possibly belonging to the NUDIX hydrolase family. By confocal microscopy and immunoblotting experiments, we demonstrate here the existence of FAD synthase in the nucleus of different experimental rat models. HPLC experiments demonstrated that isolated rat liver nuclei contain ∼300 pmol of FAD·mg(-1) protein, which was mainly protein-bound FAD. A mean FAD synthesis rate of 18.1 pmol·min(-1)·mg(-1) protein was estimated by both HPLC and continuous coupled enzymatic spectrophotometric assays. Rat liver nuclei were also shown to be endowed with a FAD pyrophosphatase that hydrolyzes FAD with an optimum at alkaline pH and is significantly inhibited by adenylate-containing nucleotides. The coordinate activity of these FAD forming and degrading enzymes provides a potential mechanism by which a dynamic pool of flavin cofactor is created in the nucleus. These data, which significantly add to the biochemical comprehension of flavin metabolism and its subcellular compartmentation, may also provide the basis for a more detailed comprehension of the role of flavin homeostasis in biologically and clinically relevant epigenetic events.

Emerging concepts in the flavinylation of succinate dehydrogenase

The Succinate Dehydrogenase (SDH) heterotetrameric complex catalyzes the oxidation of succinate to fumarate in the tricarboxylic acid (TCA) cycle and in the aerobic respiratory chains of eukaryotes and bacteria. Essential in this catalysis is the covalently-linked cofactor flavin adenine dinucleotide (FAD) in subunit1 (Sdh1) of the SDH enzyme complex. The mechanism of FAD insertion and covalent attachment to Sdh1 is unknown. Our working concept of this flavinylation process has relied mostly on foundational works from the 1990s and by applying the principles learned from other enzymes containing a similarly linked FAD. The discovery of the flavinylation factor Sdh5, however, has provided new insight into the possible mechanism associated with Sdh1 flavinylation. This review focuses on encapsulating prior and recent advances towards understanding the mechanism associated with flavinylation of Sdh1 and how this flavinylation process affects the overall assembly of SDH. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.

Detection of a quaternary organization into dimer of trimers of Corynebacterium ammoniagenes FAD synthetase at the single-molecule level and at the in cell level

Biochemical characterization of Corynebacterium ammoniagenes FADS (CaFADS) pointed to certain confusion about the stoichiometry of this bifunctional enzyme involved in the production of FMN and FAD in prokaryotes. Resolution of its crystal structure suggested that it might produce a hexameric ensemble formed by a dimer of trimers. We used atomic force microscopy (AFM) to direct imaging single CaFADS molecules bound to mica surfaces, while preserving their catalytic properties. AFM allowed solving individual CaFADS monomers, for which it was even possible to distinguish their sub-molecular individual N- and C-terminal modules in the elongated enzyme. Differences between monomers and higher stoichiometries were easily imaged, enabling us to detect formation of oligomeric species induced by ligand binding. The presence of ATP:Mg(2+) particularly induced the appearance of the hexameric assembly whose mean molecular volume resembles the crystallographic dimer of trimers. Finally, the AFM results are confirmed in cross-linking solution, and the presence of such oligomeric CaFADS species detected in cell extracts. All these results are consistent with the formation of a dimer of trimers during the enzyme catalytic cycle that might bear biological relevance.

Bacterial over-expression and purification of the 3'phosphoadenosine 5'phosphosulfate (PAPS) reductase domain of human FAD synthase: functional characterization and homology modeling

FAD synthase (FADS, EC 2.7.7.2) is a key enzyme in the metabolic pathway that converts riboflavin into the redox cofactor, FAD. Human FADS is organized in two domains: -the 3′phosphoadenosine 5′phosphosulfate (PAPS) reductase domain, similar to yeast Fad1p, at the C-terminus, and -the resembling molybdopterin-binding domain at the N-terminus. To understand whether the PAPS reductase domain of hFADS is sufficient to catalyze FAD synthesis, per se, and to investigate the role of the molybdopterin-binding domain, a soluble “truncated” form of hFADS lacking the N-terminal domain (Δ_1-328-hFADS) has been over-produced and purified to homogeneity as a recombinant His-tagged protein. The recombinant Δ_1-328-hFADS binds one mole of FAD product very tightly as the wild-type enzyme. Under turnover conditions, it catalyzes FAD assembly from ATP and FMN and, at a much lower rate, FAD pyrophosphorolytic hydrolysis. The Δ_1-328-hFADS enzyme shows a slight, but not significant, change of K_m values (0.24 and 6.23 μM for FMN and ATP, respectively) and of k_cat (4.2 × 10⁻² s⁻¹) compared to wild-type protein in the forward direction. These results demonstrate that the molybdopterin-binding domain is not strictly required for catalysis. Its regulatory role is discussed in light of changes in divalent cations sensitivity of the Δ_1-328-hFADS versus wild-type protein.

Perinatal manipulation of α-linolenic acid intake induces epigenetic changes in maternal and offspring livers

Previous studies indicated that the intake of α-linolenic acid (ALA) can alter the concentration of both ω-6 and ω-3 fatty acids in both mother and offspring, with consequences on postnatal brain development. This study describes the association between maternal ALA availability during gestation and lactation, and alterations in the Fads2 DNA methylation in both maternal and offspring livers, at the end of lactation period. Both Fads2 promoter and intron 1 DNA methylation were increased in the groups receiving postnatal flaxseed oil containing 50% ALA (mothers or pups), while bivariate analysis indicated a significant association of the Fads2 epigenetic status in the liver between each mother and its offspring. In addition, Fads2 expression was negatively correlated with promoter methylation at the individual level in maternal livers (P<0.05). This study also indicated that the interplay between ALA availability during gestation and lactation can differentially alter the expression of desaturases and elongases involved in ω-6 and ω-3 metabolic pathways. In summary, when considering the perinatal dietary ALA requirements in mice, both gestation and lactation periods should be considered as having distinct roles in modulating the metabolism of ω-6 and ω-3 fatty acids in maternal mouse livers.

Silencing of FAD synthase gene in Caenorhabditis elegans upsets protein homeostasis and impacts on complex behavioral patterns

BACKGROUND

FAD synthase is a ubiquitous enzyme that catalyses the last step of FAD biosynthesis, allowing for the biogenesis of several flavoproteins. In humans different isoforms are generated by alternative splicing, isoform 1 being localized in mitochondria. Homology searching in Caenorabditis elegans leads to the identification of two human FAD synthase homologues, coded by the single copy gene R53.1.

METHODS

The C. elegans R53.1 gene was silenced by feeding. The expression level of transcripts was established by semi-quantitative RT-PCR. Overall protein composition was evaluated by two-dimensional electrophoresis. Enzymatic activities were measured by spectrophotometry and oxygen consumption by polarography on isolated mitochondria.

RESULTS

From R53.1 two transcripts are generated by trans-splicing. Reducing by 50% the transcription efficiency of R53.1 by RNAi results in a 50% reduction in total flavin with decrease in ATP content and increase in ROS level. Significant phenotypical changes are noticed in knock-down nematodes. Among them, a significant impairment in locomotion behaviour possibly due to altered cholinergic transmission. At biochemical level, impairment of flavoenzyme activities and of some KCN-insensitive oxygen-consuming enzymes is detected. At proteomic level, at least 15 abundant proteins are affected by R53.1 gene silencing, among which superoxide dismutases.

CONCLUSION AND GENERAL SIGNIFICANCE

For the first time we addressed the existence of different isoforms of FAD-metabolizing enzymes in nematodes. A correlation between FAD synthase silencing and flavoenzyme derangement, energy shortage and redox balance impairment is apparent. In this aspect R53.1-interfered nematodes could provide an animal model system for studying human pathologies with alteration in flavin homeostasis/flavoenzyme biogenesis.

Human FAD synthase (isoform 2): a component of the machinery that delivers FAD to apo-flavoproteins

A soluble form of human FAD synthase (isoform 2; hFADS2) was produced and purified to homogeneity as a recombinant His-tagged protein. The enzyme binds 1 mole of the FAD product very tightly, although noncovalently. Complete release of FAD from the 'as isolated' protein requires extensive denaturation. A 75 : 25 mixture of apo/holoprotein could be prepared by treatment with mild chaotropes, allowing estimatation of the contribution made by bound FAD to the protein stability and evaluatation of whether structural rearrangements may be required for FAD release. Under turnover conditions, the enzyme catalyzes FAD assembly from ATP and FMN and, at a much lower rate, the pyrophosphorolytic hydrolysis of FAD. Several mechanistic features of both reactions were investigated in detail, along with their dependence on environmental conditions (pH, temperature, dependence on metals). Our data indicate that FAD release may represent the rate-limiting step of the whole catalytic cycle and that the process leading to FAD synthesis, and delivery to client apoproteins may be tightly controlled.