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

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.

Metabolism of FAD, FMN and riboflavin (vitamin B2) in the human parasitic blood fluke Schistosoma mansoni

BACKGROUND

Schistosomiasis is a parasitic disease caused by trematodes of the genus Schistosoma. The intravascular worms acquire the nutrients necessary for their survival from host blood. Since all animals are auxotrophic for riboflavin (vitamin B2), schistosomes too must import it to survive. Riboflavin is an essential component of the coenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD); these support key functions of dozens of flavoenzymes.

METHODS

Here, using a combination of metabolomics, enzyme kinetics and in silico molecular analysis, we focus on the biochemistry of riboflavin and its metabolites in Schistosoma mansoni (Sm).

RESULTS

We show that when schistosomes are incubated in murine plasma, levels of FAD decrease over time while levels of FMN increase. We show that live schistosomes cleave exogenous FAD to generate FMN and this ability is significantly blocked when expression of the surface nucleotide pyrophosphatase/phosphodiesterase ectoenzyme SmNPP5 is suppressed using RNAi. Recombinant SmNPP5 cleaves FAD with a Km of 178 ± 5.9 µM and Kcat/Km of 324,734 ± 36,347 M- 1.S- 1. The FAD-dependent enzyme IL-4I1 drives the oxidative deamination of phenylalanine to produce phenylpyruvate and H2O2. Since schistosomes are damaged by H2O2, we determined if SmNPP5 could impede H2O2 production by blocking IL-4I1 action in vitro. We found that this was not the case; covalently bound FAD on IL-4I1 appears inaccessible to SmNPP5. We also report that live schistosomes can cleave exogenous FMN to generate riboflavin and this ability is significantly impeded when expression of a second surface ectoenzyme (alkaline phosphatase, SmAP) is suppressed. Recombinant SmAP cleaves FMN with a Km of 3.82 ± 0.58 mM and Kcat/Km of 1393 ± 347 M- 1.S- 1.

CONCLUSIONS

The sequential hydrolysis of FAD by tegumental ecto-enzymes SmNPP5 and SmAP can generate free vitamin B2 around the worms from where it can be conveniently imported by the recently described schistosome riboflavin transporter SmaRT. Finally, we identified in silico schistosome homologs of enzymes that are involved in intracellular vitamin B2 metabolism. These are riboflavin kinase (SmRFK) as well as FAD synthase (SmFADS); cDNAs encoding these two enzymes were cloned and sequenced. SmRFK is predicted to convert riboflavin to FMN while SmFADS could further act on FMN to regenerate FAD in order to facilitate robust vitamin B2-dependent metabolism in schistosomes.

Metabolism of FAD, FMN and riboflavin (vitamin B2) in the human parasitic blood fluke Schistosoma mansoni

Schistosomiasis is a parasitic disease caused by trematode worms of the genus Schistosoma. The intravascular worms acquire the nutrients necessary for their survival from host blood. Since all animals are auxotrophic for riboflavin (vitamin B2), schistosomes too must import it to survive. Riboflavin is an essential component of the coenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD); these support key functions of dozens of flavoenzymes. In this work we focus on the biochemistry of riboflavin and its metabolites in Schistosoma mansoni. We show that when schistosomes are incubated in murine plasma, levels of FAD decrease over time while the levels of FMN increase. We show that live schistosomes can cleave exogenous FAD to generate FMN and this ability is significantly blocked when expression of the surface ectoenzyme SmNPP5 is suppressed using RNAi. Recombinant SmNPP5 cleaves FAD with a Km of 178 ± 5.9 µM. The FAD-dependent enzyme IL-4I1 drives the oxidative deamination of phenylalanine to produce phenylpyruvate and H2O2 in the extracellular environment. Since schistosomes can be damaged by H2O2, we determined if SmNPP5 could impede H2O2 production by blocking IL-4I1 action in vitro. We found that this was not the case, suggesting that covalently bound FAD on IL-4I1 is inaccessible to SmNPP5. We also report here that live schistosomes can cleave exogenous FMN to generate riboflavin and this ability is significantly impeded when expression of a second surface ectoenzyme (alkaline phosphatase, SmAP) is suppressed. Recombinant SmAP cleaves FMN with a Km of 3.82 ± 0.58 mM. Thus, the sequential hydrolysis of FAD by tegumental ecto-enzymes SmNPP5 and SmAP can generate free vitamin B2 around the worms from where it can be conveniently imported by, we hypothesize, the recently described schistosome riboflavin transporter SmaRT. In this work we also identified in silico schistosome homologs of enzymes that are involved in intracellular vitamin B2 metabolism. These are riboflavin kinase (SmRFK) as well as FAD synthase (SmFADS); cDNAs encoding these two enzymes were cloned and sequenced. SmRFK is predicted to convert riboflavin to FMN while SmFADS could further act on FMN to regenerate FAD in order to facilitate robust vitamin B2-dependent metabolism in schistosomes.

Regulation of respiratory complex I assembly by FMN cofactor targeting

Respiratory complex I plays a crucial role in the mitochondrial electron transport chain and shows promise as a therapeutic target for various human diseases. While most studies focus on inhibiting complex I at the Q-site, little is known about inhibitors targeting other sites within the complex. In this study, we demonstrate that diphenyleneiodonium (DPI), a N-site inhibitor, uniquely affects the stability of complex I by reacting with its flavin cofactor FMN. Treatment with DPI blocks the final stage of complex I assembly, leading to the complete and reversible degradation of complex I in different cellular models. Growing cells in medium lacking the FMN precursor riboflavin or knocking out the mitochondrial flavin carrier gene SLC25A32 results in a similar complex I degradation. Overall, our findings establish a direct connection between mitochondrial flavin homeostasis and complex I stability and assembly, paving the way for novel pharmacological strategies to regulate respiratory complex I.

New insights into the nutritional genomics of adult-onset riboflavin-responsive diseases

Riboflavin, or vitamin B2, is an essential nutrient that serves as a precursor to flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN). The binding of the FAD and/or FMN cofactors to flavoproteins is critical for regulating their assembly and activity. There are over 90 proteins in the human flavoproteome that regulate a diverse array of biochemical pathways including mitochondrial metabolism, riboflavin transport, ubiquinone and FAD synthesis, antioxidant signalling, one-carbon metabolism, nitric oxide signalling and peroxisome oxidative metabolism. The identification of patients with genetic variants in flavoprotein genes that lead to adult-onset pathologies remains a major diagnostic challenge. However, once identified, many patients with adult-onset inborn errors of metabolism demonstrate remarkable responses to riboflavin therapy. We review the structure:function relationships of mutant flavoproteins and propose new mechanistic insights into adult-onset riboflavin-responsive pathologies and metabolic dysregulations that apply to multiple biochemical pathways. We further address the vexing issue of how the inheritance of genetic variants in flavoprotein genes leads to an adult-onset disease with complex symptomologies and varying severities. We also propose a broad clinical framework that may not only improve the current diagnostic rates, but also facilitate a personalized approach to riboflavin therapy that is low cost, safe and lead to transformative outcomes in many patients.

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.

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.

Flavin Adenine Dinucleotide (FAD) Pegylated (PEG)-Complexes: Proof of Concept (PoC) of theranostic tool on a Murine Breast Cancer Model

Flavin adenine dinucleotide (FAD) plays a key role in an extensive range of cellular oxidation-reduction reactions, which is engaged in metabolic pathways. The purpose of this study was to realize pegylated flavins formulation, named FAD and FAD-PEG diacid complex as theranostic tool in cancer therapy. For this objective, a murine breast cancer model, which was induced by mouse-derived4T1 breast cancer cells was studied to assess the therapeutic efficacy of FAD (named NP1) and FAD-PEG diacid complex (named NP2). The cytokines were monitored to evaluate the serum inflammatory factors to develop the blood cell content of different groups of nude mice. The experimental model shows that an intravenous injection of FAD (NP1) can significantly reduce tumour volume, tumour index and thymus index, and decrease neutrophils (NE), monocytes (MO), eosinophils (EO), and basophils (BA). At the same time, the content of IL-1α, IL-12P70, TNF α, IL-1β and IL-6 was significantly reduced, and the content of IL-10 was significantly increased. These results provide the proof-of-concept for FAD as a smart adjuvant for cancer therapy and encourages their further development in the field of Nanomedicine.

Infant with early onset bilateral facial and bulbar weakness: Successful treatment of riboflavin in multiple acyl-CoA dehydrogenase deficiency caused by biallelic nonsense FLAD1 variants

Multiple acyl-CoA dehydrogenase deficiency (MADD) is a heterogeneous group of inborn error of metabolic disease affecting the oxidation of fatty acids and amino acids, and choline metabolism. Genes involved in electrons transfer to the mitochondrial respiratory chain typically induce MADD. Recently, FLAD1, which encodes flavin adenine dinucleotide synthase, has also been reported as a cause of MADD. Here, we present a case of a 28-month girl with progressive weakness in facial and bulbar muscle. She has been suffering from feeding difficulty and recurrent respiratory distress. Lipid storage myopathy was evident from muscle biopsy. Furthermore, whole exome sequencing identified homozygous variant of c.745C > T (p.Arg249*) in FLAD1, confirming the diagnosis of FLAD1-related MADD. The patient showed improvements in her symptoms and exhibited catch-up growth following the supplementation of riboflavin. Lipid storage myopathy with FLAD1-related MADD is potentially treatable. Therefore, we should have high clinical suspicion, even though the diagnosis is challenging.

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.

A Pegylated Flavin Adenine Dinucleotide PEG Complex to Boost Immunogenic and Therapeutic Effects in a Liver Cancer Model

Flavin adenine dinucleotide (FAD) is engaged in several metabolic diseases. Its main role is being a cofactor essential for the activity of many flavoproteins, which play a crucial role in electron transport pathways in living systems. The aim of this study was to apply a pegylated flavins formulation named FAD-PEG diacide complex as theranostic pathway in cancer therapy. For this purpose, a mouse liver cancer model induced by Hepa1-6 cells was used to evaluate the therapeutic efficacy of FAD (named NP1) and FAD-PEG diacide complex (named NP2). The cytokines were applied to screen the serum inflammatory factors, to establish the blood cell content of different groups of nude mice. The highlights follows that FAD formulations (NP1; NP2) significantly suppressed the tumor growth and reduced the tumor index without effects on the body weight of mice. Furthermore, NP2 significantly reduced the serum levels of cytokines IL-6, TNF-α and IL-12 (P70). The reported results provide the proof-of-concept for the synthesis of a smart adjuvant for liver cancer therapy and support their further development in the field of nanomedicine.

Continuous and Discontinuous Approaches to Study FAD Synthesis and Degradation Catalyzed by Purified Recombinant FAD Synthase or Cellular Fractions

Riboflavin, or vitamin B2, is the precursor of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), essential redox (and sometimes non-redox) cofactors of a large number of flavoenzymes involved in energetic metabolism, protein folding, apoptosis, chromatin remodeling, and a number of other cell regulatory processes.The cellular and subcellular steady-state concentrations of flavin cofactors, which are available for flavoprotein biogenesis and assembly, depend on carrier-mediated transport processes and on coordinated synthesizing/destroying enzymatic activities, catalyzed by enzymes whose catalytic and structural properties are still matter of investigation.Alteration of flavin homeostasis has been recently correlated to human pathological conditions, such as neuromuscular disorders and cancer, and therefore we propose here protocols useful to detect metabolic processes involved in FAD forming and destroying.Our protocols exploit the chemical-structural differences between riboflavin, FMN , and FAD , which are responsible for differences in the spectroscopic properties (mainly fluorescence) of the two cofactors (FMN and FAD); therefore, in our opinion, when applicable measurements of fluorescence changes in continuo represent the elective techniques to follow FAD synthesis and degradation. Thus, after procedures able to calibrate flavin concentrations (Subheading 3.1), we describe simple continuous and rapid procedures, based on the peculiar optical properties of free flavins, useful to determine the rate of cofactor metabolism catalyzed by either recombinant enzymes or natural enzymes present in cellular lysates/subfractions (Subheading 3.2).Fluorescence properties of free flavins can also be useful in analytical determinations of the three molecular flavin forms, based on HPLC separation, with a quite high sensitivity. Assaying at different incubation times the molecular composition of the reaction mixture is a discontinuous experimental approach to measure the rate of FAD synthesis/degradation catalyzed by cell lysates or recombinant FAD synthase (Subheading 3.3). Continuous and discontinuous approaches can, when necessary, be performed in parallel.

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.

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.

FLAD1 is up-regulated in Gastric Cancer and is a potential prediction of prognosis

Background: Gastric cancer (GC) is a common malignancy throughout the world. Biomarkers for prognosis and risk evaluation of GC are rapidly discovered. We investigated the prognostic role of FLAD1, an important protein-coding gene that affects cell cycle and survival. Methods: The expression of FLAD1 at mRNA levels in GC tumor tissues and normal tissues was mined and analyzed in Oncomine database and verified in 10 pairs of GS tissues and their adjacent normal tissues in our center by RT qPCR. The FLAD1 protein expression were detected in 106 paraffin-embedded GC tissues by immunohistochemistry (IHC). Statistical analyses were applied to evaluate the clinical significance of FLAD1. The prognostic value of FLAD1 mRNA expression was also analyzed using the Kaplan-Meier plotter (www.kmplot.com). Results: Statistics obtained from online database suggested FLAD1 mRNA was overexpressed in GC tissues. The results were further validated in 10 pairs of GS tissues and adjacent normal tissues in our center (p=0.021). IHC and survival analysis of GC samples from 106 patients showed FLAD1 was overexpressed in 63/106 (59.4%) patients and was associated to higher TNM stage (p=0.026). Multivariate analysis revealed FLAD1 was an independent prognostic factor for GC (p < 0.001). Furthermore, FLAD1 mRNA was associated to unfavorable overall survival (OS), first progression (FP), and post-progression survival (PPS) of GC (p<0.001). Conclusion: FLAD1 in GC is overexpressed at both mRNA and protein level and could be a potential biomarker for GC prognosis.

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 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.

Identification of Fatty Acid Desaturase 6 in Golden Pompano Trachinotus Ovatus (Linnaeus 1758) and Its Regulation by the PPARαb Transcription Factor

Fatty acid desaturases are rate-limiting enzymes in long-chain polyunsaturated fatty acid biosynthesis. The transcription factor peroxisome proliferator-activated receptor alpha b (PPARαb) regulates lipid metabolism in mammals, however, the mechanism whereby PPARαb regulates fatty acid desaturases is largely unknown in fish. In this study, we report the full length cDNA sequence of Trachinotus ovatus fatty acid desaturase, which encodes a 380 amino acid polypeptide, possessing three characteristic histidine domains. Phylogenetic and gene exon/intron structure analyses showed typical phylogeny: the T. ovatus fatty acid desaturase contained a highly conserved exon/intron architecture. Moreover, functional characterization by heterologous expression in yeast indicated that T. ovatus desaturase was a fatty acid desaturase, with Δ4/Δ5/Δ8 Fad activity. Promoter activity assays indicated that ToFads6 desaturase transcription was positively regulated by PPARαb. Similarly, PPARαb RNA interference decreased ToPPARαb and ToFads6 expression at the mRNA and protein levels in a time-dependent manner. Mutation analyses showed that the M2 binding site of PPARαb was functionally important for protein binding, and transcriptional activity of the ToFads6 promoter was significantly decreased after targeted mutation of M2. Electrophoretic mobile shift assays confirmed that PPARαb interacted with the binding site of the ToFads6 promoter region, to regulate ToFads6 transcription. In summary, PPARαb played a vital role in ToFads6 regulation and may promote the biosynthesis of long-chain polyunsaturated fatty acids by regulating ToFads6 expression.

A Novel Truncating FLAD1 Variant, Causing Multiple Acyl-CoA Dehydrogenase Deficiency (MADD) in an 8-Year-Old Boy

Multiple acyl-CoA dehydrogenase deficiency (MADD) or glutaric aciduria type II (GAII) is a clinically heterogeneous disorder affecting fatty acid and amino acid metabolism. Presentations range from a severe neonatal form with hypoglycemia, metabolic acidosis, and hepatomegaly with or without congenital anomalies to later-onset lipid storage myopathy. Genetic testing for MADD traditionally comprises analysis of ETFA, ETFB, and ETFDH. Patients may respond to pharmacological doses of riboflavin, particularly those with late-onset MADD due to variants in ETFDH. Increasingly other genes involved in riboflavin transport and flavoprotein biosynthesis are recognized as causing a MADD phenotype. Flavin adenine dinucleotide synthase (FADS) deficiency caused by biallelic variants in FLAD1 has been identified in nine previous cases of MADD. FLAD1 missense mutations have been associated with a riboflavin-responsive phenotype; however the effect of riboflavin with biallelic loss of function FLAD1 mutations required further investigation. Herein we describe a novel, truncating variant in FLAD1 causing MADD in an 8-year-old boy. Fibroblast studies showed a dramatic reduction in FADS protein with corresponding reduction in the FAD synthesis rate and FAD cellular content, beyond that previously documented in FLAD1-related MADD. There was apparent biochemical and clinical response to riboflavin treatment, beyond that previously reported in cases of biallelic loss of function variants in FLAD1. Early riboflavin treatment may have attenuated an otherwise severe phenotype.

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.

Discovery of antimicrobial compounds targeting bacterial type FAD synthetases

The increase of bacterial strains resistant to most of the available antibiotics shows a need to explore novel antibacterial targets to discover antimicrobial drugs. Bifunctional bacterial FAD synthetases (FADSs) synthesise the flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). These cofactors act in vital processes as part of flavoproteins, making FADS an essential enzyme. Bacterial FADSs are potential antibacterial targets because of differences to mammalian enzymes, particularly at the FAD producing site. We have optimised an activity-based high throughput screening assay targeting Corynebacterium ammoniagenes FADS (CaFADS) that identifies inhibitors of its different activities. We selected the three best high-performing inhibitors of the FMN:adenylyltransferase activity (FMNAT) and studied their inhibition mechanisms and binding properties. The specificity of the CaFADS hits was evaluated by studying also their effect on the Streptococcus pneumoniae FADS activities, envisaging differences that can be used to discover species-specific antibacterial drugs. The antimicrobial effect of these compounds was also evaluated on C. ammoniagenes, S. pneumoniae, and Mycobacterium tuberculosis cultures, finding hits with favourable antimicrobial properties.

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.