Structure of UDP-galactopyranose mutase bound to flavin mononucleotide
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Flavin mononucleotide
Mutase
Nicotinamide mononucleotide
Flavoprotein
Phosphoglycerate mutase
We have characterized the kinetics and substrate requirements of prenyl-flavin synthase from yeast. This enzyme catalyzes the addition of an isopentenyl unit to reduced flavin mononucleotide (FMN) to form an additional six-membered ring that bridges N5 and C6 of the flavin nucleus, thereby converting the flavin from a redox cofactor to one that supports the decarboxylation of aryl carboxylic acids. In contrast to bacterial enzymes, the yeast enzyme was found to use dimethylallyl pyrophosphate, rather than dimethylallyl phosphate, as the prenyl donor in the reaction. We developed a coupled assay for prenyl-flavin synthase activity in which turnover was linked to the activation of the prenyl-flavin-dependent enzyme, ferulic acid decarboxylase. The kinetics of the reaction are extremely slow: kcat = 12.2 ± 0.2 h–1, and KM for dimethylallyl pyrophosphate = 9.8 ± 0.7 μM. The KM for reduced FMN was too low to be accurately measured. The kinetics of reduced FMN consumption were studied under pre-steady state conditions. The reaction of FMN was described well by first-order kinetics with a kapp of 17.4 ± 1.1 h–1. These results indicate that a chemical step, most likely formation of the carbon–carbon bond between C6 of the flavin and the isopentenyl moiety, is substantially rate-determining in the reaction.
Enzyme Kinetics
Flavin mononucleotide
Decarboxylation
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Flavin dependent monooxygenases catalyze various reactions to play a key role in biological processes, such as catabolism, detoxification, and biosynthesis. Group D flavin dependent monooxygenases are enzymes with an Acyl-CoA dehydrogenase (ACAD) fold and use Flavin adenine dinucleotide (FAD) or Flavin mononucleotide (FMN) as a cofactor. In this research, crystal structures of Alicyclobacillus acidocaldarius protein formerly annotated as an ACAD were determined in Apo and FAD bound state. Although our structure showed high structural similarity to other ACADs, close comparison of substrate binding pocket and phylogenetic analysis showed that this protein is more closely related to other bacterial group D flavin dependent monooxygenases, such as DszC (sulfoxidase) and DnmZ and Kijd3 (nitrososynthases).
Flavin mononucleotide
Flavoprotein
Flavin adenine dinucleotide
Flavin-containing monooxygenase
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The structure of a putative protease from Bacteroides thetaiotaomicron features an unprecedented binding site for flavin mononucleotide. The flavin isoalloxazine ring is sandwiched between two tryptophan residues in the interface of the dimeric protein. We characterized the recombinant protein with regard to its affinity for naturally occurring flavin derivatives and several chemically modified flavin analogs. Dissociation constants were determined by isothermal titration calorimetry. The protein has high affinity to naturally occurring flavin derivatives, such as riboflavin, FMN, and FAD, as well as lumichrome, a photodegradation product of flavins. Similarly, chemically modified flavin analogs showed high affinity to the protein in the nanomolar range. Replacement of the tryptophan by phenylalanine gave rise to much weaker binding, whereas in the tryptophan to alanine variant, flavin binding was abolished. We propose that the protein is an unspecific scavenger of flavin compounds and may serve as a storage protein in vivo.
Flavin mononucleotide
Flavoprotein
Isothermal Titration Calorimetry
Dissociation constant
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Mutase
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UDP-galactose 4-epimerase catalyzes the interconversion of UDP-galactose and UDP-glucose. The enzyme from Escherichia coli is a dimeric protein with an overall molecular weight of 79,000 that contains NAD+ very tightly but noncovalently bound in the enzymatic active site. NAD+ is the coenzyme for epimerization and is transiently reduced to NADH in the course of catalysis. All samples of highly purified UDP-galactose 4-epimerase contain significant amounts of NADH, and that purified after overexpression in E. coli cells contains a substantial amount of NADH. To the degree that NADH replaces enzyme bound NAD+ in the coenzyme binding site, the epimerase activity is decreased. The extinction coefficient at 345 nm for NADH in its binding site is estimated to be 3.3 mM-1 cm-1. 31P NMR spectroscopic and enzymatic analyses reveal that UDP-glucose, UDP-galactose, UDP, and UMP are gradually released from the purified enzyme upon addition of UMP or P1-5'-uridine-P2-methyl diphosphate (MeUDP). It is concluded that NADH associated with the purified enzyme is a component of inactive, abortive complexes (E-NADH-uridine nucleotide) that contain tightly bound uridine nucleotides in place of the epimerization intermediate UDP-4-keto-alpha-D-hexoglucopyranose. These complexes are produced in vivo in the course of bacterial growth. The enzymatic activity of purified epimerase is increased by reaction with 1,2-naphthoquinone-4-sulfonate, which oxidizes the NADH to NAD+. Compositionally defined abortive complexes (E-NADH-uridine nucleotide) containing UMP, UDP, or UDP-hexoses (Glc/Gal) have been prepared in vitro and subjected to activation by 1,2-naphthoquinone-4-sulfonate. All are activated at rates comparable to that for the purified enzyme, although those containing UMP and UDP-hexose are more readily activated than those containing UDP. The activity of the reactivated enzyme approaches that of the most highly active epimerase that has been reported from E. coli.
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Flavin mononucleotide
Flavin adenine dinucleotide
Flavoprotein
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P450BM-3 is a self-sufficient fatty acid monooxygenase that can be expressed in Escherichia coli as either the holoenzyme or as the individual hemo- and flavoprotein domains. The flavoprotein domain (BMR) of P450BM-3 is soluble and contains an equimolar ratio of flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) and is functionally analogous to microsomal nicotinamide adenine dinucleotide phosphate (NADPH)−P450 reductases. These reductases have been proposed to have evolved through a fusion of genes encoding simple flavin-containing electron-transport proteins [Porter, T. D. (1991) Trends Biochem. Sci. 16, 154−158]. The gene encoding BMR has been divided into the coding regions for the FAD/NADPH- and FMN-binding domains. These proteins were overexpressed in E. coli and both domains were found to contain not less than 0.9 ± 0.05 mol of FAD or FMN/mol of protein. Compared to BMR, the electron-accepting properties of the recombinant flavin domains were mainly conserved. Titration of the FMN domain with sodium dithionite resulted in the conversion of the protein to the fully reduced FMNH2 form without accumulation of intermediate semiquinone forms; however, a similar titration of the FAD domain gave clear evidence for the presence of a neutral, blue flavin semiquinone during the reduction. Titrations of the reduced forms of the domains with artificial electron acceptors indicated that the electron-transferring properties of both the FAD- and FMN domains were also conserved. The rate constants of reoxidation of the fully reduced FAD and FMN domains by molecular oxygen at 20 °C were found to be 2.5 and 0.1 min-1, respectively. The cytochrome c reductase activity of BMR could be fully reconstituted with the individual domains. The data presented support the hypothesis that BMR has a discrete multidomain structure.
Flavoprotein
Flavin mononucleotide
Flavin adenine dinucleotide
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Nicotinamide adenine dinucleotide phosphate (NAD(P)H)-flavin oxidoreductases (flavin reductases) catalyze the reduction of flavin by NAD(P)H and provide the reduced form of flavin mononucleotide (FMN) to flavin-dependent monooxygenases. Based on bioinformatics analysis, we identified a putative flavin reductase gene, sso2055, in the genome of hyperthermophilic archaeon Sulfolobus solfataricus P2, and further cloned this target sequence into an expression vector. The cloned flavin reductase (EC. 1.5.1.30) was purified to homogeneity and characterized further. The purified enzyme exists as a monomer of 17.8 kDa, free of chromogenic cofactors. Homology modeling revealed this enzyme as a TIM barrel, which is also supported by circular dichroism measurements revealing a beta-sheet rich content. The optimal pH for SSO2055 activity was pH 6.5 in phosphate buffer and the highest activity observed was at 120 °C within the measurable temperature. We showed that this enzyme can use FMN and flavin adenine dinucleotide (FAD) as a substrate to generate their reduced forms. The purified enzyme is predicted to be a potential flavin reductase of flavin-dependent monooxygenases that could be involved in the biodesulfurization process of S. solfataricus P2.
Sulfolobus solfataricus
Flavin adenine dinucleotide
Flavin mononucleotide
Flavoprotein
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Flavin mononucleotide
Nitroreductase
Flavoprotein
Nicotinamide mononucleotide
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