A 2-oxoglutarate-dependent dioxygenase is integrated in DIMBOA-biosynthesis

Benzoxazinoids are secondary metabolites of grasses that function as natural pesticides. While many steps of DIMBOA biosynthesis have been elucidated, the mechanism of the introduction of OCH(3)-group at the C-7 position was unknown. Inhibitor experiments in Triticum aestivum and Zea mays suggest that a 2-oxoglutarate-dependent dioxygenase catalyses the hydroxylation reaction at C-7. Cloning and reverse genetics analysis have identified the Bx6 gene that encodes this enzyme. Bx6 is located in the Bx-gene cluster of maize.     

A nonradioactive restriction enzyme-mediated assay to detect DNA repair by Fe(II)/2-oxoglutarate-dependent dioxygenase

The Escherichia coli DNA repair enzyme AlkB belongs to the Fe(II)/2-oxoglutarate-dependent dioxygenase family. It removes methyl groups from 1-methyl adenine (1-meA) and 3-methyl cytosine (3-meC) lesions present in single-stranded DNA by oxidative decarboxylation. In the current article, we describe an in vitro assay that permits rapid detection of AlkB activity. To achieve this, we generated methylated oligonucleotide using methyl methanesulfonate and then monitored DNA repair using a methylation-sensitive restriction enzyme and novel agarose gel electrophoresis system capable of resolving small oligonucleotides. Our approach overcomes several drawbacks of NAD⁺-dependent formaldehyde dehydrogenase-coupled assay and radioisotope-based assay for determining AlkB DNA repair activity.     

Hyoscyamine 6beta-hydroxylase, a 2-oxoglutarate-dependent dioxygenase, in alkaloid-producing root cultures

Root cultures of various solanaceous plants grow well in vitro and produce large amounts of tropane alkaloids. Enzyme activity that converts hyoscyamine to 6β-hydroxyhyoscyamine is present in cell-free extracts from cultured roots of Hyoscyamus niger L. The enzyme hyoscyamine 6β-hydroxylase was purified 3.3-fold and characterized. The hydroxylation reaction has absolute requirements for hyoscyamine, 2-oxoglutarate, Fe²⁺ ions and molecular oxygen, and ascorbate stimulates this reaction. Only the l-isomer of hyoscyamine serves as a substrate; d-hyoscyamine is nearly inactive. Comparisons were made with a number of root, shoot, and callus cultures of the Atropa, Datura, Duboisia, Hyoscyamus, and Nicotiana species for the presence of the hydroxylase activity. Decarboxylation of 2-oxoglutarate during the conversion reaction was studied using [1-¹⁴C]-2-oxoglutarate. A 1:1 stoichiometry was shown between the hyoscyamine-dependent formation of CO₂ from 2-oxoglutarate and the hydroxylation of hyoscyamine. Therefore, the enzyme can be classified as a 2-oxoglutarate-dependent dioxygenase (EC 1.14.11.-). Both the supply of hyoscyamine and the hydroxylase activity determine the amounts of 6β-hydroxyhyoscyamine and scopolamine produced in alkaloid-producing cultures.     

A novel 2-oxoglutarate-dependent dioxygenase involved in vindoline biosynthesis: characterization,purification and kinetic properties

The enzyme, desacetoxyvindoline 4-hydroxylase, was purified to apparent homogeneity from Catharanthus roseus by ammonium sulfate precipitation and successive chromatography on Sephadex G-100, green 19-agarose, hydroxylapatite, -kg sepharose and Mono Q. The 4-hydroxylase was characterized by its strict specificity for position 4 of desacetoxyvindoline suggesting it to catalyze the second to last step in vindoline biosynthesis. The molecular mass of the native and denatured 4-hydroxylase was 45 kDa and 44.7 kDa, respectively, suggesting that the native enzyme is a monomer. Two-dimensional isoelectric focusing under denaturing conditions resolved the purified 4-hydroxylase into three charge isoforms of pIs 4.6, 4.7 and 4.8. The purified 4-hydroxylase exhibited no requirement for divalent cations, but inactive enzyme was reactivated in a time-dependent manner by incubation with ferrous ions. The enzyme was not inhibited by EDTA or SH-group reagents at concentrations up to 10 mM. The mechanism of action of desacetoxyvindoline 4-hydroxylase was investigated. The results of substrate interaction kinetics and product inhibition studies suggest an Ordered Ter Ter mechanism where -kg is the first substrate to bind followed by the binding of O₂ and desacetoxyvindoline. Their K _m values for -kg, O₂ and desacetoxyvindoline are 45 M, 45 M and 0.03 M, respectively. The first product to be released was deacetylvindoline followed by CO₂ and succinate, respectively.Abbreviations -kg -ketoglutarate or 2-oxoglutarate - NMT N-methyltransferase - SAM S-adenosyl-l-methionine - TLC thin layer chromatography - VBL vinblastine - VCR vincristine     

Scopoletin is biosynthesized via ortho-hydroxylation of feruloyl CoA by a 2-oxoglutarate-dependent dioxygenase in Arabidopsis thaliana

SUMMARY: Coumarins are derived via the phenylpropanoid pathway in plants. The 2H-1-benzopyran-2-one core structure of coumarins is formed via the ortho-hydroxylation of cinnamates, trans/cis isomerization of the side chain, and lactonization. Ortho-hydroxylation is a key step in coumarin biosynthesis as a branch point from lignin biosynthesis; however, ortho-hydroxylation of cinnamates is not yet fully understood. In this study, scopoletin biosynthesis was explored using Arabidopsis thaliana, which accumulates scopoletin and its beta-glucopyranoside scopolin in its roots. T-DNA insertion mutants of caffeoyl CoA O-methyltransferase 1 (CCoAOMT1) showed significant reduction in scopoletin and scopolin levels in the roots, and recombinant CCoAOMT1 exhibited 3'-O-methyltransferase activity on caffeoyl CoA to feruloyl CoA. These results suggest that feruloyl CoA is a key precursor in scopoletin biosynthesis. Ortho-hydroxylases of cinnamates were explored in the oxygenase families in A. thaliana, and one of the candidate genes in the Fe(II)- and 2-oxoglutarate-dependent dioxygenase (2OGD) family was designated as F6'H1. T-DNA insertion mutants of F6'H1 showed severe reductions in scopoletin and scopolin levels in the roots. The pattern of F6'H1 expression is consistent with the patterns of scopoletin and scopolin accumulation. The recombinant F6'H1 protein exhibited ortho-hydroxylase activity for feruloyl CoA (K(m) = 36.0 +/- 4.27 microM; k(cat) = 11.0 +/- 0.45 sec(-1)) to form 6'-hydroxyferuloyl CoA, but did not hydroxylate ferulic acid. These results indicate that Fe(II)- and 2-oxoglutarate-dependent dioxygenase is the pivotal enzyme in the ortho-hydroxylation of feruloyl CoA in scopoletin biosynthesis.     

Formation of novel flavonoids in apple (Malusxdomestica) treated with the 2-oxoglutarate-dependent dioxygenase inhibitor prohexadione-Ca

Novel flavonoids were formed in young leaves of apple (Malusxdomestica) after treatment with the dioxygenase inhibitor prohexadione-Ca, which is known to reduce the incidence and severity of fire blight caused by Erwinia amylovora and other plant diseases. The compounds were isolated and identified as luteoliflavan, luteoliflavan 5-glucoside, eriodictyol 7-glucoside and 6"-O-trans-p-coumaroyleriodictyol 3'-glucoside. These flavonoids represent a novel biosynthetic pathway in apple leading to the formation of 3-deoxyflavans. Concomitantly, the content of regularly occurring phenylpropanoids is also influenced by prohexadione-Ca with increasing amounts of hydroxycinnamic acids and decreasing flavan-3-ols and flavonols. The altered flavonoid metabolism may be related to the lowered pathogen incidence though the isolated novel flavonoids do not exhibit antibacterial activity.     

A fluorescence-based assay for 2-oxoglutarate-dependent oxygenases

A widely used generic assay for 2-oxoglutarate-dependent oxygenases relies upon monitoring the release of 14CO2 from labeled [1-14C]-2-oxoglutarate. We report an alternative assay in which depletion of 2-oxoglutarate is monitored by its postincubation derivatization with o-phenylenediamine to form a product amenable to fluorescence analysis. The utility of the procedure is demonstrated by assays with hypoxia-inducible factor hydroxylases where it was shown to give results similar to those reported with the radioactive assay, but it is more efficient and readily adapted to a multiwell format. The process should be amenable to the assay of other 2-oxoglutarate-consuming enzymes and to the discovery of inhibitors.     

Three 2-oxoglutarate-dependent dioxygenase activities of Equisetum arvense L. forming flavone and flavonol from (2S)-naringenin

Equisetum arvense L. (Equisetaceae-horsetail) accumulates various flavones and flavonols in infertile shoot. Enzyme assays conducted with crude extracts of the green tissue revealed chalcone synthase activity and also three further activities assigned to flavonoid biosynthesis and identified as flavone synthase I, flavanone 3β-hydroxylase and flavonol synthase. The latter three activities were characterized as soluble, 2-oxoglutarate-dependent dioxygenases by their typical cofactor requirements and peculiar inhibition. Notably, this is the first report of flavone synthase I which had been considered to be restricted solely to species of the Apiaceae from a distant plant taxon.     

Molecular cloning of hyoscyamine 6 beta-hydroxylase, a 2-oxoglutarate-dependent dioxygenase, from cultured roots of Hyoscyamus niger

Roots of several solanaceous plants produce anticholinergic alkaloids, hyoscyamine and scopolamine. Hyoscyamine 6 beta-hydroxylase, a 2-oxoglutarate-dependent dioxygenase (EC 1.14.11.11), catalyzes hydroxylation of hyoscyamine in the biosynthetic pathway leading to scopolamine. We report here on the isolation of cDNA clones encoding the hydroxylase from a cDNA library made from mRNA of the cultured roots of Hyoscyamus niger. The library was screened with three synthetic oligonucleotides that encode amino acid sequences of internal peptide fragments of the purified hydroxylase. Nucleotide sequence analysis of the cloned cDNA revealed an open reading frame that encodes 344 amino acids (Mr = 38,999). All 12 internal peptide fragments determined in the purified enzyme were found in the amino acid sequence deduced from the cDNA. With computer-aided comparison to other proteins we found that the hydroxylase is homologous to two synthases involved in the biosynthesis of beta-lactam antibiotics in some microorganisms and the gene products of tomato pTOM13 cDNA and maize A2 locus which had been proposed to catalyze oxidative reactions in the biosynthesis of ethylene and anthocyan, respectively. RNA blotting hybridization showed that mRNA of the hydroxylase is abundant in cultured roots and present in plant roots, but absent in leaves, stems, and cultured cells of H. niger.     

A 2-oxoglutarate-dependent dioxygenase from Ruta graveolens L. exhibits p-coumaroyl CoA 2'-hydroxylase activity (C2'H): a missing step in the synthesis of umbelliferone in plants

Coumarins are important compounds that contribute to the adaptation of plants to biotic or abiotic stresses. Among coumarins, umbelliferone occupies a pivotal position in the plant phenylpropanoid network. Previous studies indicated that umbelliferone is derived from the ortho‐hydroxylation of p‐coumaric acid by an unknown biochemical step to yield 2,4‐dihydroxycinnamic acid, which then undergoes spontaneous lactonization. Based on a recent report of a gene encoding a 2‐oxoglutarate‐dependent dioxygenase from Arabidopsis thaliana that exhibited feruloyl CoA 6′‐hydroxylase activity ( Bourgaud et al., 2006 ), we combined a bioinformatic approach and a cDNA library screen to identify an orthologous ORF (Genbank accession number JF799117 ) from Ruta graveolens L. This ORF shares 59% amino acid identity with feruloyl CoA 6′‐hydroxylase, was functionally expressed in Escherichia coli, and converted feruloyl CoA into scopoletin and p‐coumaroyl CoA into umbelliferone with equal activity. Its bi‐functionality was further confirmed in planta: transient expression of JF799117 in Nicotiana benthamiana yielded plants with leaves containing high levels of umbelliferone and scopoletin when compared to control plants, which contained barely detectable traces of these compounds. The expression of JF799117 was also tightly correlated to the amount of umbelliferone that was found in UV‐elicited R. graveolens leaves. Therefore, JF799117 encodes a p‐coumaroyl CoA 2′‐hydroxylase in R. graveolens, which represents a previously uncharacterized step in the synthesis of umbelliferone in plants. Psoralen, which is an important furanocoumarin in R. graveolens, was found to be a competitive inhibitor of the enzyme, and it may exert this effect through negative feedback on the enzyme at an upstream position in the pathway.     

A manganese-dependent dioxygenase from Arthrobacter globiformis CM-2 belongs to the major extradiol dioxygenase family.

Almost all bacterial ring cleavage dioxygenases contain iron as the catalytic metal center. We report here the first available sequence for a manganese-dependent 3,4-dihydroxyphenylacetate (3,4-DHPA) 2,3-dioxygenase and its further characterization. This manganese-dependent extradiol dioxygenase from Arthrobacter globiformis CM-2, unlike iron-dependent extradiol dioxygenases, is not inactivated by hydrogen peroxide. Also, ferrous ions, which activate iron extradiol dioxygenases, inhibit 3,4-DHPA 2,3-dioxygenase. The gene encoding 3,4-DHPA 2,3-dioxygenase, mndD, was identified from an A. globiformis CM-2 cosmid library. mndD was subcloned as a 2.0-kb SmaI fragment in pUC18, from which manganese-dependent extradiol dioxygenase activity was expressed at high levels in Escherichia coli. The mndD open reading frame was identified by comparison with the known N-terminal amino acid sequence of purified manganese-dependent 3,4-DHPA 2,3-dioxygenase. Fourteen of 18 amino acids conserved in members of the iron-dependent extradiol dioxygenase family are also conserved in the manganese-dependent 3,4-DHPA 2,3-dioxygenase (MndD). Thus, MndD belongs to the extradiol family of dioxygenases and may share a common ancestry with the iron-dependent extradiol dioxygenases. We propose the revised consensus primary sequence (G,T,N,R)X(H,A)XXXXXXX(L,I,V,M,F)YXX(D,E,T,N,A)PX(G,P) X(2,3)E for this family. (Numbers in brackets indicate a gap of two or three residues at this point in the sequence.) The suggested common ancestry is also supported by sequence obtained from genes flanking mndD, which share significant sequence identity with xylJ and xylG from Pseudomonas putida.     

A helical lid converts a sulfotransferase to a dehydratase

We report here the crystal structure of retinol dehydratase, an enzyme that catalyzes the synthesis of anhydroretinol. The enzyme is a member of the sulfotransferase superfamily and its crystal structure reveals the insertion of a helical lid into a canonical sulfotransferase fold. Site-directed mutations demonstrate that this inserted lid is necessary for anhydroretinol production but not for sulfonation; thus, insertion of a helical lid can convert a sulfotransferase into a dehydratase.     

A putative Fe2+-bound persulfenate intermediate in cysteine dioxygenase

The common reactions of dioxygen, superoxide, and hydroperoxides with thiolates are thought to proceed via persulfenate intermediates, yet these have never been visualized. Here we report a 1.4 A resolution crystal structure of the Fe(2+)-dependent enzyme cysteine dioxygenase (CDO) containing this putative intermediate trapped in its active site pocket. The complex raises the possibility that, distinct from known dioxygenases and proposed CDO mechanisms, the Fe-proximal oxygen atom may be involved in the primary oxidation event yielding a unique three-membered Fe-S-O cyclic intermediate. A nonpolar environment of the distal oxygen would facilitate isomerization of the persulfenate to the sulfinate product.     

A sorbitol oxidase that converts sorbitol to glucose in apple leaf

A new type of sorbitol oxidase which converts sorbitol to glucosein the absence of NAD or NADP was found in the leaf of the apple.The partially purified enzyme consumed 1/2 mole of oxygen inconverting one mole of sorbitol to glucose (sorbitol+1/2 O₂ glucose+H₂O). The enzyme had its optimum activity at pH 4.0,and it had a Km value of 100 mM for sorbitol and showed a weakdependency on divalent cations. This sorbitol oxidase seemsto play an important role in the utilization of accumulatedsorbitol. ¹This paper is contribution A-109, Fruit Tree Research Station (Received March 15, 1980; )     

HGF converts ErbB2/Neu epithelial morphogenesis to cell invasion

Activation of the hepatocyte growth factor receptor Met induces a morphogenic response and stimulates the formation of branching tubules by Madin-Darby canine kidney (MDCK) epithelial cells in three-dimensional cultures. A constitutively activated ErbB2/Neu receptor, NeuNT, promotes a similar invasive morphogenic program in MDCK cells. Because both receptors are expressed in breast epithelia, are associated with poor prognosis, and hepatocyte growth factor (HGF) is expressed in stroma, we examined the consequence of cooperation between these signals. We show that HGF disrupts NeuNT-induced epithelial morphogenesis, stimulating the breakdown of cell-cell junctions, dispersal, and invasion of single cells. This correlates with a decrease in junctional proteins claudin-1 and E-cadherin, in addition to the internalization of the tight junction protein ZO-1. HGF-induced invasion of NT-expressing cells is abrogated by pretreatment with a pharmacological inhibitor of the mitogen-activated protein kinase kinase (MEK) pathway, which restores E-cadherin and ZO-1 at cell-cell junctions, establishing the involvement of MEK-dependent pathways in this process. These results demonstrate that physiological signals downstream from the HGF/Met receptor synergize with ErbB2/Neu to enhance the malignant phenotype, promoting the breakdown of cell-cell junctions and enhanced cell invasion. This is particularly important for cancers where ErbB2/Neu is overexpressed and HGF is a physiological growth factor found in the stroma.     

A novel alpha-D-galactosynthase from Thermotoga maritima converts beta-D-galactopyranosyl azide to alpha-galacto-oligosaccharides

The large-scale production of oligosaccharides is a daunting task, hampering the study of the role of glycans in vivo and the testing of the efficacy of novel glycan-based drugs. Glycosynthases, mutated glycosidases that synthesize oligosaccharides in high yields, are becoming important chemo-enzymatic tools for the production of oligosaccharides. However, while β-glycosynthase can be produced with a rather well-established technology, examples of α-glycosynthases are thus far limited only to enzymes from glycoside hydrolase 29 (GH29), GH31 and GH95 families. α-L-Fucosynthases from GH29 use convenient glycosyl azide derivatives as a strategic alternative to glycosyl fluoride donors. However, the general applicability of this method to other α-glycosynthases is not trivial and remains to be confirmed. Here, β-D-galactopyranosyl azide was converted to α-galacto-oligosaccharides with good yields and high regioselectivity, catalyzed by a novel α-galactosynthase based on the GH36 α-galactosidase from the hyperthermophilic bacterium Thermotoga maritima. These results open a new avenue to the practical synthesis of biologically interesting α-galacto-oligosaccharides and demonstrate more widespread use of β-glycosyl-azide as donors, confirming their utility to expand the repertoire of glycosynthases.     

A single amino acid substitution in CFTR converts ATP to an inhibitory ligand

Cystic fibrosis (CF), one of the most common lethal genetic diseases, is caused by loss-of-function mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which encodes a chloride channel that, when phosphorylated, is gated by ATP. The third most common pathogenic mutation, a glycine-to-aspartate mutation at position 551 or G551D, shows a significantly decreased open probability (P_o) caused by failure of the mutant channel to respond to ATP. Recently, a CFTR-targeted drug, VX-770 (Ivacaftor), which potentiates G551D-CFTR function in vitro by boosting its P_o, has been approved by the FDA to treat CF patients carrying this mutation. Here, we show that, in the presence of VX-770, G551D-CFTR becomes responsive to ATP, albeit with an unusual time course. In marked contrast to wild-type channels, which are stimulated by ATP, sudden removal of ATP in excised inside-out patches elicits an initial increase in macroscopic G551D-CFTR current followed by a slow decrease. Furthermore, decreasing [ATP] from 2 mM to 20 µM resulted in a paradoxical increase in G551D-CFTR current. These results suggest that the two ATP-binding sites in the G551D mutant mediate opposite effects on channel gating. We introduced mutations that specifically alter ATP-binding affinity in either nucleotide-binding domain (NBD1 or NBD2) into the G551D background and determined that this disease-associated mutation converts site 2, formed by the head subdomain of NBD2 and the tail subdomain of NBD1, into an inhibitory site, whereas site 1 remains stimulatory. G551E, but not G551K or G551S, exhibits a similar phenotype, indicating that electrostatic repulsion between the negatively charged side chain of aspartate and the γ-phosphate of ATP accounts for the observed mutational effects. Understanding the molecular mechanism of this gating defect lays a foundation for rational drug design for the treatment of CF.