Detection of L-gulono-gamma-lactone oxidase on SDS-polyacrylamide gels by the fluorescence of its covalently bound flavin

Discovery of a Xylooligosaccharide Oxidase from Myceliophthora thermophila C1

By inspection of the predicted proteome of the fungus Myceliophthora thermophila C1 for vanillyl-alcohol oxidase (VAO)-type flavoprotein oxidases, a putative oligosaccharide oxidase was identified. By homologous expression and subsequent purification, the respective protein could be obtained. The protein was found to contain a bicovalently bound FAD cofactor. By screening a large number of carbohydrates, several mono- and oligosaccharides could be identified as substrates. The enzyme exhibits a strong substrate preference toward xylooligosaccharides; hence it is named xylooligosaccharide oxidase (XylO). Chemical analyses of the product formed upon oxidation of xylobiose revealed that the oxidation occurs at C1, yielding xylobionate as product. By elucidation of several XylO crystal structures (in complex with a substrate mimic, xylose, and xylobiose), the residues that tune the unique substrate specificity and regioselectivity could be identified. The discovery of this novel oligosaccharide oxidase reveals that the VAO-type flavoprotein family harbors oxidases tuned for specific oligosaccharides. The unique substrate profile of XylO hints at a role in the degradation of xylan-derived oligosaccharides by the fungus M. thermophila C1.

Recent progress on the characterization of aldonolactone oxidoreductases

L-Ascorbic acid (ascorbate, AsA, vitamin C) is essential for animal and plant health. Despite our dependence on fruits and vegetables to fulfill our requirement for this vitamin, the metabolic network leading to its formation in plants is just being fully elucidated. There is evidence supporting the operation of at least four biosynthetic pathways leading to AsA formation in plants. These routes use D-mannose/L-galactose, L-gulose, D-galacturonate, and myo-inositol as the main precursors. This review focuses on aldonolactone oxidoreductases, a subgroup of the vanillyl alcohol oxidase (VAO; EC 1.1.3.38) superfamily, enzymes that catalyze the terminal step in AsA biosynthesis in bacteria, protozoa, animals, and plants. In this report, we review the properties of well characterized aldonolactone oxidoreductases to date. A shared feature in these proteins is the presence of a flavin cofactor as well as a thiol group. The flavin cofactor in many cases is bound to the N terminus of the enzymes or to a recently discovered HWXK motif in the C terminus. The binding between the flavin moiety and the protein can be either covalent or non-covalent. Substrate specificity and subcellular localization differ among the isozymes of each kingdom. All oxidases among these enzymes possess dehydrogenase activity, however, exclusive dehydrogenases are also found. We also discuss recent evidence indicating that plants have both L-gulono-1,4-lactone oxidases and L-galactono-1,4-lactone dehydrogenases involved in AsA biosynthesis.

Mycobacterium tuberculosispossesses a functional enzyme for the synthesis of vitamin C,L-gulono-1,4-lactone dehydrogenase

The last step of the biosynthesis of L-ascorbic acid (vitamin C) in plants and animals is catalyzed by L-gulono-1,4-lactone oxidoreductases, which use both L-gulono-1,4-lactone and L-galactono-1,4-lactone as substrates. L-gulono-1,4-lactone oxidase is missing in scurvy-prone, vitamin C-deficient animals, such as humans and guinea pigs, which are also highly susceptible to tuberculosis. A blast search using the rat L-gulono-1,4-lactone oxidase sequence revealed the presence of closely related orthologs in a limited number of bacterial species, including several pathogens of human lungs, such as Mycobacterium tuberculosis, Pseudomonas aeruginosa, Burkholderia cepacia and Bacillus anthracis. The genome of M. tuberculosis, the etiologic agent of tuberculosis, encodes a protein (Rv1771) that shows 32% identity with the rat L-gulono-1,4-lactone oxidase protein. The Rv1771 gene was cloned and expressed in Escherichia coli, and the corresponding protein was affinity-purified and characterized. The FAD-binding motif-containing Rv1771 protein is a metalloenzyme that oxidizes L-gulono-1,4-lactone (Km 5.5 mm) but not L-galactono-1,4-lactone. The enzyme has a dehydrogenase activity and can use both cytochrome c (Km 4.7 microm) and phenazine methosulfate as exogenous electron acceptors. Molecular oxygen does not serve as a substrate for the Rv1771 protein. Dehydrogenase activity was measured in cellular extracts of a Mycobacterium bovis BCG strain. In conclusion, M. tuberculosis produces a novel, highly specific L-gulono-1,4-lactone dehydrogenase (Rv1771) and has the capacity to synthesize vitamin C.

Cloning and characterization of gluconolactone oxidase of Penicillium cyaneo-fulvum ATCC 10431 and evaluation of its use for production of D-erythorbic acid in recombinant Pichia pastoris

A D-erythorbic acid-forming soluble flavoprotein, gluconolactone oxidase (GLO), was purified from Penicillium cyaneo-fulvum strain ATCC 10431 and partially sequenced. Peptide sequences were used to isolate a cDNA clone encoding the enzyme. The cloned gene exhibits high levels of similarity with the genes encoding other known eukaryotic lactone oxidases and also with the genes encoding some putative prokaryotic lactone oxidases. Analysis of the coding sequence of the GLO gene indicated the presence of a typical secretion signal sequence at the N terminus of GLO. No other targeting or anchoring signals were found, suggesting that GLO is the first known lactone oxidase that is secreted rather than targeted to the membranes of the endoplasmic reticulum or mitochondria. Experimental evidence, including the N-terminal sequence of mature GLO and data on glycosylation and localization of the enzyme in native and recombinant hosts, supports this analysis. The GLO gene was expressed in Pichia pastoris, and recombinant GLO was produced by using the strong methanol-induced AOX1 promoter. In order to evaluate the suitability of purified GLO for production of D-erythorbic acid, we immobilized it on N-hydroxysuccinimide-activated Sepharose and found that the immobilized GLO retained full activity during immobilization but was rather unstable under reaction conditions. Our results show that both soluble and immobilized forms of GLO can, in principle, be used for production of D-erythorbic acid from D-glucono-delta-lactone or (in combination with glucose oxidase and catalase) from glucose. We also demonstrated the feasibility of glucose-D-erythorbic acid fermentation with recombinant strains coexpressing GLO and glucose oxidase genes, and we analyzed problems associated with construction of efficient D-erythorbic acid-producing hosts.

Bacterial production of D-erythroascorbic acid and L-ascorbic acid through functional expression of Saccharomyces cerevisiae D-arabinono-1,4-lactone oxidase in Escherichia coli

D-Arabinono-1,4-lactone oxidase, which catalyzes the terminal step in the biosynthesis of D-erythroascorbic acid in Saccharomyces cerevisiae, was functionally expressed in Escherichia coli inherently lacking the enzyme. The recombinant E. coli strain expressing the enzyme could overproduce D-erythroascorbic acid and L-ascorbic acid when supplied with D-arabinono-1,4-lactone and L-galactono-1,4-lactone, respectively.

An activity stain for proteins containing noncovalently bound flavin adenine dinucleotide

A procedure has been developed to allow the visualization of FAD-containing proteins on polyacrylamide gels. The technique is based on the reconstitution of apo-D-amino acid oxidase with FAD and is thus specific for this cofactor. The stain is sensitive enough to detect 5 pmol of FAD and is therefore useful for the detection of flavoproteins in systems as complex as crude tissue or bacterial extracts.

A protocol for the successful long-term enzyme replacement therapy of scurvy in guinea pigs

Gulonolactone oxidase, a key enzyme in the biosynthesis of ascorbic acid, is missing from guinea pigs and certain other scurvy-prone species. Weekly intraperitoneal injections of glutaraldehyde cross-linked immunoprecipitates of this enzyme have been shown to provide guinea pigs with the capability of synthesizing their own ascorbic acid and of surviving without an exogenous source of this vitamin. This protocol, however, was successful in only a small percentage of the animals tested. The reasons for the limited therapeutic success were investigated. Apparently, the gulonolactone oxidase-treated guinea pigs fed without ascorbic acid were receiving insufficient nutrition. By supplementing these enzyme-treated animals with vitamins A, B, D and E and selenium, we successfully maintained a high proportion of guinea pigs fed without vitamin C.

Homology of L-gulonolactone oxidase of species belonging to Mammalia, Aves, and Amphibia

Immunological cross-reactivity of L-gulonolactone oxidase of different species (rat, chicken, and bullfrog) was tested by the Ouchterlony technique. Antiserum directed against the enzyme from chicken kidney reacted with rat liver enzyme as well as with bullfrog kidney enzyme. This finding suggests that there is, at least partly, sequence homology among the enzyme from species belonging to the three classes, Mammalian, Aves, and Amphibia.