The guanosine 5'-diphosphate D-mannose: guanosine 5'-diphosphate L-galactose epimerase of Chlorella pyrenoidosa. Chemical synthesis of guanosine 5'-diphosphate L-galactose and further studies of the enzyme and the reaction it catalyzes

GDP-Mannose 3,5-Epimerase: A View on Structure, Mechanism, and Industrial Potential

GDP-mannose 3,5-epimerase (GM35E, GME) belongs to the short-chain dehydrogenase/reductase (SDR) protein superfamily and catalyses the conversion of GDP-d-mannose towards GDP-l-galactose. Although the overall reaction seems relatively simple (a double epimerization), the enzyme needs to orchestrate a complex set of chemical reactions, with no less than 6 catalysis steps (oxidation, 2x deprotonation, 2x protonation and reduction), to perform the double epimerization of GDP-mannose to GDP-l-galactose. The enzyme is involved in the biosynthesis of vitamin C in plants and lipopolysaccharide synthesis in bacteria. In this review, we provide a clear overview of these interesting epimerases, including the latest findings such as the recently characterized bacterial and thermostable GM35E representative and its mechanism revision but also focus on their industrial potential in rare sugar synthesis and glycorandomization.

Characterization of the First Bacterial and Thermostable GDP-Mannose 3,5-Epimerase

GDP-mannose 3,5-epimerase (GM35E) catalyzes the conversion of GDP-mannose towards GDP-l-galactose and GDP-l-gulose. Although this reaction represents one of the few enzymatic routes towards the production of l-sugars and derivatives, it has not yet been exploited for that purpose. One of the reasons is that so far only GM35Es from plants have been characterized, yielding biocatalysts that are relatively unstable and difficult to express heterologously. Through the mining of sequence databases, we succeeded in identifying a promising bacterial homologue. The gene from the thermophilic organism Methylacidiphilum fumariolicum was codon optimized for expression in Escherichia coli, resulting in the production of 40 mg/L of recombinant protein. The enzyme was found to act as a self-sufficient GM35E, performing three chemical reactions in the same active site. Furthermore, the biocatalyst was highly stable at temperatures up to 55 °C, making it well suited for the synthesis of new carbohydrate products with application in the pharma industry.

Two genes in Arabidopsis thaliana encoding GDP-L-galactose phosphorylase are required for ascorbate biosynthesis and seedling viability

Plants synthesize ascorbate from guanosine diphosphate (GDP)-mannose via L-galactose/L-gulose, although uronic acids have also been proposed as precursors. Genes encoding all the enzymes of the GDP-mannose pathway have previously been identified, with the exception of the step that converts GDP-L-galactose to L-galactose 1-P. We show that a GDP-L-galactose phosphorylase, encoded by the Arabidopsis thaliana VTC2 gene, catalyses this step in the ascorbate biosynthetic pathway. Furthermore, a homologue of VTC2, At5g55120, encodes a second GDP-L-galactose phosphorylase with similar properties to VTC2. Two At5g55120 T-DNA insertion mutants (vtc5-1 and vtc5-2) have 80% of the wild-type ascorbate level. Double mutants were produced by crossing the loss-of-function vtc2-1 mutant with each of the two vtc5 alleles. These show growth arrest immediately upon germination and the cotyledons subsequently bleach. Normal growth was restored by supplementation with ascorbate or L-galactose, indicating that both enzymes are necessary for ascorbate generation. vtc2-1 leaves contain more mannose 6-P than wild-type. We conclude that the GDP-mannose pathway is the only significant source of ascorbate in A. thaliana seedlings, and that ascorbate is essential for seedling growth. A. thaliana leaves accumulate more ascorbate after acclimatization to high light intensity. VTC2 expression and GDP-L-galactose phosphorylase activity rapidly increase on transfer to high light, but the activity of other enzymes in the GDP-mannose pathway is little affected. VTC2 and At5g55120 (VTC5) expression also peak in at the beginning of the light cycle and are controlled by the circadian clock. The GDP-L-galactose phosphorylase step may therefore play an important role in controlling ascorbate biosynthesis.

Characterization of a GDP-D-mannose 3'',5''-epimerase from rice

The enzymatic characterization of GDP-d-mannose 3'',5''-epimerase (GME), a key enzyme in the biosynthesis of vitamin C in plants is described. The GME gene (Genbank Accession No. AB193582) in rice was cloned, and expressed as a fusion protein in Escherichia coli. Reaction products from GDP-d-mannose, as produced by GME catalysis, were separated by recycling HPLC on an ODS column, and were determined to be GDP-l-galactose and GDP-l-gulose, based on their NMR spectra and sugar analysis. The reaction catalyzed by GME was inhibited by GDP, and was strongly accelerated by NAD(+) in contrast to the case of GME from Arabidopsis thaliana. This difference in the effect of NAD(+) on GME activity can be attributed to the NAD binding domain which is conserved in the rice gene, but not in the Arabidopsis thaliana gene. The apparent K(m) and k(cat) were determined to be 1.20x10(-5)M and 0.127s(-1), respectively, in the presence of 20microM NAD(+). The fractions of GDP-d-mannose, GDP-l-galactose and GDP-l-gulose, at equilibrium, were approximately 0.75, 0.20 and 0.05, respectively.

Structure and function of GDP-mannose-3',5'-epimerase: an enzyme which performs three chemical reactions at the same active site

GDP-mannose-3',5'-epimerase (GME) from Arabidopsis thaliana catalyzes the epimerization of both the 3' and 5' positions of GDP-alpha-D-mannose to yield GDP-beta-L-galactose. Production of the C5' epimer of GDP-alpha-D-mannose, GDP-beta-L-gulose, has also been reported. The reaction occurs as part of vitamin C biosynthesis in plants. We have determined structures of complexes of GME with GDP-alpha-D-mannose, GDP-beta-L-galactose, and a mixture of GDP-beta-L-gulose with GDP-beta-L-4-keto-gulose to resolutions varying from 2.0 to 1.4 A. The enzyme has the classical extended short-chain dehydratase/reductase (SDR) fold. We have confirmed that GME establishes an equilibrium between two products, GDP-beta-L-galactose and GDP-beta-L-gulose. The reaction proceeds by C4' oxidation of GDP-alpha-D-mannose followed by epimerization of the C5' position to give GDP-beta-L-4-keto-gulose. This intermediate is either reduced to give GDP-beta-L-gulose or the C3' position is epimerized to give GDP-beta-L-4-keto-galactose, then C4' is reduced to GDP-beta-L-galactose. The combination of oxidation, epimerization, and reduction in a single active site is unusual. Structural analysis coupled to site-directed mutagenesis suggests C145 and K217 as the acid/base pair responsible for both epimerizations. On the basis of the structure of the GDP-beta-L-gulose/GDP-beta-L-4-keto-gulose co-complex, we predict that a ring flip occurs during the first epimerization and that a boat intermediate is likely for the second epimerization. Comparison of GME with other SDR enzymes known to abstract a protein alpha to the keto function of a carbohydrate identifies key common features.

GDP-mannose 3',5'-epimerase forms GDP-L-gulose, a putative intermediate for the de novo biosynthesis of vitamin C in plants

Despite its importance for agriculture, bioindustry, and nutrition, the fundamental process of L-ascorbic acid (vitamin C) biosynthesis in plants is not completely elucidated, and little is known about its regulation. The recently identified GDP-Man 3',5'-epimerase catalyzes a reversible epimerization of GDP-D-mannose that precedes the committed step in the biosynthesis of vitamin C, resulting in the hydrolysis of the highly energetic glycosyl-pyrophosphoryl linkage. Here, we characterize the native and recombinant GDP-Man 3',5'-epimerase of Arabidopsis thaliana. GDP and GDP-D-glucose are potent competitive inhibitors of the enzyme, whereas GDP-L-fucose gives a complex type of inhibition. The epimerase contains a modified version of the NAD binding motif and is inhibited by NAD(P)H and stimulated by NAD(P)+. A feedback inhibition of vitamin C biosynthesis is observed apparently at the level of GDP-Man 3',5'-epimerase. The epimerase catalyzes at least two distinct epimerization reactions and releases, besides the well known GDP-l-galactose, a novel intermediate: GDP-L-gulose. The yield of the epimerization varies and seems to depend on the molecular form of the enzyme. Both recombinant and native enzymes co-purified with a Hsp70 heat-shock protein (Escherichia coli DnaK and A. thaliana Hsc70.3, respectively). We speculate, therefore, that the Hsp70 molecular chaperones might be involved in folding and/or regulation of the epimerase. In summary, the plant epimerase undergoes a complex regulation and could control the carbon flux into the vitamin C pathway in response to the redox state of the cell, stress conditions, and GDP-sugar demand for the cell wall/glycoprotein biosynthesis. Exogenous L-gulose and L-gulono-1,4-lactone serve as direct precursors of l-ascorbic acid in plant cells. We propose an L-gulose pathway for the de novo biosynthesis of vitamin C in plants.

Antisense suppression of l-galactose dehydrogenase in Arabidopsis thaliana provides evidence for its role in ascorbate synthesis and reveals light modulated l-galactose synthesis

l-Galactose dehydrogenase (l-GalDH), a novel enzyme that oxidizes l-Gal to l-galactono-1,4-lactone (l-GalL), has been purified from pea seedlings and cloned from Arabidopsis thaliana. l-GalL is a proposed substrate for ascorbate biosynthesis in plants, therefore the function of l-GalDH in ascorbate biosynthesis was investigated by overexpression in tobacco and antisense suppression in A. thaliana. In tobacco the highest expressing lines had a 3.5-fold increase in extractable activity, but this did not increase leaf ascorbate concentration. Arabidopsis thaliana, transformed with an antisense l-GalDH construct, produced lines with 30% of wild-type activity. These had lower leaf ascorbate concentration when grown under high light conditions. l-Gal pool size increased in antisense transformants with low l-GalDH activity, and l-Gal concentration was negatively correlated with ascorbate. The results provide direct evidence for a role of l-GalDH in ascorbate biosynthesis. Ascorbate pool size in A. thaliana is increased by acclimation to high light, but l-GalDH expression was not affected. l-Gal accumulation was higher in antisense plants acclimated to high light, indicating that the capacity to synthesize l-Gal from GDP-mannose is increased. Because the only known function of l-GalL is ascorbate synthesis, these antisense plants provide an opportunity to investigate ascorbate function with minimal effects on carbohydrate metabolism. Measurements of other antioxidants revealed an increase in ascorbate- and pyrogallol-dependent peroxidase activity in low-ascorbate lines. As ascorbate is the major hydrogen peroxide-scavenging antioxidant in plants, this could indicate a compensatory mechanism for controlling hydrogen peroxide concentration.

Partial purification and identification of GDP-mannose 3",5"-epimerase of Arabidopsis thaliana, a key enzyme of the plant vitamin C pathway

The first step in the biosynthetic pathway of vitamin C in plants is the formation, at the level of sugar nucleotide, of l-galactosyl residues, catalyzed by a largely unknown GDP-d-mannose 3",5"-epimerase. By using combined conventional biochemical and mass spectrometry methods, we obtained a highly purified preparation of GDP-d-mannose 3",5"-epimerase from an Arabidopsis thaliana cell suspension. The native enzyme is an 84-kDa dimer, composed of two apparently identical subunits. In-gel tryptic digestion of the enzyme subunit, followed by peptide sequencing and a blast search, led to the identification of the epimerase gene. The closest homolog of the plant epimerase is the BlmG gene product of Streptomyces sp., a putative NDP-d-mannose 5"-epimerase. The plant GDP-d-mannose 3",5"-epimerase is, to our knowledge, a novel member of the extended short-chain dehydrogenase/reductase family. The enzyme was cloned and expressed in Escherichia coli cells.

A high-performance liquid chromatography radio method for determination of L-ascorbic acid and guanosine 5'-diphosphate-l-galactose, key metabolites of the plant vitamin C pathway

A simple, rapid, and quantitative high-pressure liquid chromatography radio method is described for the determination of in vivo (14)C-labeled l-ascorbate, dehydro-l-ascorbate, and total l-ascorbate of Arabidopsis thaliana cell suspensions upon incubation of cultures with exogenous d-[(14)C]mannose. The same radio-HPLC conditions can be used to follow the products of in vitro enzymatic conversions of GDP-d-mannose by enzyme extracts of A. thaliana, namely GDP-l-galactose, GDP-4"-keto,6"-deoxy-d-mannose, and GDP-l-fucose. In particular, an accurate assay for GDP-d-mannose 3",5"-epimerase, a key enzyme of the plant vitamin C pathway, is presented.

BIOSYNTHESIS OF ASCORBIC ACID IN PLANTS: A Renaissance

The structure of the familiar antioxidant L-ascorbic acid (vitamin C) was described in 1933 yet remarkably, its biosynthesis in plants remained elusive until only recently. It became clear from radioisotopic labeling studies in the 1950s that plant ascorbic acid biosynthesis does not proceed in toto via a route similar to that in mammals. The description in 1996 of an Arabidopsis thaliana mutant deficient in ascorbic acid prompted renewed research effort in this area, and subsequently in 1998 a new pathway was discovered that is backed by strong biochemical and molecular genetic evidence. This pathway proceeds through the intermediates GDP-D-mannose, L-galactose, and L-galactono-1,4-lactone. Much research has focused on the properties of the terminal enzyme responsible for conversion of the aldonolactone to ascorbate, and on related enzymes in both mammals and fungi. Two of the plant biosynthetic genes have been studied at the molecular level and additional ascorbate-deficient A. thaliana mutants may hold the key to other proteins involved in plant ascorbate metabolism. An analysis of the biosynthesis of ascorbate and its analogues in algae and fungi as well as the study of alternative proposed pathways should broaden our understanding of ascorbate metabolism in plants. With a biosynthetic pathway in hand, research on areas such as the control of ascorbate biosynthesis and the physiological roles of ascorbate should progress rapidly.

L-ascorbic acid biosynthesis

Biosynthesis of L-ascorbate (vitamin C) occurs by different pathways in plants and mammals. Yeast contain D-erythroascorbate, a C5 analog of ascorbate. UDP-D-glucuronic acid is the precursor in mammals. Loss of UDP forms glucuronic acid/glucuronolactone. Reduction of these at C-1 then forms L-gulonic acid/L-gulono-1,4-lactone. The lactone is oxidized by a microsomal L-gulono-1,4-lactone oxidase to ascorbate. Only the L-gulono-1,4-lactone oxidase has been purified and cloned, and very little is known about the properties of the other enzymes. Plants form ascorbate from GDP-D-mannose via GDP-L-galactose, L-galactose, and L-galactono-1,4-lactone. The final oxidation of L-galactono-1,4-lactone to ascorbate is catalyzed by a mitochondrial L-galactono-1,4-lactone dehydrogenase located on the inner membrane and using cytochrome c as electron acceptor. GDP-mannose pyrophosphorylase and L-galactono-1,4-lactone dehydrogenase have been cloned. Yeast synthesizes D-erythroascorbate from D-arabinose and D-arabinono-1,4-lactone in a pathway analogous to that in plants. The plant, mammalian, and yeast aldonolactone oxidase/dehydrogenases that catalyze the last step in each pathway have significant sequence homology. L-Gulono-1,4-lactone oxidase is mutated and not expressed in animals, such as primates, that have lost ascorbate biosynthesis capacity. Assessment of the literature reveals that little is known about many of the enzymes involved in ascorbate biosynthesis or about the factors controlling flux through the pathways. There is also a possibility that minor alternative pathways exist in plants and mammals.

Animal and cellular models of sphingolipid storage disorders of humans

The synthesis of L-galactosylceramide is described. Data are presented indicating that this enantiomorph of D-galactocerebroside is not cleaved by galactocerebroside-beta-galactosidase obtained from mammalian tissues. The synthesis of L-glucosylceramide and beta-D-glucothiocerebroside are outlined. These compounds are also refractory to catabolism by glycosidases in mammalian tissues that catalyze the hydrolysis of naturally occurring cerebrosides. L-Hexosyl- and thioanalogs of cerebrosides and perhaps psychosines as well may be helpful for investigating the pathogenesis of Krabbe's disease and Gaucher's disease.