Chemistry and biochemistry of apiose

Biochemical Characterization of Parsley Glycosyltransferases Involved in the Biosynthesis of a Flavonoid Glycoside, Apiin

The flavonoid glycoside apiin (apigenin 7-O-[β-D-apiosyl-(1→2)-β-D-glucoside]) is abundant in apiaceous and asteraceous plants, including celery and parsley. Although several enzymes involved in apiin biosynthesis have been identified in celery, many of the enzymes in parsley (Petroselinum crispum) have not been identified. In this study, we identified parsley genes encoding the glucosyltransferase, PcGlcT, and the apiosyltransferase, PcApiT, that catalyze the glycosylation steps of apiin biosynthesis. Their substrate specificities showed that they were involved in the biosynthesis of some flavonoid 7-O-apiosylglucosides, including apiin. The expression profiles of PcGlcT and PcApiT were closely correlated with the accumulation of flavonoid 7-O-apiosylglucosides in parsley organs and developmental stages. These findings support the idea that PcGlcT and PcApiT are involved in the biosynthesis of flavonoid 7-O-apiosylglucosides in parsley. The identification of these genes will elucidate the physiological significance of apiin and the development of apiin production methods.

The apiosyltransferase celery UGT94AX1 catalyzes the biosynthesis of the flavone glycoside apiin

Apiose is a unique branched-chain pentose found in plant glycosides and a key component of the cell wall polysaccharide pectin and other specialized metabolites. More than 1,200 plant-specialized metabolites contain apiose residues, represented by apiin, a distinctive flavone glycoside found in celery (Apium graveolens) and parsley (Petroselinum crispum) in the family Apiaceae. The physiological functions of apiin remain obscure, partly due to our lack of knowledge on apiosyltransferase during apiin biosynthesis. Here, we identified UGT94AX1 as an A. graveolens apiosyltransferase (AgApiT) responsible for catalyzing the last sugar modification step in apiin biosynthesis. AgApiT showed strict substrate specificity for the sugar donor, UDP-apiose, and moderate specificity for acceptor substrates, thereby producing various apiose-containing flavone glycosides in celery. Homology modeling of AgApiT with UDP-apiose, followed by site-directed mutagenesis experiments, identified unique Ile139, Phe140, and Leu356 residues in AgApiT, which are seemingly crucial for the recognition of UDP-apiose in the sugar donor pocket. Sequence comparison and molecular phylogenetic analysis of celery glycosyltransferases suggested that AgApiT is the sole apiosyltransferase-encoding gene in the celery genome. Identification of this plant apiosyltransferase gene will enhance our understanding of the physioecological functions of apiose and apiose-containing compounds.

Carbon allocation of Spirodela polyrhiza under boron toxicity

Pectic polysaccharides containing apiose, xylose, and uronic acids are excellent candidates for boron fixation. Duckweeds are the fastest-growing angiosperms that can absorb diverse metals and contaminants from water and have high pectin content in their cell walls. Therefore, these plants can be considered excellent boron (B) accumulators. This work aimed to investigate the relationship between B assimilation capacity with apiose content in the cell wall of Spirodela polyrhiza subjected to different boric acid concentrations. Plants were grown for 7 and 10 days in ½ Schenck-Hildebrandt media supplemented with 0 to 56 mg B.L^-1, the non-structural and structural carbohydrates, and related genes were evaluated. The results showed that B altered the morphology and carbohydrate composition of this species during plant development. The optimum B concentration (1.8 mg B.L^-1) led to the highest relative growth and biomass accumulation, reduced starch, and high pectin and apiose contents, together with increased expression of UDP-apiose/UDP-xylose synthase (AXS) and 1,4-α-galacturonosyltransferase (GAUT). The toxic state (28 and 56 mg B.L^-1) increased the hexose contents in the cell wall with a concomitant reduction of pectins, apiose, and growth. The pectin content of S. polyrhiza was strongly associated with its growth capacity and regulation of B content within the cells, which have AXS as an important regulator. These findings suggest that duckweeds are suitable for B remediation, and their biomass can be used for bioenergy production.

Making and breaking of boron bridges in the pectic domain rhamnogalacturonan-II at apoplastic pH in vivo and in vitro

Cross-linking of the cell-wall pectin domain rhamnogalacturonan-II (RG-II) via boron bridges between apiose residues is essential for normal plant growth and development, but little is known about its mechanism or reversibility. We characterized the making and breaking of boron bridges in vivo and in vitro at 'apoplastic' pH. RG-II (13-26 μm) was incubated in living Rosa cell cultures and cell-free media with and without 1.2 mm H3 BO3 and cationic chaperones (Ca2+ , Pb2+ , polyhistidine, or arabinogalactan-protein oligopeptides). The cross-linking status of RG-II was monitored electrophoretically. Dimeric RG-II was stable at pH 2.0-7.0 in vivo and in vitro. In-vitro dimerization required a 'catalytic' cation at all pHs tested (1.75-7.0); thus, merely neutralizing the negative charge of RG-II (at pH 1.75) does not enable boron bridging. Pb2+ (20-2500 μm) was highly effective at pH 1.75-4.0, but not 4.75-7.0. Cationic peptides were effective at approximately 1-30 μm; higher concentrations caused less dimerization, probably because two RG-IIs then rarely bonded to the same peptide molecule. Peptides were ineffective at pH 1.75, their pH optimum being 2.5-4.75. d-Apiose (>40 mm) blocked RG-II dimerization in vitro, but did not cleave existing boron bridges. Rosa cells did not take up d-[U-14 C]apiose; therefore, exogenous apiose would block only apoplastic RG-II dimerization in vivo. In conclusion, apoplastic pH neither broke boron bridges nor prevented their formation. Thus boron-starved cells cannot salvage boron from RG-II, and 'acid growth' is not achieved by pH-dependent monomerization of RG-II. Divalent metals and cationic peptides catalyse RG-II dimerization via co-ordinate and ionic bonding respectively (possible and impossible, respectively, at pH 1.75). Exogenous apiose may be useful to distinguish intra- and extra-protoplasmic dimerization.

Practical preparation of UDP-apiose and its applications for studying apiosyltransferase

UDP-apiose, a donor substrate of apiosyltransferases, is labile because of its intramolecular self-cyclization ability, resulting in the formation of apiofuranosyl-1,2-cyclic phosphate. Therefore, stabilization of UDP-apiose is indispensable for its availability and identifying and characterizing the apiosyltransferases involved in the biosynthesis of apiosylated sugar chains and glycosides. Here, we established a method for stabilizing UDP-apiose using bulky cations as counter ions. Bulky cations such as triethylamine effectively suppressed the degradation of UDP-apiose in solution. The half-life of UDP-apiose was increased to 48.1 ± 2.4 h at pH 6.0 and 25 °C using triethylamine as a counter cation. UDP-apiose coordinated with a counter cation enabled long-term storage under freezing conditions. UDP-apiose was utilized as a donor substrate for apigenin 7-O-β-D-glucoside apiosyltransferase to produce the apiosylated glycoside apiin. This apiosyltransferase assay will be useful for identifying genes encoding apiosyltransferases.

Synthesis of UDP-apiose in Bacteria: The marine phototroph Geminicoccus roseus and the plant pathogen Xanthomonas pisi

The branched-chain sugar apiose was widely assumed to be synthesized only by plant species. In plants, apiose-containing polysaccharides are found in vascularized plant cell walls as the pectic polymers rhamnogalacturonan II and apiogalacturonan. Apiosylated secondary metabolites are also common in many plant species including ancestral avascular bryophytes and green algae. Apiosyl-residues have not been documented in bacteria. In a screen for new bacterial glycan structures, we detected small amounts of apiose in methanolic extracts of the aerobic phototroph Geminicoccus roseus and the pathogenic soil-dwelling bacteria Xanthomonas pisi. Apiose was also present in the cell pellet of X. pisi. Examination of these bacterial genomes uncovered genes with relatively low protein homology to plant UDP-apiose/UDP-xylose synthase (UAS). Phylogenetic analysis revealed that these bacterial UAS-like homologs belong in a clade distinct to UAS and separated from other nucleotide sugar biosynthetic enzymes. Recombinant expression of three bacterial UAS-like proteins demonstrates that they actively convert UDP-glucuronic acid to UDP-apiose and UDP-xylose. Both UDP-apiose and UDP-xylose were detectable in cell cultures of G. roseus and X. pisi. We could not, however, definitively identify the apiosides made by these bacteria, but the detection of apiosides coupled with the in vivo transcription of bUAS and production of UDP-apiose clearly demonstrate that these microbes have evolved the ability to incorporate apiose into glycans during their lifecycles. While this is the first report to describe enzymes for the formation of activated apiose in bacteria, the advantage of synthesizing apiose-containing glycans in bacteria remains unknown. The characteristics of bUAS and its products are discussed.

Functional Characterization of UDP-apiose Synthases from Bryophytes and Green Algae Provides Insight into the Appearance of Apiose-containing Glycans during Plant Evolution

Apiose is a branched monosaccharide that is present in the cell wall pectic polysaccharides rhamnogalacturonan II and apiogalacturonan and in numerous plant secondary metabolites. These apiose-containing glycans are synthesized using UDP-apiose as the donor. UDP-apiose (UDP-Api) together with UDP-xylose is formed from UDP-glucuronic acid (UDP-GlcA) by UDP-Api synthase (UAS). It was hypothesized that the ability to form Api distinguishes vascular plants from the avascular plants and green algae. UAS from several dicotyledonous plants has been characterized; however, it is not known if avascular plants or green algae produce this enzyme. Here we report the identification and functional characterization of UAS homologs from avascular plants (mosses, liverwort, and hornwort), from streptophyte green algae, and from a monocot (duckweed). The recombinant UAS homologs all form UDP-Api from UDP-glucuronic acid albeit in different amounts. Apiose was detected in aqueous methanolic extracts of these plants. Apiose was detected in duckweed cell walls but not in the walls of the avascular plants and algae. Overexpressing duckweed UAS in the moss Physcomitrella patens led to an increase in the amounts of aqueous methanol-acetonitrile-soluble apiose but did not result in discernible amounts of cell wall-associated apiose. Thus, bryophytes and algae likely lack the glycosyltransferase machinery required to synthesize apiose-containing cell wall glycans. Nevertheless, these plants may have the ability to form apiosylated secondary metabolites. Our data are the first to provide evidence that the ability to form apiose existed prior to the appearance of rhamnogalacturonan II and apiogalacturonan and provide new insights into the evolution of apiose-containing glycans.

Synthesis of apiose-containing oligosaccharide fragments of the plant cell wall: fragments of rhamnogalacturonan-II side chains A and B, and apiogalacturonan

Fragments of pectic polysaccharides rhamnogalacturonan-II (RG-II) and apiogalacturonan were synthesised using p-tolylthio apiofuranoside derivatives as key building blocks. Apiofuranose thioglycosides can be conveniently prepared by cyclization of the corresponding dithioacetals possessing a 2,3-O-isopropylidene group, which is required for preservation of the correct (3R) configuration of the apiofuranose ring. The remarkable stability of this protecting group in apiofuranose derivatives requires its replacement with a more reactive protecting group, such as a benzylidene acetal which was used in the synthesis of trisaccharide β-Rhap-(1→3')-β-Apif-(1→2)-α-GalAp-OMe. The X-ray crystal structure of the protected precursor of this trisaccharide has been elucidated.

Hydroxybenzoic acid derivatives in a nonhost rutaceous plant, Orixajaponica, deter both oviposition and larval feeding in a rutaceae-feeding swallowtail butterfly, Papilio xuthus L

A Rutaceae-feeding swallowtail butterfly. Papilio xuthus L., feeds on various rutaceous plants but always rejects Orixa japonica Thunb. (Rutaceae). Females were strongly deterred from laying eggs by a methanolic extract of O. japonica leaves. Larvae also rejected a diet leaf medium impregnated with O. japonica leaf extracts. Several components in the water-soluble fraction of the leaf extract were found to deter both oviposition and feeding responses. Two major deterrent compounds were characterized as 5-[[2-O-(beta-D-apiofuranosyl)-beta-D-glucopyranosyl]oxy]-2-hydroxybenzoic acid and adisyringoyl aldaric acid. These compounds induced potent deterrence of both oviposition and larval feeding by P. xuthus, which suggests a congruent chemosensory mechanism of allomonal chemicals acting on both female tarsal chemoreceptors and larval maxillary taste receptors.

(-)-2C-methyl-D-erythrono-1,4-lactone is formed after application of the terpenoid precursor 1-deoxy-D-xylulose

Application of [1,2-14C]1-deoxy-D-xylulose, the committed precursor of terpenoids, thiamine and pyridoxol, to a variety of plant species resulted in the labelling of an unknown metabolite. The isolation and purification of this metabolite from Ipomoea purpurea plants fed with 1-deoxy-D-xylulose (DX), followed by NMR analysis, resulted in the identification of its structure as (-)-2C-methyl-D-erythrono-1,4-lactone (MDEL). MDEL has been previously isolated as a stress metabolite of certain plants. A hypothetical biosynthetic scheme is given.

The deoxyxylulose phosphate pathway of terpenoid biosynthesis in plants and microorganisms

Recent studies have uncovered the existence of an alternative, non-mevalonate pathway for the formation of isopentenyl pyrophosphate and dimethylallyl pyrophosphate, the two building blocks of terpene biosynthesis.

[Synthesis of 6-O-beta-D-apiofuranosyl-beta-D-glucopyranosides of monoterpenyls]

The synthetic apiosyl-glucosides having (S)-3,7-dimethyl-1,6-octadien-3-yl (linalyl); (R)-1-methyl-1-(4-methyl-3-cyclohexen-1-yl)ethyl (terpineyl); (E) and (Z)-3,7-dimethyl-2,6-octadien-1-yl (geranyl and neryl); (S)-3,7-dimethyl-6-octen-1-yl (citronellyl); benzyl and 2-phenylethyl as aglycon moiety were prepared using the stereospecific trichloroacetimidate Schmidt method. The key intermediate diholoside 1,2,3,4-tetra-O-acetyl-6-O-[(3-C-acetoxymethyl)-2,3-di-O-acetyl-beta-D- erythrofuranosyl)-beta-D-glucopyranose was obtained by Kochetkov glycosylation of a branched-chain tetrofuranose cyanoethylidene derivative. The NMR data (1H and 13C) of the synthetic compounds and the glycosylation shifts of the apiose moiety are reported.

DL-apiose substituted with stable isotopes: synthesis, n.m.r.-spectral analysis, and furanose anomerization

The branched-chain pentose DL-apiose has been synthesized in good yield by a new and simple chemical method that can be adapted to prepare (1-13C)-, (2-13C)-, (1-2H)- and/or (2-2H)-enriched derivatives. N.m.r. spectra (1H- and 13C-) have been interpreted with the aid of selective (13C)- and (2H)-enrichment, and 2D and 13C[13C]-n.m.r. spectra. The solution composition of DL-(1-13C)apiose in 2H2O, determined by 13C-n.m.r. spectroscopy, has been found to differ from that determined previously by 1H-n.m.r. spectroscopy. Several 13C-1H and 13C-13C couplings have been measured and interpreted in terms of apiofuranose ring conformation. Ring-opening rate-constants of the four apiofuranoses [3-C-(hydroxymethyl)-alpha- and -beta-D-erythrofuranose, and 3-C-(hydroxymethyl)-alpha- and -beta-L-threofuranose] have been determined by 13C-saturation-transfer n.m.r. spectroscopy, and compared to those obtained previously for the structurally related tetrofuranoses.

Synthesis of 3-C-(hydroxymethyl)erythritol and 3-C-methylerythritol

3-C-(Hydroxymethyl)erythritol was prepared from 3-C-(hydroxymethyl)-2,3-O-isopropylidene-D-erythro-tetrofuranose (4) by hydrolysis followed by reduction, or by reduction followed by hydrolysis. Monotosylation of 4, followed by reduction with lithium aluminum hydride and hydrolysis, afforded 3-C-methylerythritol.

UDP-apiose/UDP-xylose synthase. Subunit composition and binding studies

The UDP-apiose/UDP-xylose synthase from cell suspension cultures of parsley has been purified 1400-fold by an improved method. The ratio of apiose to xylose formed from UDP-D-glucuronic acid (UDP-GlcUA) remained constant throughout the purification procedure. Dodecylsulfate-gel electrophoresis and sedimentation equilibrium measurements showed that this enzyme preparation is composed of two proteins with molecular weights of 65000 and 86000. The two proteins which are present in a molar ratio of about 1:0.7 to 1:0.9 could not be separated by ammonium sulfate fractionation, chromatography on DEAE-cellulose at different pH-values, and on omega-aminoalkyl-Sepharose, and by gel filtration on Acrylex P-100. Each protein is composed of two apparently identical subunits. The presence of only two different subunits was confirmed by end group analysis in which glycine was found as N-terminal amino acid for the larger and lysine for the smaller protein. Crosslinking with dimethylsuberimidate gave dimers of the identical subunits but no hybrids. Separation of the two proteins was achieved on DEAE-cellulose in the presence of urea. After dialysis only the 86000-Mr protein showed enzyme activity with no significant change in the apiose/xylose ratio. However, in the absence of the 65000-Mr protein enzyme stability was decreased drastically. By equilibrium dialysis it was found that 0.5 mol UDP-GlcUA are bound per mole of 86000-Mr protein. NAD+ alone was not bound, but in the presence of UDP it was also bound in a ratio of 0.5 mol/mol catalytic protein. Experiments in which sodium borohydride was added to the enzyme incubation gave no indication that the 4-keto intermediate is bound as a Schiff base to the enzyme. Also no evidence for epimerization at C-3 of the 4-ulose intermediate prior to ring contraction to apiose was found.