Mutational analysis of the active center of plant fructosyltransferases: Festuca 1-SST and barley 6-SFT

Production, modification and degradation of fructans and fructooligosacharides by enzymes originated from plants

Non-starch polysaccharides exhibit numerous beneficial health effects but compounds belonging to FODMAP (Fermentable Oligo- Di- and Monosaccharides and Polyols) has been recently connected to several gastrointestinal disorders. This review presents integrated literature data on the occurrence and types of fructans and fructooligosaccharids (classified as FODMAPs) as well as their degrading enzymes present in plants. Plants from the family Asteraceae and many monocotyledones, including families Poaceae and Liliaceae, are the most abundant sources of both fructans and fructan-degrading enzymes. So far, vast majority of publications concerning the application of these specific plants in production of bakery products is related to increase of dietary fibre content in these products. However, there is limited research on their effect on FODMAP content and fibre balance. The authors emphasize the possibility of application of enzyme rich plant extract in food production casting light on the new scientific approach to fibre modification.

Energy-Saving LED Light Affects the Efficiency of the Photosynthetic Apparatus and Carbohydrate Content in Gerbera jamesonii Bolus ex Hook. f. Axillary Shoots Multiplied In Vitro

An energy-saving light emitting diode (LED) system allows for adjustment of light quality, which affects plant development and metabolic processes in in vitro cultures. The study investigated the content of endogenous carbohydrates and the condition of the photosynthetic apparatus of Gerbera jamesonii Bolus ex Hook. f. Our aim was to analyze the effects of different LED light qualities-100% red light (R LED), 100% blue (B LED), a mixture of red and blue (7:3) (RB LED), and a fluorescent lamp as a control (Fl)-during the multiplication of axillary shoots. After 40 days, the culture measurements were performed using a non-invasive pulse amplitude modulation (PAM) fluorimeter. Sugar content was assessed with high performance liquid chromatography (HPLC). Two forms of free monosaccharides (glucose and fructose), two sugar alcohol derivatives (inositol and glycerol), and seven forms of free oligosaccharides were identified. Of those, glucose content was the highest. LEDs did not disturb the sugar metabolism in multiplied shoots. Their monosaccharides were three times more abundant than oligosaccharides; the same results were found in plants grown under control light. R light depleted the performance of the photosynthetic apparatus and caused its permanent damage. The RB LED spectrum ensured the most efficient non-photochemical quenching of the photosystem II (PS II) excitation state and high shoot quality.

Development of rubber-enriched dandelion varieties by metabolic engineering of the inulin pathway

Natural rubber (NR) is an important raw material for a large number of industrial products. The primary source of NR is the rubber tree Hevea brasiliensis, but increased worldwide demand means that alternative sustainable sources are urgently required. The Russian dandelion (Taraxacum koksaghyz Rodin) is such an alternative because large amounts of NR are produced in its root system. However, rubber biosynthesis must be improved to develop T. koksaghyz into a commercially feasible crop. In addition to NR, T. koksaghyz also produces large amounts of the reserve carbohydrate inulin, which is stored in parenchymal root cell vacuoles near the phloem, adjacent to apoplastically separated laticifers. In contrast to NR, which accumulates throughout the year even during dormancy, inulin is synthesized during the summer and is degraded from the autumn onwards when root tissues undergo a sink-to-source transition. We carried out a comprehensive analysis of inulin and NR metabolism in T. koksaghyz and its close relative T. brevicorniculatum and functionally characterized the key enzyme fructan 1-exohydrolase (1-FEH), which catalyses the degradation of inulin to fructose and sucrose. The constitutive overexpression of Tk1-FEH almost doubled the rubber content in the roots of two dandelion species without any trade-offs in terms of plant fitness. To our knowledge, this is the first study showing that energy supplied by the reserve carbohydrate inulin can be used to promote the synthesis of NR in dandelions, providing a basis for the breeding of rubber-enriched varieties for industrial rubber production.

Cloning of TaSST genes associated with water soluble carbohydrate content in bread wheat stems and development of a functional marker

We cloned TaSST genes, developed a gene-specific marker for TaSST-D1, and identified three QTL in the Doumai/Shi 4185 RIL population. TaSST-D1 is within one of the three QTL. Sucrose:sucrose-1-fructosyltransferase (1-SST), a critical enzyme in the fructan biosynthetic pathway, is significantly and positively associated with water soluble carbohydrate (WSC) content in bread wheat stems. In the present study, wheat 1-SST genes (TaSST) were isolated and located on chromosomes 4A, 7A and 7D. Sequence analysis of TaSST-D1 revealed 15 single nucleotide polymorphisms (SNP) in the third exon between cultivars with higher and lower WSC content. A cleaved amplified polymorphism sequence (CAPS) marker, WSC7D, based on the polymorphism at position 1216 (C-G) was developed to discriminate the two alleles. WSC7D was located on chromosome 7DS using a recombinant inbred line (RIL) population from a Doumai/Shi 4185 cross, and a set of Chinese Spring nullisomic-tetrasomic lines. TaSST-D1 co-segregated with the CAPS marker WSC7D and was linked to SNP marker BS00108793_51 on chromosome 7DS at a genetic distance of 6.1 cM. It explained 8.8, 10.9, and 11.3% of the phenotypic variances in trials at Beijing and Shijiazhuang as well as the averaged data from those environments, respectively. Two additional QTL (QWSC.caas-4BS and QWSC.caas-7AS) besides TaSST-D1 were mapped in the RIL population. One hundred and forty-nine Chinese wheat cultivars and advanced lines tested in four environments were used to validate a highly significant (P < 0.01) association between WSC7D and WSC content in wheat stems. WSC7D can be used as a gene-specific marker for improvement of stem WSC content in wheat breeding programs.

Cloning and characterization of acid invertase genes in the roots of the metallophyte Kummerowia stipulacea (Maxim.) Makino from two populations: Differential expression under copper stress

The roots of metallophytes serve as the key interface between plants and heavy metal-contaminated underground environments. It is known that the roots of metallicolous plants show a higher activity of acid invertase enzymes than those of non-metallicolous plants when under copper stress. To test whether the higher activity of acid invertases is the result of increased expression of acid invertase genes or variations in the amino acid sequences between the two population types, we isolated full cDNAs for acid invertases from two populations of Kummerowia stipulacea (from metalliferous and non-metalliferous soils), determined their nucleotide sequences, expressed them in Pichia pastoris, and conducted real-time PCR to determine differences in transcript levels during Cu stress. Heterologous expression of acid invertase cDNAs in P. pastoris indicated that variations in the amino acid sequences of acid invertases between the two populations played no significant role in determining enzyme characteristics. Seedlings of K. stipulacea were exposed to 0.3µM Cu(2+) (control) and 10µM Cu(2+) for 7 days under hydroponics׳ conditions. The transcript levels of acid invertases in metallicolous plants were significantly higher than in non-metallicolous plants when under copper stress. The results suggest that the expression of acid invertase genes in metallicolous plants of K. stipulacea differed from those in non-metallicolous plants under such conditions. In addition, the sugars may play an important role in regulating the transcript level of acid invertase genes and acid invertase genes may also be involved in root/shoot biomass allocation.

pKa modulation of the acid/base catalyst within GH32 and GH68: a role in substrate/inhibitor specificity?

Glycoside hydrolases of families 32 (GH32) and 68 (GH68) belong to clan GH-J, containing hydrolytic enzymes (sucrose/fructans as donor substrates) and fructosyltransferases (sucrose/fructans as donor and acceptor substrates). In GH32 members, some of the sugar substrates can also function as inhibitors, this regulatory aspect further adding to the complexity in enzyme functionalities within this family. Although 3D structural information becomes increasingly available within this clan and huge progress has been made on structure-function relationships, it is not clear why some sugars bind as inhibitors without being catalyzed. Conserved aspartate and glutamate residues are well known to act as nucleophile and acid/bases within this clan. Based on the available 3D structures of enzymes and enzyme-ligand complexes as well as docking simulations, we calculated the pKa of the acid-base before and after substrate binding. The obtained results strongly suggest that most GH-J members show an acid-base catalyst that is not sufficiently protonated before ligand entrance, while the acid-base can be fully protonated when a substrate, but not an inhibitor, enters the catalytic pocket. This provides a new mechanistic insight aiming at understanding the complex substrate and inhibitor specificities observed within the GH-J clan. Moreover, besides the effect of substrate entrance on its own, we strongly suggest that a highly conserved arginine residue (in the RDP motif) rather than the previously proposed Tyr motif (not conserved) provides the proton to increase the pKa of the acid-base catalyst.

Fructan and its relationship to abiotic stress tolerance in plants

Numerous studies have been published that attempted to correlate fructan concentrations with freezing and drought tolerance. Studies investigating the effect of fructan on liposomes indicated that a direct interaction between membranes and fructan was possible. This new area of research began to move fructan and its association with stress beyond mere correlation by confirming that fructan has the capacity to stabilize membranes during drying by inserting at least part of the polysaccharide into the lipid headgroup region of the membrane. This helps prevent leakage when water is removed from the system either during freezing or drought. When plants were transformed with the ability to synthesize fructan, a concomitant increase in drought and/or freezing tolerance was confirmed. These experiments indicate that besides an indirect effect of supplying tissues with hexose sugars, fructan has a direct protective effect that can be demonstrated by both model systems and genetic transformation.

Cloning and functional analysis of a fructosyltransferase cDNA for synthesis of highly polymerized levans in timothy (Phleum pratense L.)

Variation in the structures of plant fructans and their degree of polymerization (DP) can be explained as the result of diverse combinations of fructosyltransferases (FTs) with different properties. Although FT genes have been isolated in a range of plant species, sucrose:fructan 6-fructosyltransferase (6-SFT) cDNAs have only been functionally characterized in a few species such as wheat. A novel FT cDNA possessing 6-SFT activity has been identified and characterized from the temperate forage grass, timothy (Phleum pratense L.). The cDNA of an FT homolog, PpFT1, was isolated from cold-acclimated timothy. A recombinant PpFT1 protein expressed in Pichia pastoris showed 6-SFT/sucrose:sucrose 1-fructosyltransferase (1-SST) activity and produced linear beta(2,6)-linked levans from sucrose with higher DPs than present in graminans formed in vitro by wheat 6-SFT (Wft1). PpFT1 and Wft1 showed remarkably different acceptor substrate specificities: PpFT1 had high affinity for 6-kestotriose to produce levans and low affinity for 1-kestotriose, whereas Wft1 preferentially used 1-kestotriose as an acceptor. The affinity of the PpFT1 recombinant enzyme for sucrose as a substrate was lower than that of the Wft1 recombinant enzyme. It is also confirmed that timothy seedlings had elevated levels of PpFT1 transcripts during the accumulation of fructans under high sucrose and cold conditions. Our results suggest that PpFT1 is a novel cDNA with unique enzymatic properties that differ from those of previously cloned plant 6-SFTs, and is involved in the synthesis of highly polymerized levans in timothy.

Structural insights into glycoside hydrolase family 32 and 68 enzymes: functional implications

Glycoside hydrolases (GH) have been shown to play unique roles in various biological processes like the biosynthesis of glycans, cell wall metabolism, plant defence, signalling, and the mobilization of storage reserves. To date, GH are divided into more than 100 families based upon their overall structure. GH32 and GH68 are combined in clan GH-J, not only harbouring typical hydrolases but also non-Leloir type transferases (fructosyltransferases), involved in fructan biosynthesis. This review summarizes the recent structure-function research progress on plant GH32 enzymes, and highlights the similarities and differences compared with the microbial GH32 and GH68 enzymes. A profound analysis of ligand-bound structures and site-directed mutagenesis experiments identified key residues in substrate (or inhibitor) binding and recognition. In particular, sucrose can bind as inhibitor in Cichorium intybus 1-FEH IIa, whereas it binds as substrate in Bacillus subtilis levansucrase and Arabidopsis thaliana cell wall invertase (AtcwINV1). In plant GH32, a single residue, the equivalent of Asp239 in AtcwINV1, appears to be important for sucrose stabilization in the active site and essential in determining sucrose donor specificity.

An acceptor-substrate binding site determining glycosyl transfer emerges from mutant analysis of a plant vacuolar invertase and a fructosyltransferase

Glycoside hydrolase family 32 (GH32) harbors hydrolyzing and transglycosylating enzymes that are highly homologous in their primary structure. Eight amino acids dispersed along the sequence correlated with either hydrolase or glycosyltransferase activity. These were mutated in onion vacuolar invertase (acINV) according to the residue in festuca sucrose:sucrose 1-fructosyltransferase (saSST) and vice versa. acINV(W440Y) doubles transferase capacity. Reciprocally, saSST(C223N) and saSST(F362Y) double hydrolysis. SaSST(N425S) shows a hydrolyzing activity three to four times its transferase activity. Interestingly, modeling acINV and saSST according to the 3D structure of crystallized GH32 enzymes indicates that mutations saSST(N425S), acINV(W440Y), and the previously reported acINV(W161Y) reside very close together at the surface in the entrance of the active-site pocket. Residues in- and outside the sucrose-binding box determine hydrolase and transferase capabilities of GH32 enzymes. Modeling suggests that residues dispersed along the sequence identify a location for acceptor-substrate binding in the 3D structure of fructosyltransferases.

Insights into the catalytic properties of bamboo vacuolar invertase through mutational analysis of active site residues

Plant acid invertases, which are either associated with the cell wall or present in vacuoles, belong to family 32 of glycoside hydrolases (GH32). Homology modeling of bamboo vacuolar invertase Bobetafruct3 using Arabidopsis cell-wall invertase AtcwINV1 as a template showed that its overall structure is similar to GH32 enzymes, and that the three highly conserved motifs, NDPNG, RDP and EC, are located in the active site. This study also used site-directed mutagenesis to examine the roles of the conserved amino acid residues in these three motifs, which include Asp135, Arg259, Asp260, Glu316 and Cys317, and a conserved Trp residue (Trp159) that resides between the NDPNG and RDP motifs. The mutants W159F, W159L, E316Q and C317A retained acid invertase activity, but no invertase activity was observed for the mutant E316A or mutants with changes at Asp135, Arg259, or Asp260. The apparent K(m) values of the four mutants with invertase activity were all higher than that of the wild-type enzyme. The mutants W159L and E316Q exhibited lower k(cat) values than the wild-type enzyme, but an increase in the k(cat) value was observed for the mutants W159F and C317A. The results of this study demonstrate that these residues have individual functions in catalyzing sucrose hydrolysis.

Developing fructan-synthesizing capability in a plant invertase via mutations in the sucrose-binding box

Fructans are fructose polymers that are synthesized from sucrose by fructosyltransferases. Fructosyltransferases are present in unrelated plant families suggesting a polyphyletic origin for their transglycosylation activity. Based on sequence comparisons and enzymatic properties, fructosyltransferases are proposed to have evolved from vacuolar invertases. Between 1% and 5% of the total activity of vacuolar invertase is transglycosylating activity. We investigated the nature of the changes that can convert a hydrolysing invertase into a transglycosylating enzyme. Remarkably, replacing 33 amino acids (amino acids 143-175) corresponding to the N-terminus of the mature onion vacuolar invertase with the corresponding region of onion fructan:fructan 6G-fructosyltransferase (6G-FFT) led to a shift in activity from hydrolysis of sucrose towards transglycosylation between two sucrose molecules. The substituted N-terminal region contains the sucrose-binding box that harbours the nucleophile involved in sucrose hydrolysis (Asp164). Subsequent research into the individual amino acids responsible for the enhanced transglycosylation activity revealed that mutations in amino acids Trp161 and Asn166, can give rise to a shift towards polymerase activity. Changing the amino acid at either of these positions in the sucrose-binding box increases the transglycosylation capacity of invertases two- to threefold compared to wild type. Combining the two mutations had an additive effect on transglycosylation ability, resulting in an approximately fourfold enhancement. The mutations generated correspond with natural variation present in the sucrose-binding boxes of vacuolar invertases and fructosyltransferases. These relatively small changes that increase the transglycosylation capacity of invertases might explain the polyphyletic origin of the fructan accumulation trait.