Production of very‐high‐amylose cassava by post‐transcriptional silencing of branching enzyme genes

High amylose starch can be produced by plants deficient in the function of branching enzymes (BEs). Here we report the production of transgenic cassava (Manihot esculenta Crantz) with starches containing up to 50% amylose due to the constitutive expression of hair‐pin dsRNAs targeting the BE1 or BE2 genes. All BE1‐RNAi plant lines (BE1i) and BE2‐RNAi plant lines (BE2i) were grown up in the field, but with reduced total biomass production. Considerably high amylose content in the storage roots of BE2i plant lines was achieved. Storage starch granules of BE1i and BE2i plants had similar morphology as wild type (WT), however, the size of BE1i starch granules were bigger than that of WT. Comparisons of amylograms and thermograms of all three sources of storage starches revealed dramatic changes to the pasting properties and a higher melting temperature for BE2i starches. Glucan chain length distribution analysis showed a slight increase in chains of DP>36 in BE1i lines and a dramatic increase in glucan chains between DP 10‐20 and DP>40 in BE2i lines. Furthermore, BE2i starches displayed a B‐type X‐ray diffraction pattern instead of the A‐type pattern found in BE1i and WT starches. Therefore, cassava BE1 and BE2 function differently in storage root starch biosynthesis.     

A structural explanation for the mechanism and specificity of plant branching enzymes I and IIb

Branching enzymes (BEs) are essential in the biosynthesis of starch and glycogen and play critical roles in determining the fine structure of these polymers. The substrates of these BEs are long carbohydrate chains that interact with these enzymes via multiple binding sites on the enzyme’s surface. By controlling the branched-chain length distribution, BEs can mediate the physiological properties of starch and glycogen moieties; however, the mechanism and structural determinants of this specificity remain mysterious. In this study, we identify a large dodecaose binding surface on rice BE I (BEI) that reaches from the outside of the active site to the active site of the enzyme. Mutagenesis activity assays confirm the importance of this binding site in enzyme catalysis, from which we conclude that it is likely the acceptor chain binding site. Comparison of the structures of BE from Cyanothece and BE1 from rice allowed us to model the location of the donor-binding site. We also identified two loops that likely interact with the donor chain and whose sequences diverge between plant BE1, which tends to transfer longer chains, and BEIIb, which transfers exclusively much shorter chains. When the sequences of these loops were swapped with the BEIIb sequence, rice BE1 also became a short-chain transferring enzyme, demonstrating the key role these loops play in specificity. Taken together, these results provide a more complete picture of the structure, selectivity, and activity of BEs.     

In <Emphasis Type="Italic">vitro</Emphasis> synthesis of glycogen: the structure,properties, and physiological function of enzymatically-synthesized glycogen

This review describes a new enzymatic method for in vitro glycogen synthesis and its structure and properties. In this method, short-chain amylose is used as the substrate for branching enzymes (BE, EC 2.4.1.18). Although a kidney bean BE and Bacillus cereus BE could not synthesize high-molecular weight glucan, BEs from 6 other bacterial sources produced enzymatically synthesized glycogen (ESG). The BE from Aquifex aeolicus was the most suitable for the production of glycogen with a weight-average molecular weight (M _w) of 3,000–30,000 k. The molecular weight of the ESG is controllable by changing the concentration of the substrate amylose. Furthermore, the addition of amylomaltase (AM, EC 2.4.1.25) significantly enhanced the efficiency of this process, and the yield of ESG reached approximately 65%. Typical preparations of ESG obtained by this method were subjected to structural analyses. The average chain length, interior chain length, and exterior chain length of the ESGs were 8.2–11.6, 2.0–3.3, and 4.2–7.6, respectively. Transmission electron microscopy and intrinsic viscosity measurement showed that the ESG molecules formed spherical particles. Unlike starch, the ESGs were barely degraded by pullulanase. Solutions of ESG were opalescent (milky-white and slightly bluish), and gave a reddishbrown color on the addition of iodine. These analyses revealed that ESG shares similar molecular shapes and solution properties with natural-source glycogen. Moreover, ESG had macrophage-stimulating activity and its activity depends on the molecular weight of ESG. We successfully achieved large scale production of ESG. ESG could lead to new industrial applications, such as in the food, chemical, and pharmaceutical fields.     

Amylolytically-resistant tapioca starch modified by combined treatment of branching enzyme and maltogenic amylase

Tapioca starch was modified using branching enzyme (BE) isolated from Bacillus subtilis 168 and Bacillus stearothermophilus maltogenic amylase (BSMA), and their molecular fine structure and susceptibility to amylolytic enzymes were investigated. By BE treatment, the molecular weight decreased from 3.1 × 10⁸ to 1.7 × 10⁶, the number of shorter branch chains (DP 6–12) increased, the number of longer branch chains (DP >25) decreased, and amylose content decreased from 18.9% to 0.75%. This indicated that α–1,4 linkages of amylose and amylopectin were cleaved, and moiety of glycosyl residues were transferred to another amylose and amylopectin to produce branched glucan and BE-treated tapioca starch by forming α–1,6 branch linkages. The product was further modified with BSMA to produce highly-branched tapioca starch with 9.7% of extra branch points. When subject to digestion with human pancreatic α-amylase (HPA), porcine pancreatic α-amylase (PPA) and glucoamylase, highly-branched tapioca starch gave significantly lowered α-amylase susceptibility (7.5 times, 14.4 times and 3.9 times, respectively), compared to native tapioca starch.     

Analysis of the Functional Interaction of Arabidopsis Starch Synthase and Branching Enzyme Isoforms Reveals that the Cooperative Action of SSI and BEs Results in Glucans with Polymodal Chain Length Distribution Similar to Amylopectin

Starch synthase (SS) and branching enzyme (BE) establish the two glycosidic linkages existing in starch. Both enzymes exist as several isoforms. Enzymes derived from several species were studied extensively both in vivo and in vitro over the last years, however, analyses of a functional interaction of SS and BE isoforms are missing so far. Here, we present data from in vitro studies including both interaction of leaf derived and heterologously expressed SS and BE isoforms. We found that SSI activity in native PAGE without addition of glucans was dependent on at least one of the two BE isoforms active in Arabidopsis leaves. This interaction is most likely not based on a physical association of the enzymes, as demonstrated by immunodetection and native PAGE mobility analysis of SSI, BE2, and BE3. The glucans formed by the action of SSI/BEs were analysed using leaf protein extracts from wild type and be single mutants (Atbe2 and Atbe3 mutant lines) and by different combinations of recombinant proteins. Chain length distribution (CLD) patterns of the formed glucans were irrespective of SSI and BE isoforms origin and still independent of assay conditions. Furthermore, we show that all SS isoforms (SSI-SSIV) were able to interact with BEs and form branched glucans. However, only SSI/BEs generated a polymodal distribution of glucans which was similar to CLD pattern detected in amylopectin of Arabidopsis leaf starch. We discuss the impact of the SSI/BEs interplay for the CLD pattern of amylopectin.     

Introduction of sense and antisense cDNA for branching enzyme in the amylose-free potato mutant leads to physico-chemical changes in the starch

One isoform of the branching enzyme (BE; EC 2.4.1.18) of potato (Solarium tuberosum L.) is known and catalyses the formation of α-1,6 bonds in a glucan chain, resulting in the branched starch component amylopectin. Constructs containing the antisense or sense-orientated distal 1.5-kb part of a cDNA for potato BE were used to transform the amylose-free (amf) mutant of potato, the starch of which stains red with iodine. The expression of the endogenous BE gene was inhibited either largely or fully as judged by the decrease or absence of the BE mRNA and protein. This resulted in a low percentage of starch granules with a small blue core and large red outer layer. There was no effect on the amylose content, degree of branching or λ_max of the iodine-stained starch. However, when the physico-chemical properties of the different starch suspensions were assessed, differences were observed, which although small indicated that starch in the transformants was different from that of theamf mutant.     

Towards a better understanding of the metabolic system for amylopectin biosynthesis in plants: rice endosperm as a model tissue

Starch is made up of amylose (linear alpha-1,4-polyglucans) and amylopectin (alpha-1,6-branched polyglucans). Amylopectin has a distinct fine structure called multiple cluster structure and is synthesized by multiple subunits or isoforms of four classes of enzymes: ADPglucose pyrophosphorylase, soluble starch synthase (SS), starch branching enzyme (BE), and starch debranching enzyme (DBE). In the present paper, based on analyses of mutants and transgenic lines of rice in which each enzyme activity is affected, the contribution of the individual isoform to the fine structure of amylopectin in rice endosperm is evaluated, and a new model referred to as the "two-step branching and improper branch clearing model" is proposed to explain how amylopectin is synthesized. The model emphasizes that two sets of reactions, alpha-1,6-branch formation and the subsequent alpha-1,4-chain elongation, are catalyzed by distinct BE and SS isoforms, respectively, are fundamental to the construction of the cluster structure. The model also assesses the role of DBE, namely isoamylase or in addition pullulanase, to remove unnecessary alpha-1,6-glucosidic linkages that are occasionally formed at improper positions apart from two densely branched regions of the cluster.     

Starch- and glycogen-debranching and branching enzymes: Prediction of structural features of the catalytic (β/α)8-barrel domain and evolutionary relationship to other amylolytic enzymes

Sequence alignment and structure prediction are used to locate catalytic α-amylase-type (β/α)₈-barrel domains and the positions of their β-strands and α-helices in isoamylase, pullulanase, neopullulanase, α-amylase-pullulanase, dextran glucosidase, branching enzyme, and glycogen branching enzymes—all enzymes involved in hydrolysis or synthesis of α-1,6-glucosidic linkages in starch and related polysaccharides. This has allowed identification of the transferase active site of the glycogen debranching enzyme and the locations of β ? α loops making up the active sites of all enzymes studied. Activity and specificity of the enzymes are discussed in terms of conserved amino acid residues and loop variations. An evolutionary distance tree of 47 amylolytic and related enzymes is built on 37 residues representing the four best conserved β-strands of the barrel. It exhibits clusters of enzymes close in specificity, with the branching and glycogen debranching enzymes being the most distantly related.     

Cloning and characterization of the glycogen branching enzyme gene existing in tandem with the glycogen debranching enzyme from Pectobacterium chrysanthemi PY35

The glycogen branching enzyme gene (glgB) from Pectobacterium chrysanthemi PY35 was cloned, sequenced, and expressed in Escherichia coli. The glgB gene consisted of an open reading frame of 2196bp encoding a protein of 731 amino acids (calculated molecular weight of 83,859Da). The glgB gene is upstream of glgX and the ORF starts the ATG initiation codon and ends with the TGA stop codon at 2bp upstream of glgX. The enzyme was 43-69% sequence identical with other glycogen branching enzymes. The enzyme is the most similar to GlgB of E. coli and contained the four regions conserved among the alpha-amylase family. The glycogen branching enzyme (GlgB) was purified and the molecular weight of the enzyme was estimated to be 84kDa by SDS-PAGE. The glycogen branching enzyme was optimally active at pH 7 and 30 degrees C.     

Soluble starch synthases and starch branching enzymes from developing seeds of sorghum

Soluble starch synthases and branching enzymes have been partially purified from developing sorghum seeds. Two major fractions and one minor fraction of starch synthase were eluted on DEAE-cellulose chromatography. The minor enzyme eluted first and was similar to the early eluting major synthase in citrate-stimulated activity, faster reaction rates with glycogen primers than amylopectin primers, and in K_m for ADP-glucose (0.05 and 0.08 mM, respectively). The starch synthase peak eluted last had no citrate-stimulated activity, was equally active with glycogen and amylopectin primers, and had the highest K_m for ADP-glucose (0.10 mM). Four fractions of branching enzymes were recovered from DEAE-cellulose chromatography. One fraction eluted in the buffer wash; the other three co-eluted with the three starch synthases. All four fractions could branch amylose or amylopectin, and stimulated α-glucan synthesis catalysed by phosphorylase. Electrophoretic separation and activity staining for starch synthase of crude extracts and DEAE-cellulose fractions demonstrated complex banding patterns. The colour of the bands after iodine staining indicated that branching enzyme and starch synthase co-migrated during electrophoresis.     

Focus on Metabolism: Molecular Genetic Analysis of Glucan Branching Enzymes from Plants and Bacteria in Arabidopsis Reveals Marked Differences in Their Functions and Capacity to Mediate Starch Granule Formation

The major component of starch is the branched glucan amylopectin, the branching pattern of which is one of the key factors determining its ability to form semicrystalline starch granules. Here, we investigated the functions of different branching enzyme (BE) types by expressing proteins from maize (Zea mays BE2a), potato (Solanum tuberosum BE1), and Escherichia coli (glycogen BE [EcGLGB]) in Arabidopsis (Arabidopsis thaliana) mutant plants that are deficient in their endogenous BEs and therefore, cannot make starch. The expression of each of these three BE types restored starch biosynthesis to differing degrees. Full complementation was achieved using the class II BE ZmBE2a, which is most similar to the two endogenous Arabidopsis isoforms. Expression of the class I BE from potato, StBE1, resulted in partial complementation and high amylose starch. Expression of the glycogen BE EcGLGB restored only minimal amounts of starch production, which had unusual chain length distribution, branch point distribution, and granule morphology. Nevertheless, each type of BE together with the starch synthases and debranching enyzmes were able to create crystallization-competent amylopectin polymers. These data add to the knowledge of how the properties of the BE influence the final composition of starch and fine structure of amylopectin.Starch is composed of two glucan polymers: amylopectin and amylose. Amylopectin constitutes around 80% of the mass of most starches and is a large, branched polymer with a tree-like architecture. The positioning and frequency of branch points together with the distribution of chain lengths are thought to be critical factors allowing amylopectin to adopt a semicrystalline state. Within amylopectin molecules, clusters of unbranched chain segments align, and adjacent chains form double helices. These pack into crystalline lamellae that alternate with amorphous regions containing the branch points. Longer chain segments span from one cluster to the next (Zeeman et al., 2010).Amylopectin is synthesized by three enzyme activities. First, starch synthases (SSs) transfer the glucosyl part of ADP-Glc to the nonreducing end of existing glucan chains, forming new α-1,4 glucosidic bonds. Second, branching enzymes (BEs) cleave part of an α-1,4-linked chain and through an inter- or intramolecular transfer reaction, reattach it, creating α-1,6-branch points. This reaction creates additional nonreducing ends on which SSs can act. Third, debranching enzymes (DBEs) hydrolyze some of these branches, tailoring the structure of the polymer to promote its crystallization.Several SS and BE isoforms are involved in starch synthesis in plants. There are five conserved classes of SSs (granule-bound starch synthase [GBSS] and SS1–SS4) and two conserved classes of BEs (classes I and II; also referred to as classes B and A, respectively; Nougué et al., 2014). In addition, plants contain two classes of DBEs: isoamylases (ISAs) and limit dextrinases (LDAs; also called pullulanases). One ISA, a multimeric enzyme composed of either a mixture of ISA1 and ISA2 subunits or just ISA1 subunits, is primarily involved in amylopectin synthesis (James et al., 1995; Mouille et al., 1996; Nakamura et al., 1996; Delatte et al., 2005). The other DBEs (i.e. ISA3 and LDA) are primarily involved in starch degradation (Wattebled et al., 2005; Delatte et al., 2006).Based on the in vitro analysis of purified or recombinant proteins and the phenotypes of mutant plants, the different SS isoforms are proposed to have distinct, albeit overlapping, functions. SS1 is thought to preferentially elongate short chains produced by the branching reactions to between 8 and 12 Glc units (Delvallé et al., 2005; Fujita et al., 2006). SS2 is proposed to elongate such chains farther to about 20 Glc units, optimal for cluster formation (Edwards et al., 1999; Umemoto et al., 2002; Zhang et al., 2008). The precise role of SS3 is less clear, although it has been proposed to generate long, cluster-spanning chains (Fujita et al., 2007). SS4 has a distinct role in initiating and/or coordinating granule formation (Roldán et al., 2007; Crumpton-Taylor et al., 2013).The two different BE classes are also proposed to have distinct functions in amylopectin synthesis. In vitro analyses of maize (Zea mays), rice (Oryza sativa), and potato (Solanum tuberosum) enzymes suggest that the class I enzymes preferentially act on amylose and transfer longer chains, whereas class II enzymes preferentially act on branched substrates, such as amylopectin, and transfer shorter chains (Guan and Preiss, 1993; Rydberg et al., 2001; Nakamura et al., 2010). This knowledge derives largely from experiments where linear or branched substrates were provided to recombinant or purified enzymes and the increased degree of branching was monitored. Similar conclusions were gained by recombinant protein expression in Escherichia coli and yeast (Saccharomyces cerevisiae) strains deficient in their endogenous glycogen BEs (Guan et al., 1995; Seo et al., 2002), where chain elongation by glycogen synthases occurred concurrently with branching.Models have been proposed in which both BE classes help create the final cluster structure of amylopectin: class I BEs initiate branching by transferring long or branched chains, which are subsequently acted on by class II BEs to create more numerous shorter chains. These shorter chains are then elaborated by the SSs to create the clusters (Nakamura et al., 2010). After the branching reactions, a degree of debranching occurs, which is thought to control branch number and positioning and thereby, facilitate amylopectin crystallization (Myers et al., 2000; Zeeman et al., 2010). Several studies have shown that isa1-deficient mutants produce starch with an altered amylopectin, accumulate a related soluble polymer (phytoglycogen), or both (James et al., 1995; Mouille et al., 1996; Nakamura et al., 1996; Delatte et al., 2005).Despite the wide conservation of the two BE classes, major alterations in starch properties are only observed when genes encoding class II enzymes are mutated or repressed. Loss of class I BE activity in maize endosperm, rice endosperm, or potato tuber did not alter starch content and caused only minor differences in amylopectin structure (e.g. the distribution of chain lengths and branch points) and/or starch properties (e.g. gelatinization or digestibility; Safford et al., 1998; Blauth et al., 2002; Satoh et al., 2003; Xia et al., 2011). In contrast, loss of class II BE results in significant changes, such as decreased starch content and a high apparent amylose content. This has been observed in several species, including maize (Stinard et al., 1993), potato (Jobling et al., 1999), pea (Pisum sativum; Bhattacharyya et al., 1990), rice (Mizuno et al., 1993), barley (Hordeum vulgare; Regina et al., 2010), and wheat (Triticum aestivum; Regina et al., 2006). The high apparent amylose content was caused at least in part by the accumulation of less-frequently branched amylopectin that stains with a higher wavelength of maximal absorption (λ_max) than that of the wild type (Boyer et al., 1976). In potato, this phenotype was enhanced by the simultaneous suppression of BE1 (Schwall et al., 2000), a result also shown recently in barley (Carciofi et al., 2012).Arabidopsis (Arabidopsis thaliana) has three genes annotated as BEs, At3g20440 (BE1), At5g03650 (BE2), and At2g36390 (BE3), but it seems that only BE2 and BE3 are active. Both BE2 and BE3 are class II BEs, making Arabidopsis somewhat unusual in not possessing a class I BE. The gene annotated as BE1 encodes a related protein that falls into a separate clade to either class I or II BEs (Dumez et al., 2006; Han et al., 2007; Wang et al., 2010). It was initially suggested that plants with mutations in this gene had a wild-type phenotype (Dumez et al., 2006), but subsequent work indicated that homozygous be1 mutation causes embryo lethality (hence, its alternative name EMBRYO DEFECTIVE2729; Wang et al., 2010). Thus, the function of the protein encoded at At3g20440 is currently unknown but unlikely to be a functional BE.The be2 and be3 single mutants have phenotypes that closely resemble the wild type, indicating that there is a high degree of redundancy between the enzymes. However, be2be3 double mutants lack starch (Dumez et al., 2006). Instead, the plants accumulate large amounts of maltose and other linear malto-oligosaccharides (MOSs). This is presumably because linear chains produced by the SSs are cleaved by starch-degrading enzymes (α- and β-amylases; Dumez et al., 2006). The altered metabolism of these double-mutant plants impedes growth, and they are smaller and paler than the wild type. The precise reason for this is unclear.In addition to mutagenesis, there have been several studies where BEs were overexpressed in transgenic plants. Overexpression of the E. coli glycogen BE (EcGLGB) in potato tubers or rice endosperm resulted in an increased degree of branching of amylopectin (Shewmaker et al., 1994; Kortstee et al., 1996; Kim et al., 2005). Overexpression of endogenous plant BE2 genes has also been performed in both rice and potato, increasing the proportion of shorter amylopectin chains (Tanaka et al., 2004; Brummell et al., 2015), and rice, leading to the accumulation of highly branched, water-soluble polysaccharides (Tanaka et al., 2004). Transgenic expression of genes from different photosynthetic organisms has also shown the degree of functional conservation within the plant BE classes. Sawada et al. (2009) showed that class II BE from Chlorella kessleri could rescue the BE2b-deficient phenotype in rice endosperm.The aim of this work was to investigate the capacity of different types of BEs to mediate starch granule formation by assessing their ability to function in the context of an otherwise intact starch biosynthesis pathway. To do this, we used the Arabidopsis be2be3 double mutants as a line in which to express three types of BEs. We chose BE2a from maize (required for leaf starch synthesis and similar to the endogenous Arabidopsis proteins; Yandeau-Nelson et al., 2011), BE1 from potato (represents the plant class I BEs that Arabidopsis lacks; Safford et al., 1998), and GLGB (the BE from E. coli involved in glycogen biosynthesis). This approach differs from previous investigations, because the activity of each BE type (working in planta with the same set of SSs and DBEs) can be assessed, and the results can be directly compared. In addition, we sought to address whether a glycogen BE was sufficient for starch production—in other words, whether the remaining starch biosynthetic enzymes are capable of generating a crystallization competent polymer, even when partnered with a BE with a different specificity. In previously described transgenic plants expressing E. coli GLGB, the endogenous plant BEs were still present (Shewmaker et al., 1994; Kortstee et al., 1996; Kim et al., 2005).In the transgenic lines generated here, we analyzed glucan synthesis, starch structure, and composition. Our results show that all three BE types can mediate starch granule production but to differing degrees. In each case, the structure of amylopectin and the amylose content depend on the type of BE present, as does starch granule morphology. We discuss the reasons for these differences in relation to previously reported BE properties.     

Properties of the Glucan Branching Enzyme of the Hyperthermophilic Bacterium Aquifex aeolicus

Abstract

Glucan branching enzymes are responsible for the synthesis of α(1→6) glycosidic bonds in glycogen and amylopectin. The glucan branching enzyme of the hyperthermophile Aquifex aeolicus is the most thermoactive and thermostable glucan branching enzyme described. The gene encoding this glucan branching enzyme was overexpressed in E. coli and purified using γ-cyclodextrin affinity chromatography. Subsequently, the enzyme was stable up to 90°C. Its thermostability may be explained by the relatively high number of aromatic amino acid residues present, in combination with a relatively low number glutamine/asparagine residues. The Km for amylose was 4µM and the Vmax was 4.9 U/mg of protein (at optimal pH and temperature). The side-chain distribution of the branched glucan formed from amylose was determined.     

Heterologous expression and characterization of a novel branching enzyme from the thermoalkaliphilic anaerobic bacterium Anaerobranca gottschalkii

The gene encoding the branching enzyme (BE) from the thermoalkaliphilic, anaerobic bacterium Anaerobranca gottschalkii was fused with a twin arginine translocation protein secretory-pathway-dependent signal sequence from Escherichia coli and expressed in Staphylococcus carnosus. The secreted BE was purified using hydrophobic interaction and gel filtration chromatography. The monomeric enzyme (72 kDa) shows maximal activity at 50°C and pH 7.0. With amylose the BE displays high transglycosylation and extremely low hydrolytic activity. The conversion of amylose and linear dextrins was analysed by applying high-performance anion exchange chromatography and quantitative size-exclusion chromatography. Amylose (10⁴–4×10⁷ g/mol) was converted to a major extent to products displaying molecular masses of 10⁴–4×10⁵ g/mol, indicating that the enzyme could be applicable for the production of starch or dextrins with narrow molecular mass distributions. The majority of the transferred oligosaccharides, determined after enzymatic hydrolysis of the newly synthesized α-1,6 linkages, ranged between 10³ and 10⁴ g/mol, which corresponds to a degree of polymerisation (DP) of 6–60. The minimal donor chain length is DP 16. Furthermore, the obtained results support the hypotheses of a random endocleavage mechanism of BE and the occurrence of interchain branching.     

Structural features of the glycogen branching enzyme encoding genes from aspergilli

A maltose binding protein, p78, was purified to homogeneity from Aspergillus nidulans by a single column chromatography step on cross-linked amylose. The partial amino acid sequence was highly homologous to the glycogen branching enzymes (GBEs) of human and yeast, and p78 did show branching enzyme activity. The genomic gene and its cDNA encoding GBE (p78) were isolated from the A. nidulans genomic and cDNA libraries. Furthermore, a cDNA encoding A. oryzae GBE was entirely sequenced. A. nidulans GBE shared overall and significant amino acid sequence identity with GBEs from A. oryzae (83.9%), Saccharomyces cerevisiae (61.1%) and human (63.0%), and with starch branching enzymes from green plants (55–56%).     

Expression and characterization of alpha-(1,4)-glucan branching enzyme Rv1326c of Mycobacterium tuberculosis H37Rv

Glycogen branching enzyme (GlgB, EC 2.4.1.18) catalyzes the third step of glycogen biosynthesis by the cleavage of an alpha-(1,4)-glucosidic linkage and subsequent transfer of cleaved oligosaccharide to form a new alpha-(1,6)-branch. A single glgB gene Rv1326c is present in Mycobacterium tuberculosis. The predicted amino acid sequence of GlgB of M. tuberculosis has all the conserved regions of alpha-amylase family proteins. The overall amino acid identity to other GlgBs ranges from 48.5 to 99%. The glgB gene of M. tuberculosis was cloned and expressed in Escherichia coli. The recombinant protein was purified to homogeneity using metal affinity and ion exchange chromatography. The recombinant protein is a monomer as evidenced by gel filtration chromatography, is active as an enzyme, and uses amylose as the substrate. Enzyme activity was optimal at pH 7.0, 30 degrees C and divalent cations such as Zn2+ and Cu2+ inhibited activity. CD spectroscopy, proteolytic cleavage and mass spectroscopy analyses revealed that cysteine residues of GlgB form structural disulfide bond(s), which allow the protein to exist in two different redox-dependent conformational states. These conformations have different surface hydrophobicities as evidenced by ANS-fluorescence of oxidized and reduced GlgB. Although the conformational change did not affect the branching enzyme activity, the change in surface hydrophobicity could influence the interaction or dissociation of different cellular proteins with GlgB in response to different physiological states.     

Branching enzyme assay: selective quantitation of the alpha 1,6-linked glucosyl residues involved in the branching points

Methods previously described for glycogen or amylopectin branching enzymatic activity are insufficiently sensitive and not quantitative. A new, more sensitive, specific, and quantitative one was developed. It is based upon the quantitation of the glucose residues joined by alpha 1,6 bonds introduced by varying amounts of branching enzyme. The procedure involved the synthesis of a polysaccharide from Glc-1-P and phosphorylase in the presence of the sample to be tested. The branched polysaccharide was then purified and the glucoses involved in the branching points were quantitated after degradation with phosphorylase and debranching enzymes. This method appeared to be useful, not only in enzymatic activity determinations but also in the study of the structure of alpha-D-glucans when combined with those of total polysaccharide quantitation, such as iodine and phenol-sulfuric acid.     

Characterization of substrate and product specificity of the purified recombinant glycogen branching enzyme of Rhodothermus obamensis

Background

Glycogen and starch branching enzymes catalyze the formation of α(1 → 6) linkages in storage polysaccharides by rearrangement of preexisting α-glucans. This reaction occurs through the cleavage of α(1 → 4) linkage and transfer in α(1 → 6) of the fragment in non-reducing position. These enzymes define major elements that control the structure of both glycogen and starch.

Methods

The kinetic parameters of the branching enzyme of Rhodothermus obamensis (RoBE) were established after in vitro incubation with different branched or unbranched α-glucans of controlled structure.

Results

A minimal chain length of ten glucosyl units was required for the donor substrate to be recognized by RoBE that essentially produces branches of DP 3–8. We show that RoBE preferentially creates new branches by intermolecular mechanism. Branched glucans define better substrates for the enzyme leading to the formation of hyper-branched particles of 30–70 nm in diameter (dextrins). Interestingly, RoBE catalyzes an additional α-4-glucanotransferase activity not described so far for a member of the GH13 family.

Conclusions

RoBE is able to transfer α(1 → 4)-linked-glucan in C4 position (instead of C6 position for the branching activity) of a glucan to create new α(1 → 4) linkages yielding to the elongation of linear chains subsequently used for further branching. This result is a novel case for the thin border that exists between enzymes of the GH13 family.

General significance

This work reveals the original catalytic properties of the thermostable branching enzyme of R. obamensis. It defines new approach to produce highly branched α-glucan particles in vitro.     

Effects of the activities of key enzymes involved in starch biosynthesis on the fine structure of amylopectin in developing rice (Oryza sativa L.) endosperms

The dynamic changes of the activities of enzymes involving in starch biosynthesis, including ADP-glucose pyrophosphorylase (AGPase), soluble starch synthases (SSS), starch branching enzyme (SBE) and starch debranching enzymes (DBE) were studied, and changes of fine structure of amy- lopectin were characterized by isoamylase treatment during rice grain development, using trans anti-waxy gene rice plants. The relationships between the activities of those key enzymes were also analyzed. The amylose synthesis was significantly inhibited in transgenic Wanjing 9522, but the total starch content and final grain weight were less affected as compared with those of non-transgenic Wanjing 9522 rice cultivar. Analyses on the changes of activities of enzymes involving in starch bio- synthesis showed that different enzyme activities were expressed differently during rice endosperm development. Soluble starch synthase is relatively highly expressed in earlier stage of endosperm de- velopment, whilst maximal expression of granule-bound starch synthase (GBSS) occurred in mid-stage of endosperm development. No obvious differences in changes of the activities of AGPase and SBE between two rice cultivars investigated, except the DBEs. Distribution patterns of branches of amy- lopectin changed continually during the development of rice grains and varied between two rice culti- vars. It was suggested that amylopectin synthesis be prior to the synthesis of amylose and different enzymes have different roles in controlling syntheses of branches of amylopectin.     

胚乳中淀粉的合成

禾本科植物胚乳内所含有的淀粉根据其结构、组成可以分为两类：直链淀粉（由α-1,4糖苷键连接的多聚D-葡萄糖）和支链淀粉（在以α-1,4糖苷键连接的主链上通过形成α-1,6糖苷键引入支链的多聚D-葡萄糖）。前者是以一种线性无序状态存在,而支链淀粉则是构成淀粉半晶体结构的主要成分。其中,除了负责合成作为糖基直接供体的ADP—Glc的酶AGPase外,直链淀粉中链的延伸反应由GBSSI完成,而支链淀粉的合成则相对复杂,需要SS、SBE、DBE、SP等一些酶的协同调控来共同完成。本文综述了胚乳中淀粉合成过程中所涉及的一些关键酶的研究进展,并对此研究领域进行了展望。