TreX from Sulfolobus solfataricus ATCC 35092 Displays Isoamylase and 4-α-Glucanotransferase Activities

A treX in the trehalose biosynthesis gene cluster of Sulfolobus solfataricus ATCC 35092 has been reported to produce TreX, which hydrolyzes the α-1,6-branch portion of amylopectin and glycogen. TreX exhibited 4-α-D-glucan transferase activity, catalyzing the transfer of α-1,4-glucan oligosaccharides from one molecule to another in the case of linear maltooligosaccharides (G3–G7), and it produced cyclic glucans from amylopectin and amylose like 4-α-glucanotransferase. These results suggest that TreX is a novel isoamylase possessing the properties of 4-α-glucanotransferase.     

Reaction Kinetics of Substrate Transglycosylation Catalyzed by TreX of Sulfolobus solfataricus and Effects on Glycogen Breakdown

We studied the activity of a debranching enzyme (TreX) from Sulfolobus solfataricus on glycogen-mimic substrates, branched maltotetraosyl-β-cyclodextrin (Glc₄-β-CD), and natural glycogen to better understand substrate transglycosylation and the effect thereof on glycogen debranching in microorganisms. The validation test of Glc₄-β-CD as a glycogen mimic substrate showed that it followed the breakdown process of the well-known yeast and rat liver extract. TreX catalyzed both hydrolysis of α-1,6-glycosidic linkages and transglycosylation at relatively high (>0.5 mM) substrate concentrations. TreX transferred maltotetraosyl moieties from the donor substrate to acceptor molecules, resulting in the formation of two positional isomers of dimaltotetraosyl-α-1,6-β-cyclodextrin [(Glc₄)₂-β-CD]; these were 6¹,6³- and 6¹,6⁴-dimaltotetraosyl-α-1,6-β-CD. Use of a modified Michaelis-Menten equation to study substrate transglycosylation revealed that the k_cat and K_m values for transglycosylation were 1.78 × 10³ s⁻¹ and 3.30 mM, respectively, whereas the values for hydrolysis were 2.57 × 10³ s⁻¹ and 0.206 mM, respectively. Also, enzyme catalytic efficiency (the k_cat/K_m ratio) increased as the degree of polymerization of branch chains rose. In the model reaction system of Escherichia coli, glucose-1-phosphate production from glycogen by the glycogen phosphorylase was elevated ∼1.45-fold in the presence of TreX compared to that produced in the absence of TreX. The results suggest that outward shifting of glycogen branch chains via transglycosylation increases the number of exposed chains susceptible to phosphorylase action. We developed a model of the glycogen breakdown process featuring both hydrolysis and transglycosylation catalyzed by the debranching enzyme.     

Structural rationale for the short branched substrate specificity of the glycogen debranching enzyme GlgX

Glycogen serves as major energy storage in most living organisms. GlgX, with its gene in the glycogen degradation operon, functions in glycogen catabolism by selectively catalyzing the debranching of polysaccharide outer chains in bacterial glycosynthesis. GlgX hydrolyzes α‐1,6‐glycosidic linkages of phosphorylase‐limit dextrin containing only three or four glucose subunits produced by glycogen phosphorylase. To understand its mechanism and unique substrate specificity toward short branched α‐polyglucans, we determined the structure of GlgX from Escherichia Coli K12 at 2.25 Å resolution. The structure reveals a monomer consisting of three major domains with high structural similarity to the subunit of TreX, the oligomeric bifunctional glycogen debranching enzyme (GDE) from Sulfolobus. In the overlapping substrate binding groove, conserved residues Leu270, Asp271, and Pro208 block the cleft, yielding a shorter narrow GlgX cleft compared to that of TreX. Residues 207–213 form a unique helical conformation that is observed in both GlgX and TreX, possibly distinguishing GDEs from isoamylases and pullulanases. The structural feature observed at the substrate binding groove provides a molecular explanation for the unique substrate specificity of GlgX for G4 phosphorylase‐limit dextrin and the discriminative activity of TreX and GlgX toward substrates of varying lengths. Proteins 2010. © 2010 Wiley‐Liss, Inc.     

Structural insight into the bifunctional mechanism of the glycogen-debranching enzyme TreX from the archaeon Sulfolobus solfataricus

TreX is an archaeal glycogen-debranching enzyme that exists in two oligomeric states in solution, as a dimer and tetramer. Unlike its homologs, TreX from Sulfolobus solfataricus shows dual activities for alpha-1,4-transferase and alpha-1,6-glucosidase. To understand this bifunctional mechanism, we determined the crystal structure of TreX in complex with an acarbose ligand. The acarbose intermediate was covalently bound to Asp363, occupying subsites -1 to -3. Although generally similar to the monomeric structure of isoamylase, TreX exhibits two different active-site configurations depending on its oligomeric state. The N terminus of one subunit is located at the active site of the other molecule, resulting in a reshaping of the active site in the tetramer. This is accompanied by a large shift in the "flexible loop" (amino acids 399-416), creating connected holes inside the tetramer. Mutations in the N-terminal region result in a sharp increase in alpha-1,4-transferase activity and a reduced level of alpha-1,6-glucosidase activity. On the basis of geometrical analysis of the active site and mutational study, we suggest that the structural lid (acids 99-97) at the active site generated by the tetramerization is closely associated with the bifunctionality and in particular with the alpha-1,4-transferase activity. These results provide a structural basis for the modulation of activities upon TreX oligomerization that may represent a common mode of action for other glycogen-debranching enzymes in higher organisms.     

Association of bi-functional activity in the N-terminal domain of glycogen debranching enzyme

Glycogen debranching enzyme (GDE) in mammals and yeast exhibits α-1,4-transferase and α-1,6-glucosidase activities within a single polypeptide chain and facilitates the breakdown of glycogen by a bi-functional mechanism. Each enzymatic activity of GDE is suggested to be associated with distinct domains; α-1,4-glycosyltransferase activity with the N-terminal domain and α-1,6-glucosidase activity with the C-terminal domain. Here, we present the biochemical features of the GDE from Saccharomyces cerevisiae using the substrate glucose(n)-β-cyclodextrin (Gn-β-CD). The bacterially expressed and purified GDE N-terminal domain (aa 1–644) showed α-1,4-transferase activity on maltotetraose (G4) and G4-β-CD, yielding various lengths of (G)_n. Surprisingly, the N-terminal domain also exhibited α-1,6-glucosidase activity against G1-β-CD and G4-β-CD, producing G1 and β-CD. Mutational analysis showed that residues D535 and E564 in the N-terminal domain are essential for the transferase activity but not for the glucosidase activity. These results indicate that the N-terminal domain (1–644) alone has both α-1,4-transferase and the α-1,6-glucosidase activities and suggest that the bi-functional activity in the N-domain may occur via one active site, as observed in some archaeal debranching enzymes.     

Enzymatic synthesis of dimaltosyl-beta-cyclodextrin via a transglycosylation reaction using TreX, a Sulfolobus solfataricus P2 debranching enzyme

Di-O-α-maltosyl-β-cyclodextrin ((G2)₂-β-CD) was synthesized from 6-O-α-maltosyl-β-cyclodextrin (G2-β-CD) via a transglycosylation reaction catalyzed by TreX, a debranching enzyme from Sulfolobus solfataricus P2. TreX showed no activity toward glucosyl-β-CD, but a transfer product (1) was detected when the enzyme was incubated with maltosyl-β-CD, indicating specificity for a branched glucosyl chain bigger than DP2. Analysis of the structure of the transfer product (1) using MALDI-TOF/MS and isoamylase or glucoamylase treatment revealed it to be dimaltosyl-β-CD, suggesting that TreX transferred the maltosyl residue of a G2-β-CD to another molecule of G2-β-CD by forming an α-1,6-glucosidic linkage. When [¹⁴C]-maltose and maltosyl-β-CD were reacted with the enzyme, the radiogram showed no labeled dimaltosyl-β-CD; no condensation product between the two substrates was detected, indicating that the synthesis of dimaltosyl-β-CD occurred exclusively via transglycosylation of an α-1,6-glucosidic linkage. Based on the HPLC elution profile, the transfer product (1) was identified to be isomers of 6¹,6³- and 6¹,6⁴-dimaltosyl-β-CD. Inhibition studies with β-CD on the transglycosylation activity revealed that β-CD was a mixed-type inhibitor, with a K_i value of 55.6 μmol/mL. Thus, dimaltosyl-β-CD can be more efficiently synthesized by a transglycosylation reaction with TreX in the absence of β-CD. Our findings suggest that the high yield of (G2)₂-β-CD from G2-β-CD was based on both the transglycosylation action mode and elimination of the inhibitory effect of β-CD.     

Synthesis of N-Carbamyl-d-2-thienyl-glycine and d-2-Thienylglycine by Microbial Hydantoinase

A debranching enzyme purified from germinating rice endosperm hydrolyzed oligosaccharides having maltosyl or maltotriosyl branches (B₄-B₆) moderately. Hydrolysis of maltosylmaltose by a “pullulanase” of higher plant origin has been scarcely reported, while our enzyme debranched maltosylmaltose like microbial pullulanase. Additionally, the enzyme slowly hydrolyzed isopanose to glucose and maltose.

Gel-filtration analyses of hydrolysis products of polysaccharides with the enzyme suggested that while it hydrolyzed α-1,6-linkages of pullulan at random, it hydrolyzed amylopectin and glycogen at the outer α-1,6-linkages preferentially In the hydrolysis products of glycogen with the enzyme for a longer incubation time, large molecular-weight glucans still remained. This indicated that the enzyme was able to hydrolyze a few of the α-1,6-linkages of glycogen.     

A histochemical study of liver enzymes involved in glycogen metabolism in the X-irradiated rat

Summary The liver enzymes responsible for the breakdown and synthesis of glycogen from glucose have been investigated cytochemically in rats exposed to 1200 rads of x-irradiation. It was found that significant changes occur in their activities and that amylophosphorylase and amylo-1,6-glucosidase (debranching enzyme), both of which are responsible for the conversion of glycogen to glucose, are markedly inhibited by radiation. A significant inhibition of the activity of 1,41,6 transglucosidase (branching enzyme) was also observed. In contrast, the activity of UDPG-glycogen transglucosylase, which is responsible for the in vivo synthesis of 1,4-polysaccharides, was found to be stimulated.     

Specificity of Debranching Enzymes

THE debranching enzymes are a most important group because, with the amylases and phosphorylases, they completely degrade starch and glycogen to the monosaccharide level. Some of the earlier preparations of debranching enzymes were not homogeneous and their use led to erroneous conclusions about their specificity. These views were later qualified by studies using more highly purified preparations. For example, some preparations of yeast isoamylase hydrolysed α-1,6-D-glucosidic linkages in both glycogen and oligosaccharide α-dextrins¹, but the latter activity is now no longer ascribed to isoamylase².     

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.     

The molecular heterogeneity of purified human liver lysosomal alpha-glucosidase (acid alpha-glucosidase).

An α-glucosidase active at acid pH and presumably lysosomal in origin has been purified from human liver removed at autopsy. The enzyme has both α-1,4-glucosidase and α-1,6-glucosidase activities. The K_m of maltose for the enzyme is 8.9 mm at the optimal pH of 4.0. The K_m of glycogen at the optimal pH of 4.5 is 2.5% (9.62 mm outerchain end groups). Isomaltose has a K_m of 33 mm when α-1,6-glucosidase activity is tested at pH 4.2. The enzyme exists in several active charge isomer forms which have pI values between 4.4 and 4.7. These forms do not differ in their specific activities. Electrophoresis in polyacrylamide gels under denaturing conditions indicates that the protein is composed of two subunits whose approximate molecular weights are 88,000 and 76,000. An estimated molecular weight of 110,000 was obtained by nondenaturing polyacrylamide gel electrophoresis. When the protein was chromatographed on Bio-Gel P-200 it was separated into two partially resolved active peaks which did not differ in their charge isomer constitution or in subunit molecular weights. One peak gave a strongly positive reaction for carbohydrate by the periodic acid-Schiff method and the other did not. Both had the same specific activity. The enzyme was antigenic in rabbits, and the antibodies so obtained could totally inhibit the hydrolytic action of the enzyme on glycogen but were markedly less effective in inhibiting activity toward isomaltose and especially toward maltose. Using these antibodies it was found that liver and skeletal muscle samples from patients with the “infantile” form or with the “adult” form of Type II glycogen storage disease, all of whom lack the lysosomal α-glucosidase, do not have altered, enzymatically inactive proteins which are immunologically cross-reactive with antibodies for the α-glucosidase of normal human liver.     

Purification and Some Properties of Debranching Enzyme of Germinating Rice Endosperm

A debranching enzyme was extracted from the endosperm of germinating rice seeds and purified through three steps, namely cyclohexaamylose-coupled Sepharose 6B, Ultrogel AcA-44 and Bio-Gel P-150 column chromatography. This disc-electrophoretically homogeneous enzyme showed a specific activity of 43 units/mg of protein (30°C) with a pH optimum of 5.5. The isoelectric point was 4.9, unlike that (pI 3.5) of debranching enzyme of ungerminated rice seeds. Our enzyme hydrolyzed pullulan rapidly, and glutinous rice starch and waxy corn starch moderately. The enzyme was also able to act on phytoglycogen and glycogen unlike debranching enzymes originating in some plants.     

Purification and properties of a thermostable pullulanase from Clostridium thermosulfurogenes EM1 which hydrolyses both α-1,6 and α-1,4-glycosidic linkages

Summary A novel thermostable pullulanase, secreted by the thermophilic anaerobic bacterium Clostridium thermosulfurogenes EM1, was purified and characterized. Applying anion exchange chromatography and gel filtration the enzyme was purified 47-fold and had a specific activity of 200 units/mg. The molecular mass of this thermostable enzyme was determined to be 102 000 daltons and consisted of a single subunit. The enzyme was able to attack specifically the -1,6-glycosidic linkages in pullulan and caused its complete hydrolysis to maltotriose. Surprisingly and unlike the enzyme from Klebsiella pneumoniae, the purified enzyme from this anaerobic thermophile exhibited, in addition to its debranching and pullulanase activity, an -1,4 hydrolysing activity as well. By the action of this single polypeptide chain various branched and linear polysaccharides were completely converted to two major products, namely maltose and maltotriose. The K _m values of this enzyme for pullulan and amylose were determined to be 1.33 mg/ml and 0.38 mg/ml, respectively. This debranching enzyme displays a temperature optimum at 60°–65° C and a pH optimum at 5.5–6.0. The application of this new class of pullulanase (pullulanase type II) in industry will significantly enhance the starch saccharification process. Offprint requests to: G. Antranikian     

TreX from Sulfolobus solfataricus ATCC 35092 displays isoamylase and 4-alpha-glucanotransferase activities

A treX in the trehalose biosynthesis gene cluster of Sulfolobus solfataricus ATCC 35092 has been reported to produce TreX, which hydrolyzes the alpha-1,6-branch portion of amylopectin and glycogen. TreX exhibited 4-alpha-D-glucan transferase activity, catalyzing the transfer of alpha-1,4-glucan oligosaccharides from one molecule to another in the case of linear maltooligosaccharides (G3-G7), and it produced cyclic glucans from amylopectin and amylose like 4-alpha-glucanotransferase. These results suggest that TreX is a novel isoamylase possessing the properties of 4-alpha-glucanotransferase.     

Glycogen debranching enzyme in bovine brain

Glycogen debranching enzyme was partially purified from bovine brain using a substrate for measuring the amylo-1,6-glucosidase activity. Bovine cerebrum was homogenized, followed by cell-fractionation of the resulting homogenate. The enzyme activity was found mainly in the cytosolic fraction. The enzyme was purified 5,000-fold by ammonium sulfate precipitation, anion-exchange chromatography, gel-filtration, anion-exchange HPLC, and gel-permeation HPLC. The enzyme preparation had no alpha-glucosidase or alpha-amylase activities and degraded phosphorylase limit dextrin of glycogen with phosphorylase. The molecular weight of the enzyme was 190,000 and the optimal pH was 6.0. The brain enzyme differed from glycogen debranching enzyme of liver or muscle in its mode of action on dextrins with an alpha-1,6-glucosyl branch, indicating an amino acid sequence different from those of the latter two enzymes. It is likely that the enzyme is involved in the breakdown of brain glycogen in concert with phosphorylase as in the cases of liver and muscle, but that this proceeds in a somewhat different manner. The enzyme activity decreased in the presence of ATP, suggesting that the degradation of brain glycogen is controlled by the modification of the debranching enzyme activity as well as the phosphorylase.     

Glycogenolytic enzymes in sporulating yeast.

During meiosis in Saccharomyces cerevisiae, the polysaccharide glycogen is first synthesized and then degraded during the period of spore maturation. We have detected, in sporulating yeast strains, an enzyme activity which is responsible for the glycogen catabolism. The activity was absent in vegetative cells, appeared coincidently with the beginning of glycogenolysis and the appearance of mature ascospores, and increased progressively until spourlation was complete. The specific activity of glycogenolytic enzymes in the intact ascus was about threefold higher than in isolated spores. The glycogenolysis was not due to combinations of phosphorylase plus phosphatase or amylase plus maltase. Nonsporulating cells exhibited litle or no glycogen catabolism and contained only traces of glycogenolytic enzyme, suggesting that the activity is sporulation specific. The partially purified enzyme preparation degraded amylose and glycogen, releasing glucose as the only low-molecular-weight product. Maltotriose was rapidly hydrolyzed; maltose was less susceptible. Alpha-methyl-D-glucoside, isomaltose, and linear alpha-1,6-linked dextran were not attacked. However, the enzyme hydrolyzed alpha-1,6-glucosyl-Schardinger dextrin and increased the beta-amylolysis of beta-amylase-limit dextrin. Thus, the preparation contains alpha-1,4- and alpha-1,6-glucosidase activities. Sephadex G-150 chromatography partially resolved the enzyme into two activities, one of which may be a glucamylase and the other a debranching enzyme.     

Enzymatic characterization of an extracellular glucoamylase from Tricholoma matsutake and its cloning and secretory expression in Pichia pastoris

ABSTRACT

A glucoamylase from the ectomycorrhizal fungus Tricholoma matsutake (TmGLA) was purified 33.2-fold to homogeneity as a single monomeric glycoprotein with a molecular mass of 63.9 kDa. Maximum activity was observed at 60°C and pH 5.0. The enzyme is active down to 50°C and in the pH range of 4.0–6.0, and its activity is strongly inhibited by Ag⁺. It degrades α-1,4- and α-1,6-glycosidic linkages in various polysaccharides. Its gene (TmGlu1) was cloned using information from the enzyme’s internal amino acid sequences and the whole genome sequence of T. matsutake NBRC 30605. The deduced amino acid sequence showed clear homology with those of GH family 15 proteins. Pichia pastoris transformed with TmGlu1 secreted the active enzyme in a glycosylated form, and its characteristics were the same as the native enzyme.     

Behaviour of Endomycopsis fibuligera glucoamylase towards raw starch

An extracellular glucoamylase [exo-1,4-α-d-glucosidase, 1,4-α-d-glucan glucohydrolase, EC 3.2.1.3] of Endomycopsis fibuligera has been purified and some of its properties studied. It had a very high debranching activity (0.63). The enzyme was completely adsorbed onto raw starch at all the pH values tested (pH 2.0–7.6). Amylase inhibitor from Streptomyces sp. did not prevent the adsorption of glucoamylase onto raw starch although the enzyme did not digest raw starch in the presence of amylase inhibitor. Sodium borate (0.1 m) eluted only 35% of the adsorbed enzyme from raw starch. The optimum pH for raw starch digestion was 4.5 whereas that of boiled soluble starch hydrolysis was 5.5. Waxy starches were more easily digested than non-waxy starches, and root starches were slowly digested by this enzyme.     

Cloning and characterization of glycogen-debranching enzyme from hyperthermophilic archaeon Sulfolobus shibatae

A gene encoding a putative glycogen-debranching enzyme in Sulfolobus shibatae (abbreviated as SSGDE) was cloned and expressed in Escherichia coli. The recombinant enzyme was purified to homogeneity by heat treatment and Ni-NTA affinity chromatography. The recombinant SSGDE was extremely thermostable, with an optimal temperature at 85 degrees C. The enzyme had an optimum pH of 5.5 and was highly stable from pH 4.5 to 6.5. The substrate specificity of SSGDE suggested that it possesses characteristics of both amylo-1,6-glucosidase and alpha-1,4-glucanotransferase. SSGDE clearly hydrolyzed pullulan to maltotrlose, and 6-O-alpha-maltosyl-beta-cyclodextrin (G2-beta-CD) to maltose and beta-cyclodextrin. At the same time, SSGDE transferred maltooligosyl residues to the maltooligosaccharides employed, and maltosyl residues to G2-beta-CD. The enzyme preferentially hydrolyzed amylopectin, followed in a decreasing order by glycogen, pullulan, and amylose. Therefore, the present results suggest that the glycogen-debranching enzyme from S. shibatae may have industrial application for the efficient debranching and modification of starch to dextrins at a high temperature.     

A debranching enzyme deficiency in endosperms of the sugary-1 mutants of maize

Many of the sugary-1 mutants of maize (Zea mays L.) have the highly branched water-soluble polysaccharide, phytoglycogen, in quantities equal to or greater than starch as an endosperm storage product in mature seeds. We find that all sugary mutants investigated are deficient in debranching enzyme [α-(1, 6)-glucosidase] activity in endosperm tissue 23 days postpollination and suggest that this deficiency is the primary biochemical lesion leading to phytoglycogen accumulation in sugary endosperms. This would indicate that the amylopectin component of starch depends on an equilibrium between the activities of branching enzymes introducing α-1,6 branch points into the linear α-1,4 glucans and debranching enzymes. The debranching enzyme activities from nonsugary endosperms can be separated into three peaks on a hydroxyapatite column. The sugary endosperm extracts lack one of these peaks of activity while the other two fractions have much reduced activity. The embryos of developing seeds (23 days after pollination) from both sugary and nonsugary genotypes have equivalent debranching activity. The debranching enzyme activity of developing endosperms is proportional to the number of copies (0 to 3) of the nonmutant (Su) allele present suggesting that the Su allele may be the structural gene for this debranching enzyme, although this is not definitive. This identification of debranching enzyme activity as being the biochemical lesion in sugary endosperms is consistent with several previous observations on the mutant.