Expression of a Functional Recombinant Human Glycogen Debranching Enzyme (hGDE) in N. benthamiana Plants and in Hairy Root Cultures

BACKGROUND

Glycogen storage disease type III (GSDIII, Cori/Forbes disease) is a metabolic disorder due to the deficiency of the Glycogen Debranching Enzyme (GDE), a large monomeric protein (about 176 kDa) with two distinct enzymatic activities: 4-α-glucantransferase and amylo-α-1,6-glucosidase. Several mutations along the amylo-alpha-1,6-glucosidase,4-alphaglucanotransferase (Agl) gene are associated with loss of enzymatic activity. The unique treatment for GSDIII, at the moment, is based on diet. The potential of plants to manufacture exogenous engineered compounds for pharmaceutical purposes, from small to complex protein molecules such as vaccines, antibodies and other therapeutic/prophylactic entities, was shown by modern biotechnology through "Plant Molecular Farming".

OBJECTIVE AND METHODS

In an attempt to develop novel protein-based therapeutics for GSDIII, the Agl gene, encoding for the human GDE (hGDE) was engineered for expression as a histidinetagged GDE protein both in Nicotiana benthamiana plants by a transient expression approach, and in axenic hairy root in vitro cultures (HR) from Lycopersicum esculentum and Beta vulgaris.

RESULTS

In both plant-based expression formats, the hGDE protein accumulated in the soluble fraction of extracts. The plant-derived protein was purified by affinity chromatography in native conditions showing glycogen debranching activity.

CONCLUSION

These investigations will be useful for the design of a new generation of biopharmaceuticals based on recombinant GDE protein that might represent, in the future, a possible therapeutic option for GSDIII.

Characterization of the Paenibacillus beijingensis DSM 24997 GtfD and its glucan polymer products representing a new glycoside hydrolase 70 subfamily of 4,6-α-glucanotransferase enzymes

Previously we have reported that the Gram-negative bacterium Azotobacter chroococcum NCIMB 8003 uses the 4,6-α-glucanotransferase GtfD to convert maltodextrins and starch into a reuteran-like polymer consisting of (α1→4) glucan chains connected by alternating (α1→4)/(α1→6) linkages and (α1→4,6) branching points. This enzyme constituted the single evidence for this reaction and product specificity in the GH70 family, mostly containing glucansucrases encoded by lactic acid bacteria (http://www.CAZy.org). In this work, 4 additional GtfD-like proteins were identified in taxonomically diverse plant-associated bacteria forming a new GH70 subfamily with intermediate characteristics between the evolutionary related GH13 and GH70 families. The GtfD enzyme encoded by Paenibacillus beijingensis DSM 24997 was characterized providing the first example of a reuteran-like polymer synthesizing 4,6-α-glucanotransferase in a Gram-positive bacterium. Whereas the A. chroococcum GtfD activity on amylose resulted in the synthesis of a high molecular polymer, in addition to maltose and other small oligosaccharides, two reuteran-like polymer distributions are produced by P. beijingensis GtfD: a high-molecular mass polymer and a low-molecular mass polymer with an average Mw of 27 MDa and 19 kDa, respectively. Compared to the A. chroooccum GtfD product, both P. beijingensis GtfD polymers contain longer linear (α1→4) sequences in their structure reflecting a preference for transfer of even longer glucan chains by this enzyme. Overall, this study provides new insights into the evolutionary history of GH70 enzymes, and enlarges the diversity of natural enzymes that can be applied for modification of the starch present in food into less and/or more slowly digestible carbohydrate structures.

Molecular mutagenesis at Tyr-101 of the amylomaltase transcribed from a gene isolated from soil DNA

The wild-type (WT) amylomaltase gene was directly isolated from soil DNA and cloned into a pET19b vector to express in E. coli BL21(DE3). The ORF of this gene consisted of 1,572 bp, encoding an enzyme of 523 amino acids. Though showing 99% sequence identity to amylomaltse from Thermus thermophilus ATCC 33923, this enzyme is unique in its alkaline optimum pH. In order to alter amylomaltase with less coupling or hydrolytic activity to enhance cycloamylose (CA) formation through cyclization reaction, site-directed mutagenesis of the second glucan binding site involving in CA production was performed at Tyr-101. The result revealed that the mutated Y101S enzyme showed a small increase in cyclization activity while significantly decreased disproportionation, coupling and hydrolytic activities. Mutation also resulted in the change in substrate specificity for disproportionation reaction. The WT enzyme preferred maltotriose, while the activity of mutated enzyme was the highest with maltopentaose substrate. Product analysis by HPAEC-PAD demonstrated that the main CAs of the WT amylomaltase were CA29-CA37. Y101S mutation did not change the product pattern, however, the amount of CAs formed by the mutated enzyme tended to increase especially at long incubation time.

Transglycosylation Activity ofDictyoglomus thermophilumAmylase A

Amylase A from Dictyoglomus thermophilum is a thermophilic enzyme and has about 40% identity with 4-alpha-glucanotransferase (GTase) from Thermococcus litoralis, and both of these enzymes belong to family 57 glycosyl hydrolase. Since the transglycosylation activity of T. litoralis GTase has been well characterized, the substrate specificity and reaction products of amylase A from D. thermophilum were examined. alpha-1,4 Glucan was produced from maltooligosaccharides, and glucoamylase-resistant molecules (cycloamyloses) were produced from longer chain amylose (average molecular mass 200 kDa). It has been reported that amylase A from D. thermophilum hydrolyzes starch, but in this study it was found that the enzyme was also able to use maltooligosaccharides and long chain amylose as substrate and has transglycosylation activity.

Purification of Glycogen Debranching Enzyme from Porcine Brain: Evidence for Glycogen Catabolism in the Brain

Amylo-1,6-glucosidase from porcine brain was purified to homogeneity by ammonium sulfate fractionation, followed by sequential steps of liquid chromatography on DEAE-Sephacel, Sephacryl S-300, and Super Q. The purified enzyme had both maltooligosaccharide transferase and amylo-1,6-glucosidase activities within a single polypeptide chain, and the combination of these two activities removed the branches of phosphorylase limit dextrin. Based on these results, the purified enzyme was identified as a glycogen debranching enzyme (GDE). The molecular weight of the brain GDE was 170,000 by gel-filtration and 165,000 by reducing SDS-PAGE. The pH profile of maltooligosaccharide transferase activity coincided with that of the amylo-1,6-glucosidase activity (pH optimum at 6.0). The existence of GDE as well as glycogen phosphorylase in the brain explains brain glycogenolysis fully and supports the hypothesis that glycogen is a significant source of energy in this organ.

Function of second glucan binding site including tyrosines 54 and 101 in Thermus aquaticus amylomaltase

Amylomaltase from Thermus aquaticus catalyzes three types of transglycosylation reaction, as well as a weak hydrolytic reaction of alpha-1,4 glucan. From our previous study [Fujii et al., Appl. Environ. Microbiol., 71, 5823-5827 (2005)], tyrosine 54 (Y54) was identified as an amino acid controlling the reaction specificity of this enzyme. Since Y54 is not located around the active site but in the proposed second glucan binding site that is 14 A away from catalytic residues, the functions of Y54 and the second glucan binding site are of great interest. In this study, we introduced mutations into another tyrosine (Y101) in the second glucan binding site. The obtained mutated enzymes were subjected to all four types of enzyme assay and the effects of mutations on the reaction specificities of these enzymes were comprehensively investigated. These studies indicated that the amino acid substitution at Y54 or Y101 for removing their aromatic side chain increases cyclization activity (intra-molecular transglycosylation reaction) but decreases disproportionation, coupling and hydrolytic activities (inter-molecular reactions). The superimposition of the reported structures of the enzyme with and without substrate analog revealed the occurrence of a conformational change in which a donor binding site becomes open. From lines of evidence, we conclude that the binding of glucan substrate to the second glucan binding site through an interaction with the aromatic side chains of Y54 and Y101 is a trigger for the enzyme to take a completely active conformation for all four types of activity, but prevents the cyclization reaction to occur since the flexibility of the glucan is restricted by such binding.

Activation of 4-α-Glucanotransferase Activity of Porcine Liver Glycogen Debranching Enzyme with Cyclodextrins

Glycogen debranching enzyme (GDE) is a single polypeptide chain containing distinct active sites for 4-alpha-glucanotransferase and amylo-alpha-1,6-glucosidase activities. Debranching of phosphorylase limit dextrin from glycogen is carried out by cooperation of the two activities. We examined the effects of cyclodextrins (CDs) on debranching activity of porcine liver GDE using a fluorogenic branched dextrin, Glcalpha1-4Glcalpha1-4Glcalpha1-4(Glcalpha1-4Glcalpha1-4Glcalpha1-4Glcalpha1-6)Glcalpha1-4Glcalpha1-4Glcalpha1-4Glcalpha1-4GlcPA (B5/84), as a substrate. B5/84 was hydrolyzed by the hydrolytic action of 4-alpha-glucanotransferase to B5/81 and maltotriose. The fluorogenic product was further hydrolyzed by the amylo-alpha-1,6-glucosidase activity to the debranched product, Glcalpha1-4Glcalpha1-4Glcalpha1-4Glcalpha1-4Glcalpha1-4Glcalpha1-4Glcalpha1-4GlcPA (G8PA), and glucose. alpha-, beta- and gamma-CDs accelerated the liberation of B5/81 from B5/84, indicating that the 4-alpha-glucanotransferase activity was activated by CDs to remove the maltotriosyl residue from the maltotetraosyl branch. This led to acceleration of B5/84 debranching. The extent of 4-alpha-glucanotransferase activation increased with CD concentration before reaching a constant value. This suggests that there is an activator binding site and that the binding of CDs stimulates 4-alpha-glucanotransferase activity. In the porcine liver, glycogen degradation may be partially stimulated by the binding of a glycogen branch to this activator binding site.

Characterisation of disproportionating enzyme from wheat endosperm

Disproportionating enzyme or D-enzyme (EC 2.4.1.25) is an alpha-1,4 glucanotransferase which catalyses cleavage and transfer reactions involving alpha-1,4 linked glucans altering (disproportionating) the chain length distribution of pools of oligosaccharides. While D-enzyme has been well characterised in some plants, e.g. potato and Arabidopsis, very little is known about its abundance and function in cereals which constitute the major source of starch worldwide. To address this we have investigated D-enzyme in wheat (Triticum aestivum). Two putative D-enzyme cDNA clones have been isolated from tissue-specific cDNA libraries. TaDPE1-e, from an endosperm cDNA library, encodes a putative polypeptide of 575 amino acid residues including a predicted transit peptide of 41 amino acids. The second cDNA clone, TaDPE1-l, from an Aegilops taushii leaf cDNA library, encodes a putative polypeptide of 579 amino acids including a predicted transit peptide of 45 amino acids. The mature polypeptides TaDPE1-e and TaDPE1-l were calculated to be 59 and 60 kDa, respectively, and had 96% identity. The putative polypeptides had significant identity with deduced D-enzyme sequences from corn and rice, and all the expected conserved residues were present. Protein analysis revealed that D-enzyme is present in the amyloplast of developing endosperm and in the germinating seeds. D-enzyme was partially purified from wheat endosperm and shown to exhibit disproportionating activity in vitro by cleaving maltotriose to produce glucose as well as being able to use maltoheptaose as the donor for the addition of glucans to the outer chains of glycogen and amylopectin.

Characterisation of disproportionating enzyme from wheat endosperm

Disproportionating enzyme or D-enzyme (EC 2.4.1.25) is an alpha-1,4 glucanotransferase which catalyses cleavage and transfer reactions involving alpha-1,4 linked glucans altering (disproportionating) the chain length distribution of pools of oligosaccharides. While D-enzyme has been well characterised in some plants, e.g. potato and Arabidopsis, very little is known about its abundance and function in cereals which constitute the major source of starch worldwide. To address this we have investigated D-enzyme in wheat (Triticum aestivum). Two putative D-enzyme cDNA clones have been isolated from tissue-specific cDNA libraries. TaDPE1-e, from an endosperm cDNA library, encodes a putative polypeptide of 575 amino acid residues including a predicted transit peptide of 41 amino acids. The second cDNA clone, TaDPE1-l, from an Aegilops taushii leaf cDNA library, encodes a putative polypeptide of 579 amino acids including a predicted transit peptide of 45 amino acids. The mature polypeptides TaDPE1-e and TaDPE1-l were calculated to be 59 and 60 kDa, respectively, and had 96% identity. The putative polypeptides had significant identity with deduced D-enzyme sequences from corn and rice, and all the expected conserved residues were present. Protein analysis revealed that D-enzyme is present in the amyloplast of developing endosperm and in the germinating seeds. D-enzyme was partially purified from wheat endosperm and shown to exhibit disproportionating activity in vitro by cleaving maltotriose to produce glucose as well as being able to use maltoheptaose as the donor for the addition of glucans to the outer chains of glycogen and amylopectin.

Crystallization and preliminary X-ray crystallographic study of disproportionating enzyme from potato

Disproportionating enzyme (D-enzyme; EC 2.4.1.25) is a 59 kDa protein that belongs to the alpha-amylase family. D-enzyme catalyses intramolecular and intermolecular transglycosylation reactions of alpha-1,4 glucan. A crystal of the D-enzyme from potato was obtained by the hanging-drop vapour-diffusion method. Preliminary X-ray data showed that the crystal diffracts to 2.0 A resolution and belongs to space group C222(1), with unit-cell parameters a = 69.7, b = 120.3, c = 174.2 A.

A synergistic reaction mechanism of a cycloalternan-forming enzyme and a d-glucosyltransferase for the production of cycloalternan in Bacillus sp. NRRL B-21195

Cycloalternan-forming enzyme (CAFE) was first described as the enzyme that produced cycloalternan from alternan. In this study, we found that a partially purified preparation of CAFE containing two proteins catalyzed the synthesis of cycloalternan from maltooligosaccharides, whereas the purified CAFE alone was unable to do so. In addition to the 117 kDa CAFE itself, the mixture also contained a 140 kDa protein. The latter was found to be a disproportionating enzyme (DE) that catalyzes transfer of a D-glucopyranosyl residue from the non-reducing end of one maltooligosaccharide to the non-reducing end of another, forming an isomaltosyl residue at the non-reducing end. CAFE then transfers the isomaltosyl residue to the non-reducing end of another isomaltosyl maltooligosaccharide, to form an alpha-isomaltosyl-(1-->3)-alpha-isomaltosyl-(1-->4)-maltooligosaccharide, and subsequently catalyzes a cyclization to produce cycloalternan. Thus, DE and CAFE act synergistically to produce cycloalternan directly from maltodextrin or starch.

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.

The Saccharomyces cerevisiae YPR184w gene encodes the glycogen debranching enzyme

The YPR184w gene encodes a 1536-amino acid protein that is 34-39% identical to the mammal, Drosophila melanogaster and Caenorhabditis elegans glycogen debranching enzyme. The N-terminal part of the protein possesses the four conserved sequences of the alpha-amylase superfamily, while the C-terminal part displays 50% similarity with the C-terminal of other eukaryotic glycogen debranching enzymes. Reliable measurement of alpha-1,4-glucanotransferase and alpha-1, 6-glucosidase activity of the yeast debranching enzyme was determined in strains overexpressing YPR184w. The alpha-1, 4-glucanotransferase activity of a partially purified preparation of debranching enzyme preferentially transferred maltosyl units than maltotriosyl. Deletion of YPR184w prevents glycogen degradation, whereas overexpression had no effect on the rate of glycogen breakdown. In response to stress and growth conditions, the transcriptional control of YPR184w gene, renamed GDB1 (for Glycogen DeBranching gene), is strictly identical to that of other genes involved in glycogen metabolism.

High expression of glycogen-debranching enzyme in Escherichia coli and its competent purification method

Glycogen-debranching enzyme (GDE) gene from Saccharomyces cerevisiae was cloned and expressed into Escherichia coli. A 99.3% homology was found between the nucleotide sequences of GDE gene harbored in the recombinant E. coli plasmid (pTrc99A) and the open reading frame (902039-906646 position) of the 4608-bp fragment of S. cerevisiae chromosome XVI. We investigated the best conditions for GDE expression. When the cultivation temperature of recombinant E. coli strains was lowered to 25 degrees C and the isopropyl-beta-d-thiogalactopyranoside (IPTG) concentration used for induction was decreased to as low as 0.02 mM, a total of about 33 mg of recombinant GDE can be isolated from a liter culture as estimated by amylo-1,6-glucosidase activity. Consecutively, we developed a new method for purifying GDE. The method requires only a single-step purification via beta-cyclodextrin-immobilized Sepharose 6B (beta-CD Sepharose 6B) affinity chromatography and renders a 90% recovery of the enzyme. Moreover, the purified recombinant GDE is a homogeneous protein and possesses the same characteristics as those of S. cerevisiae. With the highly expressed GDE in recombinant E. coli and a rapid and effective purification method, we successfully resolved the hurdle always faced for obtaining an ample amount of purified GDE. The availability of GDE, hence, may allow advancement on GDE studies and provide new prospects for GDE on biotechnological application.

Maltose metabolism in the hyperthermophilic archaeon Thermococcus litoralis: purification and characterization of key enzymes

Maltose metabolism was investigated in the hyperthermophilic archaeon Thermococcus litoralis. Maltose was degraded by the concerted action of 4-alpha-glucanotransferase and maltodextrin phosphorylase (MalP). The first enzyme produced glucose and a series of maltodextrins that could be acted upon by MalP when the chain length of glucose residues was equal or higher than four, to produce glucose-1-phosphate. Phosphoglucomutase activity was also detected in T. litoralis cell extracts. Glucose derived from the action of 4-alpha-glucanotransferase was subsequently metabolized via an Embden-Meyerhof pathway. The closely related organism Pyrococcus furiosus used a different metabolic strategy in which maltose was cleaved primarily by the action of an alpha-glucosidase, a p-nitrophenyl-alpha-D-glucopyranoside (PNPG)-hydrolyzing enzyme, producing glucose from maltose. A PNPG-hydrolyzing activity was also detected in T. litoralis, but maltose was not a substrate for this enzyme. The two key enzymes in the pathway for maltose catabolism in T. litoralis were purified to homogeneity and characterized; they were constitutively synthesized, although phosphorylase expression was twofold induced by maltodextrins or maltose. The gene encoding MalP was obtained by complementation in Escherichia coli and sequenced (calculated molecular mass, 96,622 Da). The enzyme purified from the organism had a specific activity for maltoheptaose, at the temperature for maximal activity (98 degrees C), of 66 U/mg. A Km of 0.46 mM was determined with heptaose as the substrate at 60 degrees C. The deduced amino acid sequence had a high degree of identity with that of the putative enzyme from the hyperthermophilic archaeon Pyrococcus horikoshii OT3 (66%) and with sequences of the enzymes from the hyperthermophilic bacterium Thermotoga maritima (60%) and Mycobacterium tuberculosis (31%) but not with that of the enzyme from E. coli (13%). The consensus binding site for pyridoxal 5'-phosphate is conserved in the T. litoralis enzyme.

Thermus aquaticus ATCC 33923 amylomaltase gene cloning and expression and enzyme characterization: production of cycloamylose

The amylomaltase gene of the thermophilic bacterium Thermus aquaticus ATCC 33923 was cloned and sequenced. The open reading frame of this gene consisted of 1,503 nucleotides and encoded a polypeptide that was 500 amino acids long and had a calculated molecular mass of 57,221 Da. The deduced amino acid sequence of the amylomaltase exhibited a high level of homology with the amino acid sequence of potato disproportionating enzyme (D-enzyme) (41%) but a low level of homology with the amino acid sequence of the Escherichia coli amylomaltase (19%). The amylomaltase gene was overexpressed in E. coli, and the enzyme was purified. This enzyme exhibited maximum activity at 75 degrees C in a 10-min reaction with maltotriose and was stable at temperatures up to 85 degrees C. When the enzyme acted on amylose, it catalyzed an intramolecular transglycosylation (cyclization) reaction which produced cyclic alpha-1,4-glucan (cycloamylose), like potato D-enzyme. The yield of cycloamylose produced from synthetic amylose with an average molecular mass of 110 kDa was 84%. However, the minimum degree of polymerization (DP) of the cycloamylose produced by T. aquaticus enzyme was 22, whereas the minimum DP of the cycloamylose produced by potato D-enzyme was 17. The T. aquaticus enzyme also catalyzed intermolecular transglycosylation of maltooligosaccharides. A detailed analysis of the activity of T. aquaticus ATCC 33923 amylomaltase with maltooligosaccharides indicated that the catalytic properties of this enzyme differ from those of E. coli amylomaltase and the plant D-enzyme.

Thermotoga maritima maltosyltransferase, a novel type of maltodextrin glycosyltransferase acting on starch and malto-oligosaccharides

A novel enzyme acting on starch and malto-oligosaccharides was identified and characterised. The non-hydrolytic enzyme, designated maltosyltransferase (MTase), of the hyperthermophilic bacterium Thermotoga maritima MSB8 disproportionates malto-oligosaccharides via glycosyl transfer reactions. The enzyme has a unique transfer specificity strictly confined to the transfer of maltosyl units. Incubation of MTase with starch or its constituents. i.e. amylose and amylopectin, led to the formation of a set of multiples of maltose (i.e. maltose, maltotetraose, maltohexaose etc.). Malto-oligosaccharides with a degree of polymerization (DP) X were disproportionated to products with a DP of X +/- 2n (with X > or = 3 and n = 0,1,2,...). Maximum activity in a 10-min assay was recorded at pH 6.5 and 85-90 degrees C. The enzyme displayed extraordinary resistance to thermal inactivation. For example, at 90, 85, and 70 degrees C (pH 6.5, 0.34 mg ml-1 protein), MTase half-lives of about 2.5 h, 17 h, and 21 days, respectively, were recorded. The gene for MTase, designated mmtA, was isolated from a gene library of T. maritima strain MSB8. Analysis of the MTase primary structure as deduced from the nucleotide sequence of mmtA revealed that the enzyme is not closely related to known protein sequences. However, low-level local similarity between MTase and the alpha-amylase enzyme family (glycosyl hydrolase family 13) was detected, including conserved acidic residues essential for catalysis. Therefore, MTase should be assigned to this family. Based on detailed sequence analyses and comparison with amylolytic enzymes of known crystal structure we propose that MTase contains a (beta/alpha)8-fold as the core supersecondary structure which is typical for the alpha-amylase family. On the other hand, MTase is unique in that it lacks several residues highly conserved throughout this family. Also, MTase possesses an extraordinarily large domain B (a domain typical for the alpha-amylase family, inserted between beta-strand 3 and alpha-helix 3 of the (beta/alpha)8-barrel fold).

4-alpha-glucanotransferase from the hyperthermophilic archaeon Thermococcus litoralis--enzyme purification and characterization, and gene cloning, sequencing and expression in Escherichia coli

4-Alpha-Glucanotransferase was purified from cells of Thermococcus litoralis, a hyperthermophilic archaeon. The molecular mass of the enzyme was estimated to be approximately 87 kDa by gel filtration. The optimal temperature for its activity was 90 degrees C. The enzyme catalyzed the transglycosylation of maltooligosaccharides, yielding maltooligosaccharides of various lengths and glucose. When maltoheptaose was used as the substrate, glucoamylase-resistant and glucoamylase-sensitive saccharides were produced. On incubation of amylose with the T. litoralis enzyme, glucoamylase-resistant but alpha-amylase-sensitive molecules were produced, but the amount of reducing sugar showed only slight increases. These results indicate that the T. litoralis enzyme catalyzes not only intermolecular transglycosylation to produce linear alpha-1,4-glucan, but also intramolecular transglycosylation to produce cyclic alpha-1,4-glucan (cycloamylose), similarly to potato 4-alpha-glucanotransferase (called disproportionating enzyme). The gene encoding the T. litoralis 4-alpha-glucanotransferase was cloned, sequenced and expressed in Escherichia coli. The nucleotide sequence of the gene encoded a 659-amino acid protein with a calculated molecular mass of 77,883 Da. The amino acid sequence of the T. litoralis enzyme showed high similarity with those of alpha-amylases of Pyrococcus furiosus, a hyperthermophilic archaeon, and Dictyoglomus thermophilum, an extremely thermophilic bacterium, but little similarity with those of other known 4-alpha-glucanotransferases.

Potato D-enzyme catalyzes the cyclization of amylose to produce cycloamylose, a novel cyclic glucan

Potato D-enzyme was purified from recombinant Escherichia coli, and its action on synthetic amylose (average Mr of 320,000) was analyzed. D-enzyme treatment resulted in a decrease in the ability of the amylose to form a blue complex with iodine. Analysis of the products indicated that the enzyme catalyzes an intramolecular transglycosylation reaction on amylose to produce cyclic alpha-1,4-glucan (cycloamylose). Confirmation of the cyclic structure was achieved by demonstrating the absence of reducing and nonreducing ends, resistance to hydrolysis by glucoamylase (an exoamylase), and by "time of flight" mass spectrometry. The degree of polymerization of cycloamylose products was determined by time of flight mass spectrometry analysis and by high-performance anion-exchange chromatography following partial acid hydrolysis of purified cycloamylose molecules and was found to range from 17 to several hundred. The yield of cycloamylose increased with time and reached >95%. D-enzyme did not act upon purified cycloamylose, but if glucose was added as an acceptor molecule, smaller cyclic and linear molecules were produced. The mechanism of the cyclization reaction, the possible role of the enzyme in starch metabolism, and the potential applications for cycloamylose are discussed.

Disproportionating enzyme (4-alpha-glucanotransferase; EC 2.4.1.25) of potato. Purification, molecular cloning, and potential role in starch metabolism

Disproportionating enzyme (D-enzyme, 4-alpha-glucanotransferase; EC 2.4.1.25) has been purified to homogeneity from potato tubers and its activity characterized. The enzyme catalyzes the transfer of maltooligosaccharides from one 1,4-alpha-D-glucan molecule to another, or to glucose. Maltooligosaccharides are effective donor molecules, but short chain amylose and amylopectin may also function as donors. Enzyme activity is not affected by inorganic phosphate, 3-phosphoglycerate, or hexose phosphates. A cDNA clone encoding the enzyme was isolated using oligonucleotide probes derived from partial peptide sequences of the purified enzyme. The identity of the cDNA clone was confirmed by expression in Escherichia coli resulting in D-enzyme activity. The amino acid sequence deduced from the cDNA shows significant homology with a 4-alpha-glucanotransferase from Streptococcus. The deduced sequence indicates the presence of an amino-terminal plastid transit peptide of 52 amino acid residues and a mature polypeptide of 524 residues. D-enzyme mRNA is present in leaves, stems, roots, and stolons but is most abundant in developing and mature tubers. The amount of mRNA in leaves increases in response to light and to sucrose added to the medium. These results are discussed in terms of the function of D-enzyme in potato starch metabolism.

Purification and characterization of a novel thermostable 4-alpha-glucanotransferase of Thermotoga maritima cloned in Escherichia coli

Maltodextrin glycosyltransferase (4-alpha-glucanotransferase) of the extremely thermophilic ancestral bacterium Thermotoga maritima has been purified from an Escherichia coli clone expressing the corresponding T. maritima MSB8 chromosomal gene. T. maritima 4-alpha-glucanotransferase, an approximately 53-kDa monomeric enzyme, is the most thermophilic glycosyltransferase described to date. It retained more than 90% of its maximum activity at temperatures from 55 degrees C up to 80 degrees C. The proposed action modus is the transfer of 1,4-alpha-glucanosyl chains, thus resulting in the disproportionation of 1,4-alpha-glucans. It converted soluble starch, amylopectin, and amylose, thereby changing the iodine staining properties of these substrates. The addition of low-molecular-mass malto-oligosaccharides, which act as glucanosyl acceptor molecules, enhanced the reaction and resulted in the formation of a series of linear maltohomologues from two to more than nine glucose units in size. Use of either of the malto-oligosaccharides maltotetraose, maltopentaose, maltohexaose, or maltoheptaose as sole substrate also yielded linear maltohomologues. On the other hand, maltose and maltotriose were not disproportionated by 4-alpha-glucanotransferase, although both were good acceptors for glucanosyl transfer. Glucose did not function as an acceptor in transfer reactions. Glucose also never appeared as a reaction product. The chain length of glucanosyl segments transferred ranged from two to probably far more than six glucose residues. Comparison of the N-terminal amino acid sequence of 4-alpha-glucanotransferase with other published protein sequences revealed significant similarity to sequences near the N-termini of various eucaryotic maltases and bacterial cyclodextrin glycosyltransferases, suggesting its relatedness on the molecular level with other starch- and maltodextrin-converting enzymes.

Properties of yeast debranching enzyme and its specificity toward branched cyclodextrins

Debranching enzyme was purified from Saccharomyces cerevisiae by DEAE-cellulose, omega-aminobutyl agarose and hydroxyapatite column chromatography. The activity of the eluent was monitored by the iodine-staining method which detects both the direct and indirect debranching enzymes. The elution profiles at every step showed a single peak with no shoulder. The crude and the purified enzyme preparations gave a single activity band with the same mobility on PAGE. The crude product produced 80% glucose compared to reducing sugar from glycogen-phosphorylase-limited dextrin while the partially purified and purified preparations produced 100% glucose. The activity of the purified enzyme was characterized and compared with that of the rabbit muscle enzyme by using various branched cyclodextrins as substrates. Both enzymes hydrolyzed 6-O-alpha-D-glucosyl cyclodextrins to glucose and cyclodextrins, but did not act on 6-O-alpha-maltosyl cyclomaltoheptaose. The yeast enzyme gave rise to glucose as a sole reducing sugar from 6-O-alpha-maltotriosyl cyclomaltoheptaose and 6-O-alpha-maltotetraosyl cyclomaltoheptaose, indicating that maltosyl and maltotriosyl transfers, respectively, had occurred, prior to the action of amylo-1,6-glucosidase. 6-O-alpha-D-Glucosyl cyclomaltoheptaose and 6-O-alpha-D-glucosyl cyclomalto-octaose, respectively, were better substrates than glycogen-phosphorylase-limited dextrin for the yeast and muscle enzymes. The yeast enzyme released glucose at a similar rate from 6-O-alpha-maltotriosyl cyclomaltoheptaose as from 6-O-alpha-maltotetraosyl cyclomaltoheptaose, but considerably lower rates than that from limit dextrin. The yeast debranching enzyme appears to be exclusively oligo-1,4----1,4-glucantransferase-amylo-1,6-glucosidase and does not have isoamylase.

Molecular characterization of malQ, the structural gene for the Escherichia coli enzyme amylomaltase

The structural gene for the Escherichia coli enzyme amylomaltase, malQ, is the second gene in the malPQ operon. The nucleotide sequence of malQ shows that the gene encodes an Mr 78360 protein close to the experimentally determined Mr of purified amylomaltase (72000-74000). The malQ initiation codon was identified by sequence analysis of clustered deletions around the 5' end of the gene. One of these deletions removed the first 5 bases from the malQ coding sequence. Strains carrying a plasmid with this truncated malQ gene under lacZ promoter control and out-of-frame with the first four codons of lacZ were Mal-. The Mal+ phenotype could be restored by inserting small, random fragments of E. coli chromosomal DNA into the unique EcoRI site. Nucleotide sequencing showed that the inserts either joined the lacZ and malQ sequences in frame, or contained a new translation start signal and coding sequence in frame with malQ. These results indicate that amylomaltase could be useful as a reporter protein in gene fusion studies.

Glycogen debranching enzyme: purification, antibody characterization, and immunoblot analyses of type III glycogen storage disease

Type III glycogen storage disease is caused by a deficiency of glycogen debranching-enzyme activity. Many patients with this disease have both liver and muscle involvement, whereas others have only liver involvement without clinical or laboratory evidence of myopathy. To improve our understanding of the molecular basis of the disease, debranching enzyme was purified 238-fold from porcine skeletal muscle. In sodium dodecyl sulfate-polyacrylamide gel electrophoresis the purified enzyme gave a single band with a relative molecular weight of 160,000 that migrated to the same position as purified rabbit-muscle debranching enzyme. Antiserum against porcine debranching enzyme was prepared in rabbit. The antiserum reacted against porcine debranching enzyme with a single precipitin line and demonstrated a reaction having complete identity to those of both the enzyme present in crude muscle and the enzyme present in liver extracts. Incubation of antiserum with purified porcine debranching enzyme inhibited almost all enzyme activity, whereas such treatment with preimmune serum had little effect. The antiserum also inhibited debranching-enzyme activity in crude liver extracts from both pigs and humans to the same extent as was observed in muscle. Immunoblot analysis probed with anti-porcine-muscle debranching-enzyme antiserum showed that the antiserum can detect debranching enzyme in both human muscle and human liver. The bands detected in human samples by the antiserum were the same size as the one detected in porcine muscle. Five patients with Type III and six patients with other types of glycogen storage disease were subjected to immunoblot analysis. Although anti-porcine antiserum detected specific bands in all liver and muscle samples from patients with other types of glycogen storage disease (Types I, II, and IX), the antiserum detected no cross-reactive material in any of the liver or muscle samples from patients with Type III glycogen storage disease. These data indicate (1) immunochemical similarity of debranching enzyme in liver and muscle and (2) that deficiency of debranching-enzyme activity in Type III glycogen storage disease is due to absence of debrancher protein in the patients that we studied.

Purification and properties of debranching enzyme from dogfish muscle

Glycogen debranching enzyme (4-alpha-glucanotransferase amylo-1,6-glucosidase, EC 2.4.1.25 + 3.2.1.33) was purified 140-fold from dogfish muscle in a rapid, high-yield procedure that takes advantage of a strong binding of the enzyme to glycogen, and its quantitative adsorption to concanavalin A-Sepharose only when the polysaccharide is present. The final product was hrophoresis in the presence and absence of dodecyl sulfate. A molecular weight of 162,000 +/- 5000 was determined by sedimentation equilibrium analysis in good agreement with the value of 160,000 estimated by gel electrophoresis, but a low-sedimentation constant of 6.5 S suggests that the enzyme is asymmetric. The molecule appears to be made up of a single polypeptide chain with no evidence for multiple repeating sequences: it could not be dissociated into smaller fragments by dodecyl sulfate even after complete carboxymethylation; tryptic cleavage of the native protein yielded only two fragments of molecular weight 20,000 and 140,000 without loss of enzymatic activity. The amino acid composition of the enzyme is reported; no covalently bound phosphate or carbohydrate could be detected. All 32 sulfhydryl groups present were titrated with 5,5'-dithiobis(2-nitrobenzoic acid) under denaturing conditions; eight reacted readily in the native enzyme without loss of catalytic activity, while substitution of eight additional ones lowered the activity by 50%. Inactivation was greatly reduced by glycogen; the polysaccharide also influenced markedly the electrophoretic behavior of the enzyme and large filamentous aggregates were formed when solutions of both were mixed. Purified debranching enzyme releases 3 mumol of glucose min-1 mg-1 at 19 degrees C, pH 6.0, from a glycogen limit dextrin and one-tenth this amount when the native polysaccharide is used as substrate; glycogen is quantitatively degraded in the presence of phosphorylase. None of the usual sugar phosphates or nucleotide effectors of glycolysis affected enzymatic activity. No phosphorylation by either dogfish or rabbit skeletal muscle protein kinase or phosphorylase kinase could be demonstrated, nor any direct interaction with phosphorylase as measured by SH-group reactivity, enzymatic activity, or rate of phosphorylase b to a conversion. Purification of the 160,000 molecular weight M-line protein of skeletal muscle resulted in the quantitative removal of debranching enzyme, indicating that the two proteins are different.

The action pattern of amylomaltase from Escherichia coli

Amylomaltase, the inducible 4-alpha-glucanotransferase of Escherichia coli strain ML, has been purified to homogeneity. Its specific activity with a commercial maltose substrate was 500 mkat/kg protein (30 mumol glucose formed min-1 mg protein-1). The purified enzyme, dependent on buffer concentration, exists in interconvertible low-molecular-weight (apparent molecular weight 71000) and high-molecular-weight (apparent molecular weight 370000) forms. The specificity of amylomaltase has been redefined. Hitherto, the enzyme was thought to be a glucosyltransferase, catalysing the transfer of single glucosyl units, and maltose has been regarded as its most important substrate. Amylomaltase is now shown to exhibit both glucosyl-transfer and 4-alpha-glucanosyl-transfer specificity. 4-alpha-Glucanosyl chains containing up to at least nine glucosyl units can be transferred. However, it is concluded that the transfer reaction by which amylomaltase action was originally expressed, does not take place, i.e., Maltose + maltose in equilibrium Maltotriose + glucose and that maltose has a restricted role as a substrate. This may be due to the inability of maltose to function as a donor substrate, serving only as an acceptor substrate. It is confirmed that when a maltodextrin serves as a donor, that portion of the molecule transfered by the enzyme is that containing the nonreducing-end-group. Enzyme action on chromatographically pure maltose is characterized by a lag phase in the time course of glucose release. The lag pahse is overcome by addition of 'priming' (catalytic) concentrations of maltotriose or higher maltodextrins. An autocatalytic reaction mechanism involving the generation of primer molecules is proposed to explain the action of the enzyme on maltose. The redefined action pattern of amylomaltase is consistent with the redefined role of the enzyme in the utilization of exogenous and endogenous 1,4-alpha-glucans by E. coli.

Purification and properties of a debranching enzyme from Escherichia coli

The debranching enzyme (EC 3.2.1.-) from Escherichia coli K12 was purified 312-fold with a 21% yield, DEAE-cellulose and DEAE-Sephadex chromatography were used for purification. The preparation was homogeneous and showed only a single band of protein and activity upon polyacrylamide gel electrophoresis. The enzyme hydrolyzed 1,6-alpha-glucosidic linkages in phosphorylase and beta-amylase limit dextrins prepared from glycogen and amylopectin. Small branched oligosaccharides were also hydrolyzed. Amylopectin was also completely hydrolyzed but the enzyme showed only a very low activity with glycogen as the substrate. The enzyme cannot be classified as a pullulanase because it has practically no activity with pullulan. But it also differs from the bacterial isoamylases described in other studies because of its inability to hydrolyze glycogen. The optimal pH is about 5.6. The optimal growth conditions for the synthesis of the enzyme by E. coli were also examined in the present studies.