Efficient purification, characterization and partial amino acid sequencing of two alpha-1,4-glucan lyases from fungi

Production of 1,5-anhydro-d-fructose by an α-glucosidase belonging to glycoside hydrolase family 31

α-1,4-Glucan lyases [glycoside hydrolase family (GH) 31] catalyze an elimination reaction to form 1,5-anhydro-d-fructose (AF), while GH31 α-glucosidases normally catalyze a hydrolytic reaction. We determined that a small amount of AF was produced by GH31 Aspergillus niger α-glucosidase from maltooligosaccharides by elimination reaction, likely via an oxocarbenium ion intermediate.

Crystal structure of α-1,4-glucan lyase, a unique glycoside hydrolase family member with a novel catalytic mechanism

α-1,4-Glucan lyase (EC 4.2.2.13) from the red seaweed Gracilariopsis lemaneiformis cleaves α-1,4-glucosidic linkages in glycogen, starch, and malto-oligosaccharides, yielding the keto-monosaccharide 1,5-anhydro-D-fructose. The enzyme belongs to glycoside hydrolase family 31 (GH31) but degrades starch via an elimination reaction instead of hydrolysis. The crystal structure shows that the enzyme, like GH31 hydrolases, contains a (β/α)8-barrel catalytic domain with B and B' subdomains, an N-terminal domain N, and the C-terminal domains C and D. The N-terminal domain N of the lyase was found to bind a trisaccharide. Complexes of the enzyme with acarbose and 1-dexoynojirimycin and two different covalent glycosyl-enzyme intermediates obtained with fluorinated sugar analogues show that, like GH31 hydrolases, the aspartic acid residues Asp(553) and Asp(665) are the catalytic nucleophile and acid, respectively. However, as a unique feature, the catalytic nucleophile is in a position to act also as a base that abstracts a proton from the C2 carbon atom of the covalently bound subsite -1 glucosyl residue, thus explaining the unique lyase activity of the enzyme. One Glu to Val mutation in the active site of the homologous α-glucosidase from Sulfolobus solfataricus resulted in a shift from hydrolytic to lyase activity, demonstrating that a subtle amino acid difference can promote lyase activity in a GH31 hydrolase.

Inhibition spectrum studies of microthecin and other anhydrofructose derivatives using selected strains of Gram-positive and -negative bacteria, yeasts and moulds, and investigation of the cytotoxicity of microthecin to malignant blood cell lines

AIMS

To prepare 1,5-anhydro-d-fructose (AF) derivatives, test their microbial inhibition spectrum, and to further examine the most effective AF derivative against Pseudomonas aeruginosa and malignant blood cell lines.

METHODS AND RESULTS

Microthecin and nine other AF derivatives were synthesized from AF. The 10 compounds were tested in vitro against Gram-positive (GP) and Gram-negative (GN) bacteria, yeasts and moulds using a well diffusion method and in a Bioscreen growth analyser. Of the test compounds, microthecin exhibited the most significant antibacterial activity at 100-2000 ppm against both GP and GN bacteria, including Ps. aeruginosa. Further tests with three malignant blood cell lines (Mutu, Ramos, Raji) and one normal cell line indicated that microthecin was a cell toxin, with a cell mortality >85% at 50 ppm. The other nine AF derivatives demonstrated low or no antimicrobial activity.

CONCLUSIONS

Microthecin was active 100-2000 ppm against GP and GN bacteria including Ps. aeruginosa, but was inactive against yeasts and moulds. Microthecin was also a cytotoxin to some mammalian cell lines.

SIGNIFICANCE AND IMPACT OF THE STUDY

Microthecin might have potential for development as a novel drug against Ps. aeruginosa and to target cancer cells. It might also be developed as a food processing aid to control bacterial growth.

Polysaccharide Lyases: Recent Developments as Biotechnological Tools

Polysaccharide lyases, which are polysaccharide cleavage enzymes, act mainly on anionic polysaccharides. Produced by prokaryote and eukaryote organisms, these enzymes degrade (1,4) glycosidic bond by a beta elimination mechanism and have unsaturated oligosaccharides as major products. New polysaccharides are cleaved only by their specific polysaccharide lyases. From anionic polysaccharides controlled degradations, various biotechnological applications were investigated. This review catalogues the degradation of bacterial, plant and animal polysaccharides (neutral and anionic) by this family of carbohydrate acting enzymes.

The anhydrofructose pathway and its possible role in stress response and signaling

Anhydrofructose (AF) pathway describes the catabolism of alpha-1,4-glucans of glycogen, starch and maltosaccharides to various metabolites via the central intermediate AF. The reaction sequence of the pathway consists of more than 10 enzymatic steps. This pathway occurs in certain bacteria, fungi, algae and mammals. In this communication, the AF pathway and its regulatory mechanisms in these organisms are presented and the metabolites of this pathway as antioxidants and antimicrobials in biotic and abiotic stress responses and in carbon starvation signaling are discussed.

Enzymatic description of the anhydrofructose pathway of glycogen degradation

The anhydrofructose pathway describes the degradation of glycogen and starch to metabolites via 1,5-anhydro-D-fructose (1,5AnFru). Enzymes that form 1,5AnFru, ascopyrone P (APP), and ascopyrone M (APM) have been reported from our laboratory earlier. In the present study, APM formed from 1,5AnFru was found to be the intermediate to the antimicrobial microthecin. The microthecin forming enzyme from the fungus Phanerochaete chrysosporium proved to be aldos-2-ulose dehydratase (AUDH, EC 4.2.1.-), which was purified and characterized for its enzymatic and catalytic properties. The purified AUDH showing a molecular mass of 97.4 kDa on SDS-PAGE was partially sequenced. Total 332 amino acid residues in length were obtained, representing some 37% of the AUDH protein. The obtained amino acid sequences showed no homology to known proteins but to an unannotated DNA sequence in Scaffold 62 of the published genome of the fungus. The alignment revealed three introns of the identified AUDH gene (Audh; ph.chr), thus the first gene coding for a neutral sugar dehydratase is identified. AUDH was found to be a bi-functional enzyme, being able to dehydrate 1,5AnFru to APM and further isomerizing the APM formed to microthecin. The optimal pH for the formation of APM and microthecin was pH 5.8 and 6.8, respectively. AUDH showed 5 fold higher activity toward 1,5AnFru than toward its analogue glucosone, when tested at concentrations from 0.6 mM to 0.2 M. Based on the characteristic UV absorbance of microthecin (230 nm) and APM (262 nm) assay methods were developed for the microthecin forming enzymes.

Enzymatic description of the anhydrofructose pathway of glycogen degradation; I. Identification and purification of anhydrofructose dehydratase, ascopyrone tautomerase and alpha-1,4-glucan lyase in the fungus Anthracobia melaloma

The anhydrofructose pathway describes the degradation of glycogen and starch to metabolites via 1,5-anhydro-d-fructose (1,5AnFru). The enzyme catalyzing the first reaction step of this pathway, i.e., alpha-1,4-glucan lyase (EC 4.2.1.13), has been purified, cloned and characterized from fungi and red algae in our laboratory earlier. In the present study, two 1,5AnFru metabolizing enzymes were discovered in the fungus Anthracobia melaloma for the formation of ascopyrone P (APP), a fungal secondary metabolite exhibiting antibacterial and antioxidant activity. These are 1,5AnFru dehydratase (AFDH) and ascopyrone tautomerase (APTM). AFDH catalyzed the conversion of 1,5AnFru to ascopyrone M (APM), a compound that has been earlier presumed to occur biologically, while APTM isomerized the APM formed to APP. Both enzymes were purified 400-fold by (NH(4))(2)SO(4) fractionation, hydrophobic interaction, ion-exchange and gel filtration chromatography. The purified AFDH showed a molecular mass of 98 kDa on SDS-PAGE and 230 kDa by gel filtration. The corresponding values for APTM was 60 and 140 kDa. Spectrophotometric and HPLC methods were developed for the assay of these two enzymes. To confirm that A. melaloma possessed all enzymes needed for conversion of glycogen to APP, an alpha-1,4-glucan lyase from this fungus was isolated and partially sequenced. Based on this work, a scheme of the enzymatic description of the anhydrofructose pathway in A. melaloma was proposed.

Investigation of the effectiveness of Ascopyrone P as a food preservative

Ascopyrone P (APP), a novel antibacterial from fungi, was evaluated as a food preservative. Efficacy was generally assessed by comparing the time taken for test strains to grow to 10(6) CFU/g in food +/- APP. In chilled chicken soup, 2000 mg kg-1 APP prevented Bacillus cereus, Listeria monocytogenes, Pseudomonas fluorescens, Salmonella and Escherichia coli reaching this threshold for >60 days. Good activity was also observed at 500-1000 mg kg-1 but not against L. monocytogenes. No activity was observed against Saccharomyces cerevisiae. Activity was reduced at 20 degrees C, although 2000 mg kg-1 was still effective against B. cereus and P. fluorescens. APP was less effective in chilled cooked meat systems and ineffective in raw meat. In a cooked meat system at 8 degrees C, bacteriostatic effect was generally observed at 2000 mg kg-1 against Salmonella typhimurium, E. coli and P. fluorescens but not against L. monocytogenes or Lactobacillus sake. Activity against Gram-negative enteric bacteria was enhanced by low temperature. In milk, 2000 mg l-1 was effective against P. fluorescens at chilled but not ambient temperature. APP was ineffective against yeasts and the mould Byssochlamys in apple juice. A minimum of 2000 mg kg-1 APP would appear to be necessary for antibacterial efficacy in food, although low-temperature storage may help. Observed variations in sensitivity may be related to APP stability, which decreases >pH 5.5. Toxicology testing is needed before consideration of APP for food use.

Ascopyrone P, a novel antibacterial derived from fungi

AIMS

To assess the antimicrobial efficacy of ascopyrone P (APP), a secondary metabolite formed by the fungi Anthracobia melaloma, Plicaria anthracina, Plic. leiocarpa and Peziza petersi belonging to the order Pezizales.

METHODS AND RESULTS

In vitro testing using a well diffusion procedure showed that APP at a high concentration (approximately 5%) inhibited the growth of Gram-positive and Gram-negative bacteria. Using an automated microbiology reader, growth curve analysis showed that 2000-4000 mg l(-1) APP caused total or significant bacterial inhibition after incubation for 24 h at 30 degrees C. Against certain yeast strains, 1000- 2000 mg l(-1) APP enhanced growth, although at higher concentrations inhibition of some yeasts was observed. Clostridium and fungal strains were not sensitive to 2000 mg l(-1) APP. No significant cidal effect was observed after 2 h against Listeria monocytogenes or Escherichia coli. Results were identical whether the APP samples tested had been produced enzymatically or chemically.

CONCLUSIONS

At a level of 2000 mg l(-1), APP demonstrated growth inhibitory activity against a broad range of bacteria, but not yeasts or moulds.

SIGNIFICANCE AND IMPACT OF THE STUDY

A possible application for this novel natural antimicrobial is in food preservation, to control the growth of Gram-negative and Gram-positive bacteria in raw and cooked foods. Effective dosage levels would be 500-4000 mg kg(-1), depending on food type. The efficacy, organoleptic and safety aspects of this compound in food still need to be assessed.

1,5-Anhydro-D-fructose; a versatile chiral building block: biochemistry and chemistry

There is a steadily increasing need to expand sustainable resources, and carbohydrates are anticipated to play an important role in this respect, both for bulk and fine chemical preparation. The enzyme alpha-(1-->4)-glucan lyase degrades starch to 1,5-anhydro-D-fructose. This compound, which has three different functional properties, a prochiral center together with a permanent pyran ring, renders it a potential chiral building block for the synthesis of valuable and potentially biologically active compounds. 1,5-Anhydro-D-fructose is found in natural materials as a degradation product of alpha-(1-->4)-glucans. The occurrence of lyases and the metabolism of 1,5-anhydro-D-fructose are reviewed in the biological part of this article. In the chemical part, the elucidated structure of 1,5-anhydro-D-fructose will be presented together with simple stereoselective conversions into hydroxy/amino 1,5-anhydro hexitols and a nojirimycin analogue. Synthesis of 6-O-acylated derivatives of 1,5-anhydro-D-fructose substituted with long fatty acid residues is carried out using commercially available enzymes. Those reactions lead to compounds with potential emulsifying properties. The use of protected derivatives of 1,5-anhydro-D-fructose for the synthesis of natural products is likewise reviewed. The potential utilization of this chemical building block is far from being exhausted. Since 1,5-anhydro-D-fructose now is accessible in larger amounts through a simple-enzyme catalyzed degradation of starch by alpha-(1-->4)-glucan lyase, the application of 1,5-anhydro-D-fructose may be considered a valuable contribution to the utilization of carbohydrates as the most abundant resource of sustainable raw materials.

Purification and properties of a glucuronan lyase from Sinorhizobium meliloti M5N1CS (NCIMB 40472)

A glucuronan lyase extracted from Sinorhizobium meliloti strain M5N1CS was purified to homogeneity by anion-exchange chromatography. The purified enzyme corresponds to a monomer with a molecular mass of 20 kDa and a pI of 4.9. A specific activity was found only for polyglucuronates leading to the production of 4,5-unsaturated oligoglucuronates. The enzyme activity was optimal at pH 6.5 and 50 degrees C. Zn(2+), Cu(2+), and Hg(2+) (1 mM) inhibited the enzyme activity. No homology of the enzyme N-terminal amino acid sequence was found with any of the previously published protein sequences. This enzyme purified from S. meliloti strain M5N1CS corresponding to a new lyase was classified as an endopolyglucuronate lyase.

N-linked oligosaccharide processing enzyme glucosidase II produces 1,5-anhydrofructose as a side product

alpha-1,4-Glucan lyase cleaves alpha-1,4-linkages of nonreducing termini of alpha-1,4-glucans to produce 1,5-anhydrofructose (1,5-AnFru). The enzymes isolated from fungi and algae show high homology with glycoside hydrolase family 31. Purification of alpha-1,4-glucan lyase from rat liver using DEAE Cellulose chromatography resulted in separation of two enzymatic active fractions, one was bound to the column and the other was in the flow-through. Partial amino acid sequence determined from the lyase, retained on the anion exchange column, were identical with that of the N:-linked oligosaccharide processing enzyme glucosidase II. The lyase showed similar enzymatic properties as the microsomal glucosidase such as inhibition by 1-deoxynojirimycin and castanospermine. On the other hand, glucosidase II purified from rat liver microsomes produced not only glucose but also a small amount of 1,5-AnFru using maltose as substrate. Furthermore, CHO cells overexpressing pig liver glucosidase II showed a 1.5- to 2-fold higher lyase activity compared to the nontransfected CHO cells. Conversely, no lyase activity was detectable either in PHAR2.7, the glucosidase II-deficient mutant from a mouse lymphoma cell line, or in Saccharomyces cerevisiae strain YG427 having the glucosidase II gene disrupted. These data demonstrate that glucosidase II possesses an additional enzymatic activity of releasing 1,5-AnFru from maltose.

Characterization of exo-(1,4)-alpha glucan lyase from red alga Gracilaria chorda. Activation, inactivation and the kinetic properties of the enzyme

Exo-(1,4)-alpha glucan lyase (GLase) was purified from a red alga Gracilaria chorda. The enzyme was activated 1.3-fold in the presence of Ca(2+) and Cl(-) ions. The ions also stabilized the enzyme increasing the temperature of its maximum activity from 45 degrees C to 50 degrees C. GLase was inactivated by chemical modification with carbodiimide and a carboxyl group of the enzyme was shown essential to the lyase activity. A tryptophanyl residue(s) was also shown to be important for the activity and was probably involved in substrate binding. K(m) values of the enzyme were 2.3 mM for maltose, 0.4 mM for maltotriose and 0.1 mM for maltooligosaccharides of degree of polymerization (dp) 4-7, and the k(0) values for the oligosaccharides were similar (42-53 s(-1)). The analysis of these kinetic parameters showed that the enzyme has four subsites to accommodate oligosaccharides. The subsite map of GLase was unique, since subsite 1 and subsite 2 have large positive and small negative affinities, respectively. The subsite map of this type has not been found in other enzymes with exo-action on alpha-1,4-glucan. The K(m) and k(0) values for the polysaccharides were lower (0.03 mM) and higher (60-100 s(-1)), respectively, suggesting the presence of another affinity site specific to the polysaccharides.

alpha-1,4-glucan lyases producing 1,5-anhydro-D-fructose from starch and glycogen have sequence similarity to alpha-glucosidases

In the past few years a novel enzyme alpha-1,4-glucan lyase (EC 4.2. 2.13), which releases 1,5-anhydrofructose from starch and glycogen, has been cloned and characterized from red algae and fungi. Accumulated evidence indicates that the lytic degradation of starch and glycogen also occurs in other organisms. The present review focuses on the biochemical and molecular aspects of eight known alpha-1,4-glucan lyases and their genes from red algae and fungi. While the amino acid sequence identity is 75-80% among the alpha-1, 4-glucan lyases from each of the taxonomic groups, the identity between the algal and fungal alpha-1,4-glucan lyases is only 25-28%. Notably database searches disclosed that the alpha-1,4-glucan lyases have a clear identity of 23-28% with alpha-glucosidases of glycoside hydrolase family 31, thus for the first time linking enzymes from the class of hydrolases with that of lyases. The alignment of lyases and alpha-glucosidases revealed seven well-conserved regions, three of which have been reported to be involved in catalysis and substrate binding in alpha-glucosidases. The shared substrate and inhibitor specificity and sequence similarity of alpha-1,4-glucan lyases with alpha-glucosidases suggest that related structural elements are involved in the two different catalytic mechanisms.

A group of alpha-1,4-glucan lyases and their genes from the red alga Gracilariopsis lemaneiformis: purification, cloning, and heterologous expression

We present here the first report of a group of alpha-1,4-glucan lyases (EC 4.2.2.13) and their genes. The lyases produce 1, 5-anhydro-D-fructose from starch and related oligomers and polymers. The enzymes were isolated from the red alga Gracilariopsis lemaneiformis from the Pacific coasts of China and USA, and the Atlantic Coast of Venezuela. Three lyase isozymes (GLq1, GLq2 and GLq3) from the Chinese subspecies, two lyase isozymes (GLs1 and GLs2) from the USA subspecies and one lyase (GLa1) from the Venezuelan subspecies were identified and investigated. GLq1, GLq3, GLs1 and GLa1 were purified and partially sequenced. Based on the amino acid sequences obtained, three lyase genes or their cDNAs (GLq1, GLq2 and GLs1) were cloned and completely sequenced and two other genes (GLq3 and GLs2) were partially sequenced. The coding sequences of the lyase genes GLq1, GLq2 and GLs1 are 3267, 3276 and 3279 bp, encoding lyases of 1088, 1091 and 1092 amino acids, respectively. The deduced molecular masses of the mature lyases from the coding sequences are 117030, 117667 and 117790 Da, respectively, close to those determined by mass spectrometry using purified lyases. The amino acid sequence identity is more than 70% among the six algal lyase isozymes. The algal GLq1 gene was expressed in Pichia pastoris and Aspergillus niger, and the expression product was identical to the wild-type enzyme.