Purification and properties of an extracellular glucoamylase from a diastatic strain of Saccharomyces cerevisiae

A re-evaluation of diastatic Saccharomyces cerevisiae strains and their role in brewing

Abstract

Diastatic strains of Saccharomyces cerevisiae possess the unique ability to hydrolyze and ferment long-chain oligosaccharides like dextrin and starch. They have long been regarded as important spoilage microbes in beer, but recent studies have inspired a re-evaluation of the significance of the group. Rather than being merely wild-yeast contaminants, they are highly specialized, domesticated yeasts belonging to a major brewing yeast lineage. In fact, many diastatic strains have unknowingly been used as production strains for decades. These yeasts are used in the production of traditional beer styles, like saison, but also show potential for creation of new beers with novel chemical and physical properties. Herein, we review results of the most recent studies and provide a detailed account of the structure, regulation, and functional role of the glucoamylase-encoding STA1 gene in relation to brewing and other fermentation industries. The state of the art in detecting diastatic yeast in the brewery is also summarized. In summary, these latest results highlight that having diastatic S. cerevisiae in your brewery is not necessarily a bad thing.

Key Points

•Diastatic S. cerevisiae strains are important spoilage microbes in brewery fermentations.

•These strains belong to the ‘Beer 2’ or ‘Mosaic beer’ brewing yeast lineage.

•Diastatic strains have unknowingly been used as production strains in breweries.

•The STA1-encoded glucoamylase enables efficient maltotriose use.

Electronic supplementary material

The online version of this article (10.1007/s00253-020-10531-0) contains supplementary material, which is available to authorized users.

A deletion in the STA1 promoter determines maltotriose and starch utilization in STA1+ Saccharomyces cerevisiae strains

Diastatic strains of Saccharomyces cerevisiae are common contaminants in beer fermentations and are capable of producing an extracellular STA1-encoded glucoamylase. Recent studies have revealed variable diastatic ability in strains tested positive for STA1, and here, we elucidate genetic determinants behind this variation. We show that poorly diastatic strains have a 1162-bp deletion in the promoter of STA1. With CRISPR/Cas9-aided reverse engineering, we show that this deletion greatly decreases the ability to grow in beer and consume dextrin, and the expression of STA1. New PCR primers were designed for differentiation of highly and poorly diastatic strains based on the presence of the deletion in the STA1 promoter. In addition, using publically available whole genome sequence data, we show that the STA1 gene is prevalent among the 'Beer 2'/'Mosaic Beer' brewing strains. These strains utilize maltotriose efficiently, but the mechanisms for this have been unknown. By deleting STA1 from a number of highly diastatic strains, we show here that extracellular hydrolysis of maltotriose through STA1 appears to be the dominant mechanism enabling maltotriose use during wort fermentation in STA1+ strains. The formation and retention of STA1 seems to be an alternative evolutionary strategy for efficient utilization of sugars present in brewer's wort. The results of this study allow for the improved reliability of molecular detection methods for diastatic contaminants in beer and can be exploited for strain development where maltotriose use is desired.

Purification and biochemical characterisation of glucoamylase from a newly isolated Aspergillus niger: relation to starch processing

Herein, we investigate a glucoamylase from newly isolated Aspergillus niger. The enzyme was purified, using fractionation, followed by anion-exchange chromatography and then characterised. The molecular mass of the enzyme was estimated to be ∼62,000Da, using SDS-PAGE and 57151Da, based on mass spectrometry results. The pI of the protein, and optimum pH/temperature of enzyme activity were 4.4, 5 and 70°C, respectively and the kinetic parameters (Km, Vmax and kcat) were determined to be 0.33 (mgml(-1)), 0.095 (Uμg(-1)min(-1)) and 158.3 (s(-1)) for soluble starch, respectively. The glucoamylase nature of the enzyme was also confirmed using TLC and a specific substrate. Metal ions Fe(3+), Al(3+) and Hg(2+) had the highest inhibitory effect, while Ag(2)(+), Ca(2+), Zn(2+), Mg(2+) and Cd(2+) and EDTA showed no significant effect on the enzyme activity. In addition, thermal stability of the enzyme increased in the presence of starch and calcium ion. Based on the results, the purified glucoamylase appeared to be a newly isolated enzyme.

Purification and Characterization of a Glucoamylase from Humicola grisea

A thermostable extracellular glucoamylase from the thermophilic fungus Humicola grisea was purified to homogeneity. Its molecular mass and isoelectric point were 74 kDa and 8.4, respectively. The enzyme contained 5% carbohydrate, showed maximal activities at pH 6.0 and 60(deg)C, and was stable at 55(deg)C and pH 6.0 for 2 h. The K(infm) of soluble starch hydrolysis at 50(deg)C and pH 6.0 was 0.14 mg/ml. The purified enzyme was remarkably insensitive to glucose.

Improving the amylolytic activity of Saccharomyces cerevisiae glucoamylase by the addition of a starch binding domain

Glucoamylase produced by amylolytic strains of Saccharomyces cerevisiae (var. diastaticus) lacks a starch binding domain that is present in homologous glucoamylases from Aspergillus niger and other filamentous fungi. The absence of the binding domain makes the enzyme inefficient against raw starch and hence unsuitable for most biotechnological applications. We have constructed a hybrid glucoamylase-encoding gene by in-frame fusion of the S. cerevisiae STA1 gene and DNA fragment that encodes the starch binding domain of A. niger glucoamylase. The hybrid enzyme resulting from expression of the chimeric gene in S. cerevisiae has substrate binding capability and hydrolyses insoluble starch, properties not present in the original yeast enzyme.

Purification and characterization of a thermostable glucoamylase from Chaetomium thermophilum

Thermostable amylolytic enzymes are currently being investigated to improve industrial processes of starch degradation. A thermostable extracellular glucoamylase (exo-1, 4-alpha-D-glucanohydrolase, E.C.3.2.1.3) from the culture supernatant of a thermophilic fungus Chaetomium thermophilum was purified to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) homogeneity by using ammonium sulfate fraction, DEAE-Sepharose Fast Flow chromatography, and Phenyl-Sepharose Fast Flow chromatography. SDS-PAGE of the purified enzyme showed a single protein band of molecular weight 64 kDa. The glucoamylase exhibited optimum catalytic activity at pH 4.0 and 65 degrees C. It was thermostable at 50 degrees C and 60 degrees C, and retained 50% activity after 60 min at 65 degrees C. The half-life of the enzyme at 70 degrees C was 20 min. N-terminal amino acid sequencing (15 residues) was AVDSYIERETPIAWN. Different metal ions showed different effects on the glucoamylase activity. Ca2+, Mg2+, Na+, and K+ enhanced the enzyme activity, whereas Fe2+, Ag+, and Hg2+ cause obvious inhibition. These properties make it applicable to other biotechnological purposes.

Purification and characterization of a thermostable glucoamylase fromAspergillus fumigatus

A thermostable glucoamylase from Aspergillus fumigatus was purified to homogeneity. It was a glycoprotein with 23% carbohydrate content and an apparent molecular mass of 42 kDa. The enzyme showed maximal activities at pH 4.5-5.5 and 65°C and preferentially attacked polysacharides, such as starch, glycogen, amylopectin, and amylose, rather than maltose and maltoriose. The Kmand Vmaxof soluble starch hydrolysis at 40°C and pH 5.0 were 0.1 mg ·mL-1and 161 µmol glucose equivalents liberated ·min-1·mg protein-1, respectively. The purified enzyme was remarkably insensitive to glucose. It was not affected by 500 mM D-glucose and retained about 80% of its original activity in the presence of 1000 mM of this sugar.Key words: amylase, Aspergillus fumigatus, enzyme purification, glucose insensitive, thermostableglucoamylase.

Coregulation of starch degradation and dimorphism in the yeast Saccharomyces cerevisiae

Saccharomyces cerevisiae, the exemplar unicellular eukaryote, can only survive and proliferate in its natural habitats through constant adaptation within the constraints of a dynamic ecosystem. In every cell cycle of S. cerevisiae, there is a short period in the G1 phase of the cell cycle where "sensing" transpires; if a sufficient amount of fermentable sugars is available, the cells will initiate another round of vegetative cell division. When fermentable sugars become limiting, the yeast can execute the diauxic shift, where it reprograms its metabolism to utilize nonfermentable carbon sources. S. cerevisiae can also initiate the developmental program of pseudohyphal formation and invasive growth response, when essential nutrients become limiting. S. cerevisiae shares this growth form-switching ability with important pathogens such as the human pathogen, Candida albicans, and the corn smut pathogen Ustilago maydis. The pseudohyphal growth response of S. cerevisiae has mainly been implicated as a means for the yeast to search for nutrients. An important observation made was that starch-degrading S. cerevisiae strains have the added ability to form pseudohyphae and grow invasively into a starch-containing medium. More significantly, it was also shown that the STA1-3 genes encoding three glucoamylase isozymes responsible for starch hydrolysis in S. cerevisiae are coregulated with a gene, MUC1, essential for pseudohyphal and invasive growth. At least two putative transcriptional activators, Mss10p and Mss11p, are involved in this regulation. The Muc1p is a putative integral membrane-bound protein similar to mammalian mucin-like proteins that have been implicated in the ability of cancer cells to invade other tissues. This provided us with an excellent example of integrative control between nutrient sensing, signaling, and differential development.

Expression of a plant protein by Neurospora crassa

Heterologous expression of plant genes may serve as an important alternative for producing plant proteins. We have investigated the ability of the fungus Neurospora crassa to secrete zeamatin, a protein produced by Zea mays. Zeamatin was induced after being fused to glucoamylase, an extracellular hydrolase produced by N. crassa. Glucoamylase induction and other culture parameters were monitored in untransformed N. crassa grown in shaken liquid culture. A DNA plasmid, pGEZ, was constructed by inserting zeamatin-encoding cDNA into an expression cassette containing the promoter, a truncated open reading frame, and the terminator sequence of the N. crassa glucoamylase gene. Zeamatin-encoding cDNA was modified at the N terminus to include a kex-2 protease site, allowing cleavage of the chimeric product in the secretory pathway. Strains containing the chimeric gene construct were grown in liquid culture and induced for glucoamylase and zeamatin production. Zeamatin antibody detected a protein in a Western blot of concentrated culture supernatants that comigrated with authentic zeamatin. Secreted zeamatin was active in inhibiting the growth of Candida albicans in an agar diffusion assay, indicating that zeamatin had been correctly synthesized, processed, and secreted by N. crassa.

Cultural conditions for production of glucoamylase from Lactobacillus amylovorus ATCC 33621

Lactobacillus amylovorus ATCC 33621 is an actively amylolytic bacterial strain which produces a cell-bound glucoamylase (EC 3.2.1.3). Conditions of growth and glucoamylase production were investigated using dextrose-free de Man-Rogosa-Sharpe (MRS) medium in a 1.5 l fermenter, with varying dextrin concentration (0.1-1.5% (w/v)), pH (4.5-6.5) and temperature (25-55 degrees C). Cell extracts were prepared by subjecting cells to treatment with a French Pressure cell in order to release intracellular proteins. Glucoamylase activity was then assayed. The effects of pH (4.0-9.0), temperature (15-85 degrees C) and substrate (dextrin and starch, 0-2% w/v) concentration on crude enzyme activity were investigated. Optimal growth was obtained in MRS medium containing 1% (w/v) dextrin, at pH 5.5 and 37 degrees C. Glucoamylase production was maximal at the late logarithmic phase of growth, during 16-18 h. Crude enzyme had a pH optimum of 6.0 and temperature optimum of 60 degrees C. With starch as the substrate, maximal activity was obtained at a concentration of 1.5% (w/v). The effects of ions and inhibitors on glucoamylase activity were also investigated. Enzyme activity was not significantly influenced by Ca2+ and EDTA at 1 mmol l-1 concentration; however Pb2+ and Co2+ were found to inhibit the activity at concentrations of 1 mmol l-1. The crude enzyme was found to be thermolabile when glucoamylase activity decreased after about 10 min exposure at 60 degrees C. This property can be exploited in the brewing of low calorie beers where only mild pasteurization treatments are used to inactivate enzymes. The elimination of residual enzyme effect would prevent further maltodextrin degradation and sweetening during long-term storage, thus helping to stabilize the flavour of beer.

Identification, characterization, and partial purification of glucoamylase from Aspergillus niger (syn A. ficuum) NRRL 3135

The crude extracellular extract of Aspergillus niger (syn A. ficuum) NRRL 3135 contains glucoamylase (exo-1,4-alpha-D-glucanohydrolase, EC 3.2.1.2). The enzyme, a glycoprotein, was purified 7-fold by ion-exchange chromatography, chromatofocusing, and conconavalin A affinity chromatography. The molecular weight of the enzyme was estimated to be 90 kDa by SDS-PAGE and gel permeation chromatography. The pI of the enzyme was 3.4. The temperature optimum of the enzyme was 60 degrees C and the pH optimum was 5.0. The Vmax values for soluble starch, maltose, maltotriose, maltotretraose, maltopentaose, and isomaltose were 55.2, 11.7, 32.3, 47.8, 59.2, 12.5 nKat glucose/sec, respectively and the Km values for the same substrates were 0.09%, 0.67 mM, 0.76 mM, 0.76 mM, 0.68 mM, and 122.01 mM, respectively.

Regional sequence homologies in starch-degrading enzymes

The enzymatic hydrolysis of starch, consisting of linear (amylose) and branched (amylopectin) glucose polymers, is catalyzed by alpha-, beta- and glucoamylases (gamma-amylases), cyclodextrinases, alpha-glucosidases, and debranching enzymes. Saccharomyces cerevisiae cannot utilize starch. Our laboratory has previously co-expressed the Bacillus amyloliquefaciens alpha-amylase (AMY) and the Saccharomyces diastaticus glucoamylase (STA2) genes in S. cerevisiae. A gene encoding a debranching enzyme (pullulanase) from Klebsiella pneumoniae ATCC15050 was cloned and its nucleotide sequence determined. This gene will be co-expressed with the alpha- and gamma-amylase to produce an amylolytic S. cerevisiae strain. Extensive data base comparisons of the K. pneumoniae pullulanase amino-acid sequence with the amino-acid sequences of other debranching enzymes and alpha-, beta- and gamma-amylases (from bacteria, yeasts, higher fungi and higher eukaryotes), indicated that these debranching enzymes have amino-acid regions similar to those found in alpha-amylases. The conserved regions in alpha-amylases comprise key residues that are implicated in substrate binding, catalysis, and calcium binding and are as follows. Region 1: DVVINH; region 2: GFRLDAAKH and region 4: FVDNHD. When comparing conserved regions, no similarity could be detected between debranching enzymes and beta- and gamma-amylases.

Isolation and purification of amyloglucosidase from Halobacterium sodomense

Amyloglucosidase from Halobacterium sodomense was purified by a combination of hydrophobic interaction chromatography and immobilized metal ion affinity chromatography at analytical and preparative scale with 75% recovery. The enzyme was found to be a dimer of two different subunits with molecular weights of 72,000 and 82,000 D, respectively, combining in a 175,000 D native protein. The specific activity, KM, and amino acid composition of the enzyme was determined.

Role of maltase in the utilization of sucrose by Candida albicans

Two isoenzymes of maltase (EC 3.2.1.20) were purified to homogeneity from Candida albicans. Isoenzymes I and II were found to have apparent molecular masses of 63 and 66 kDa on SDS/PAGE with isoelectric points of 5.0 and 4.6 respectively. Both isoenzymes resembled each other in similar N-terminal sequence, specificity for the alpha(1-->4) glycosidic linkage and immune cross-reactivity on Western blots using a maltase II antigen-purified rabbit antibody. Maltase was induced by growth on sucrose whereas beta-fructofuranosidase activity could not be detected under similar conditions. Maltase I and II were shown to be unglycosylated enzymes by neutral sugar assay, and more than 90% of alpha-glucosidase activity was recoverable from spheroplasts. These data, in combination with other results from this laboratory [Geber, Williamson, Rex, Sweeney and Bennett (1992) J. Bacteriol. 174, 6992-6996] showing lack of a plausible leader sequence in genomic or mRNA transcripts, suggest an intracellular localization of the enzyme. To establish further the mechanism of sucrose assimilation by maltase, the existence of a sucrose-inducible H+/sucrose syn-transporter was demonstrated by (1) the kinetics of sucrose-induced [14C]sucrose uptake, (2) recovery of intact [14C]sucrose from ground cells by t.l.c. and (3) transport of 0.83 mol of H+/mol of [14C]sucrose. In total, the above is consistent with a mechanism whereby sucrose is transported into C. albicans to be hydrolysed by an intracellular maltase.

Production of the STA2-encoded glucoamylase in Saccharomyces cerevisiae is subject to feed-back control

Three modes of production of the extracellular glucoamylase (GA) in Saccharomyces cerevisiae have been identified; repressed, basal and induced. The repressed mode is found with cells grown in rich media containing non-limiting concentrations of monosaccharides or disaccharides, including GA-hydrolysable maltose, as a sole carbon source. Both the basal and the induced modes (spanned by some seven-fold differences in the rate of GA production) can be displayed by either glucose-limited or glycerol-plus ethanol-consuming cultures; the induced mode is switched over to the basal one due to a feed-back inhibition by extracellularly accumulated GA. It is proposed that the feed-back control involved in GA production can be attenuated by starch which can thus 'induce' higher rates of GA production compared to the basal mode.

The glucoamylase multigene family in Saccharomyces cerevisiae var. diastaticus: an overview

Saccharomyces cerevisiae has been used widely both as a model system for unraveling the biochemical, genetic, and molecular details of gene expression and the secretion process, and as a host for the production of heterologous proteins of biotechnological interest. The potential of starch as a renewable biological resource has stimulated research into amylolytic enzymes and the broadening of the substrate range of S. cerevisiae. The enzymatic hydrolysis of starch, consisting of linear (amylose) and branched glucose polymers (amylopectin), is catalyzed by alpha- and beta-amylases, glucoamylases, and debranching enzymes, e.g., pullulanases. Starch utilization in the yeast S. cerevisiae var. diastaticus depends on the expression of the three unlinked genes, STA1 (chr. IV), STA2 (chr. II), and STA3 (chr. XIV), each encoding one of the extracellular glycosylated glucoamylases isozymes GAI, GAII, or GAIII, respectively. The restriction endonuclease maps of STA1, STA2, and STA3 are identical. These genes are absent in S. cerevisiae, but a related gene, SGA1, encoding an intracellular, sporulation-specific glucoamylase (SGA), is present. SGA1 is homologous to the middle and 3' regions of the STA genes, but lacks a 5' sequence that encodes the domain for secretion of the extracellular glucoamylases. The STA genes are positively regulated by the presence of three GAM genes. In addition to positive regulation, the STA genes are regulated negatively at three levels. Whereas strains of S. diastaticus are capable of expressing the STA genes, most strains of S. cerevisiae contain STA10, whose presence represses the expression of the STA genes in an undefined manner. The STA genes are also repressed in diploid cells, presumably by the MATa/MAT alpha-encoded repressor. STA gene expression is reduced in liquid synthetic media, it is carbon catabolite repressed by glucose, and is inhibited in petite mutants.

Localization of glucoamylase genes of Saccharomyces cerevisiae by pulsed field gel electrophoresis

The chromosomal locations of four glucoamylase-specifying genes in the yeast Saccharomyces cerevisiae have been determined. Chromosomes were separated by pulsed field gel electrophoresis and blots were probed with radiolabelled STA2 and marker DNA from specific yeast chromosomes. The three genes encoding extracellular glucoamylases, STA1 (DEX2), STA2 (DEX1) and STA3 (DEX3) are located on chromosomes IV, II and XIV, respectively. SGA, specifying the sporulation-specific glucoamylase, was positioned on chromosome IX.

A genetic analysis of glucoamylase activity in the diastatic yeast Saccharomyces cerevisiae NCYC 625

The wild diastatic yeast Saccharomyces cerevisiae NCYC 625 has been shown to be homozygous for the glucoamylase-specifying gene STA2. spoII-1-mapping has positioned STA2 on chromosome II. Expression of STA2 is suppressed in some but not all diploids capable of sporulation, and is also inhibited by unlinked nuclear suppressor genes (SGL) found in some S. cerevisiae tester strains. EMS-induced glucoamylase-negative mutants often contain STA2-suppressor mutations. Depending on the allelic status of GEP1, a nuclear gene which also appears able to antagonise SGL-mediated suppression, STA2 expression can be blocked in petite mutants.