Survival of barley (1→3, 1→w4)-β-glucanase isoenzymes during kilning and mashing

Proteomic profiling of barley spent grains guides enzymatic solubilization of the remaining proteins

Within the brewing industry, malted barley is being increasingly replaced by raw barley supplemented with exogenous enzymes to lessen reliance on the time-consuming, high water and energy cost of malting. Regardless of the initial grain of choice, malted or raw, the resultant bulk spent grains are rich in proteins (up to 25% dry weight). Efficient enzymatic solubilization of these proteins requires knowledge of the protein composition within the spent grains. Therefore, a comprehensive proteomic profiling was performed on spent grains derived from (i) malted barley (spent grain A, SGA) and (ii) enzymatically treated raw barley (spent grain B, SGB); data are available via ProteomeXchange with identifier PXD008090. Results from complementary shotgun proteomics and 2D gel electrophoresis showed that the most abundant proteins in both spent grains were storage proteins (hordeins and embryo globulins); these were present at an average of two fold higher in spent grain B. Quantities of other major proteins were generally consistent in both spent grains A and B. Subsequent in silico protein sequence analysis of the predominant proteins facilitated knowledge-based protease selection to enhance spent grain solubilization. Among tested proteases, Alcalase 2.4 L digestion resulted in the highest remaining protein solubilization with 9.2 and 11.7% net dry weight loss in SGA and SGB respectively within 2 h. Thus, Alcalase alone can significantly reduce spent grain side stream, which makes it a possible solution to increase the value of this low-value side stream from the brewing and malt extract beverage manufacturing industry.

Variation in barley (1 → 3, 1 → 4)-β-glucan endohydrolases reveals novel allozymes with increased thermostability

Novel barley (1 → 3, 1 → 4)-β-glucan endohydrolases with increased thermostability. Rapid and reliable degradation of (1 → 3, 1 → 4)-β-glucan to produce low viscosity wort is an essential requirement for malting barley. The (1 → 3, 1 → 4)-β-glucan endohyrolases are responsible for the primary hydrolysis of cell wall β-glucan. The variation in β-glucanase genes HvGlb1 and HvGlb2 that encode EI and EII, respectively, were examined in elite and exotic germplasm. Six EI and 14 EII allozymes were identified, and significant variation was found in β-glucanase from Hordeum vulgare ssp. spontaneum (wild barley), the progenitor of modern cultivated barley. Allozymes were examined using prediction methods; the change in Gibbs free energy of the identified amino acid substitutions to predict changes in enzyme stability and homology modelling to examine the structure of the novel allozymes using the existing solved EII structure. Two EI and four EII allozymes in wild barley accessions were predicted to have improved barley β-glucanase thermostability. One novel EII candidate was identified in existing backcross lines with contrasting HvGlb2 alleles from wild barley and cv Flagship. The contrasting alleles in selected near isogenic lines were examined in β-glucanase thermostability analyses. The EII from wild barley exhibited a significant increase in β-glucanase thermostability conferred by the novel HvGlb2 allele. Increased β-glucanase thermostability is heritable and candidates identified in wild barley could improve malting and brewing quality in new varieties.

Novel Barley (1→3,1→4)-β-Glucan Endohydrolase Alleles Confer Increased Enzyme Thermostability

Barley (1→3,1→4)-β-glucan endohydrolases (β-glucanases; EI and EII) are primarily responsible for hydrolyzing high molecular weight (1→3,1→4)-β-glucans (β-glucan) during germination. Incomplete endosperm modification during malting results in residual β-glucan that can contribute to increased wort viscosity and beer chill haze. Four newly identified forms of EI and EII and the reference enzymes EI-a and EII-a were expressed in Escherichia coli, and the recombinant proteins were characterized for enzyme kinetics and thermostability. EI and EII variants that exhibited higher residual β-glucanase activity than EI-a and EII-a after heat treatment also exhibited increased substrate affinity and decreased turnover rates. The novel EII-l form exhibited significantly increased thermostability compared with the reference EII-a when activity was measured at elevated temperature. EII-l exhibited a T₅₀ value, which indicates the temperature at which 50% of β-glucanase activity remains, 1.3 °C higher than that of EII-a. The irreversible thermal inactivation difference between EII-a and EII-l after 5 min of heat treatment at 56 °C was 11.9%. The functional significance of the three amino acid differences between EII-a and EII-l was examined by making combinatorial mutations in EII-a using site-directed mutagenesis. The S20G and D284E amino acid substitutions were shown to be responsible for the increase in EII-1 thermostability.

Differences in the thermostabilities of barley (1----3,1----4)-beta-glucanases are only partly determined by N-glycosylation

Barley (1----3,1----4)-beta-glucan 4-glucanohydrolase (EC 3.2.1.73) isoenzyme EII carries 4% by weight carbohydrate and is more stable at elevated temperatures than isoenzyme EI, which has no associated carbohydrate. The relationship between carbohydrate content and thermostability has been investigated by treatment of the two isoenzymes with N-glycopeptidase F (EC 3.5.1.52). Removal of carbohydrate from isoenzyme EII results in a decrease in the enzyme's thermostability, but treatment of isoenzyme EI with the N-glycopeptidase F has no effect. In addition, removal of a single N-glycosylation site in isoenzyme EII (Asn190-Ala-Ser) by site-directed mutagenesis of the corresponding cDNA led to a reduction in thermostability, while the introduction of this site into isoenzyme EI enhanced stability. We conclude that N-glycosylation of Asn190 enhances the stability of isoenzyme EII at elevated temperatures, but that other factors related to their primary structures also contribute to the differences in thermostabilities of the barley (1----3,1----4)-beta-glucanases.

Structure and tissue-specific regulation of genes encoding barley (1----3, 1----4)-beta-glucan endohydrolases

Two genes encode (1----3, 1----4)-beta-glucan 4-glucanohydrolase (EC 3.2.1.73) isoenzymes in barley. A gene for isoenzyme EI has been isolated from a barley genomic library and the nucleotide sequence of a 4643 bp fragment determined. The gene is located on barley chromosome 5 while the gene for (1----3, 1----4)-beta-glucanase isoenzyme EII is carried on chromosome 1. The isoenzyme EI gene contains a single 2514 bp intron that is inserted in codon 25 of a sequence encoding a signal peptide of 28 amino acids. The coding region of the mature enzyme is characterized by a high G+C content, which results from an extreme bias towards the use of these nucleotides in the wobble base position of codons. Determination of the nucleotide sequence of the gene has enabled the complete primary structure of the enzyme to be deduced: isoenzyme EI shows 92% positional identity with the primary sequence of (1----3, 1----4)-beta-glucanase isoenzyme EII at both the nucleotide and amino acid level. However, the nucleotide sequences of the two genes diverge markedly in their 3' untranslated regions. Expression sites of the two genes were defined by Northern analysis using oligonucleotide probes specific for these 3' untranslated regions and by amplifying specific cDNAs through the polymerase chain reaction. In the tissues examined, transcription of the isoenzyme EII gene is restricted to the aleurone layer of germinated grain. In contrast, the gene for isoenzyme EI is transcribed at relatively high levels in young leaves, but also in the scutellum and aleurone of germinated grain.

Hybrid bacillus endo-(1-3,1-4)-beta-glucanases: construction of recombinant genes and molecular properties of the gene products

Hybrid beta-glucanase genes were constructed by the reciprocal exchange of the two halves of the isolated beta-glucanase genes from Bacillus amyloliquefaciens and B. macerans. The beta-glucanase hybrid enzyme 1 (H1) contains the 107 amino-terminal residues of mature B. amyloliquefaciens beta-glucanase and the 107 carboxyl-terminal amino acid residues of B. macerans beta-glucanase. The reciprocal beta-glucanase hybrid enzyme 2 (H2) consists of the 105 amino-terminal residues from the B. macerans enzyme and the carboxyl-terminal 107 amino acids from B. amyloliquefaciens. The biochemical properties of the two hybrid enzymes differ significantly from each other as well as from both parental beta-glucanases. Hybrid beta-glucanase H1 exhibits increased thermostability in comparison to other beta-glucanases, especially in an acidic environment. This hybrid enzyme has maximum activity between pH 5.6 and 6.6, whereas the pH-optimum for enzymatic activity of B. amyloliquefaciens beta-glucanase was found to be at pH 6 to 7 and for B. macerans at pH 6.0 to 7.5. Hybrid enzyme 1 being more heat stable than both parental enzymes represents a case of intragenic heterosis. Hybrid beta-glucanase 2 (H2) was found to be more thermolabile than the naturally occurring beta-glucanases it was derived from and the pH-optimum for enzymatic activity was determined to be between pH 7 and pH 8.