Functional and structural characterization of an endo-β-1,3-glucanase from Euglena gracilis

Endo-β-1,3-glucanases from several organisms have attracted much attention in recent years because of their capability for in vitro degrading β-1,3-glucan as a critical step for both biofuels production and short-chain oligosaccharides synthesis. In this study, we biochemically characterized a putative endo-β-1,3-glucanase (EgrGH64) belonging to the family GH64 from the single-cell protist Euglena gracilis. The gene coding for the enzyme was heterologously expressed in a prokaryotic expression system supplemented with 3% (v/v) ethanol to optimize the recombinant protein right folding. Thus, the produced enzyme was highly purified by immobilized-metal affinity and gel filtration chromatography. The enzymatic study demonstrated that EgrGH64 could hydrolyze laminarin (K_M 23.5 mg ml^-1,k_cat 1.20 s^-1) and also, but with less enzymatic efficiency, paramylon (K_M 20.2 mg ml^-1,k_cat 0.23 ml mg^-1 s^-1). The major product of the hydrolysis of both substrates was laminaripentaose. The enzyme could also use ramified β-glucan from the baker's yeast cell wall as a substrate (K_M 2.10 mg ml^-1, k_cat 0.88 ml mg^-1 s^-1). This latter result, combined with interfacial kinetic analysis evidenced a protein's greater efficiency for the yeast polysaccharide, and a higher number of hydrolysis sites in the β-1,3/β-1,6-glucan. Concurrently, the enzyme efficiently inhibited the fungal growth when used at 1.0 mg/mL (15.4 μM). This study contributes to assigning a correct function and determining the enzymatic specificity of EgrGH64, which emerges as a relevant biotechnological tool for processing β-glucans.

Basic β-1,3-Glucanase from Drosera binata Exhibits Antifungal Potential in Transgenic Tobacco Plants

The basic β-1,3-glucanase of the carnivorous plant Drosera binata was tested as a purified protein, as well as under the control of a double CaMV35S promoter in transgenic tobacco for its capability to inhibit the growth of Trichoderma viride, Rhizoctonia solani, Alternaria solani, and Fusarium poae in an in-vitro assay. The purified protein inhibited tested phytopathogens but not the saprophytic fungus T. viride. Out of the analysed transgenic plants, lines 13, 16, 19, and 22 exhibited high DbGluc1 transcript abundance normalised to the actin transcript. Because of DbGluc1 transgene expression, lines 13 and 16 showed a 1.7-fold increase and lines 19 and 22 showed more than a 2-fold increase in total β-1,3-glucanase activity compared to the non-transgenic control. In accordance with the purified β-1,3-glucanase in-vitro antifungal assay, crude protein extracts of lines 19 and 22 significantly inhibited the growth of phytopathogens (14–34%). Further analyses revealed that the complementary action of transgenic β-1,3-glucanase and 20% higher activity of endogenous chitinase(s) in these lines were crucial for maximising the antifungal efficiency of crude protein extracts.

Novel Anti-Fungal d-Laminaripentaose-Releasing Endo-β-1,3-glucanase with a RICIN-like Domain from Cellulosimicrobium funkei HY-13

Endo-β-1,3-glucanase plays an essential role in the deconstruction of β-1,3-d-glucan polysaccharides through hydrolysis. The gene (1650-bp) encoding a novel, bi-modular glycoside hydrolase family 64 (GH64) endo-β-1,3-glucanase (GluY) with a ricin-type β-trefoil lectin domain (RICIN)-like domain from Cellulosimicrobium funkei HY-13 was identified and biocatalytically characterized. The recombinant enzyme (rGluY: 57.5 kDa) displayed the highest degradation activity for laminarin at pH 4.5 and 40 °C, while the polysaccharide was maximally decomposed by its C-terminal truncated mutant enzyme (rGluYΔRICIN: 42.0 kDa) at pH 5.5 and 45 °C. The specific activity (26.0 U/mg) of rGluY for laminarin was 2.6-fold higher than that (9.8 U/mg) of rGluYΔRICIN for the same polysaccharide. Moreover, deleting the C-terminal RICIN domain in the intact enzyme caused a significant decrease (>60%) of its ability to degrade β-1,3-d-glucans such as pachyman and curdlan. Biocatalytic degradation of β-1,3-d-glucans by inverting rGluY yielded predominantly d-laminaripentaose. rGluY exhibited stronger growth inhibition against Candida albicans in a dose-dependent manner than rGluYΔRICIN. The degree of growth inhibition of C. albicans by rGluY (approximately 1.8 μM) was approximately 80% of the fungal growth. The superior anti-fungal activity of rGluY suggests that it can potentially be exploited as a supplementary agent in the food and pharmaceutical industries.

Efficient endo-β-1,3-glucanase expression in Pichia pastoris for co-culture with Agrobacterium sp. for direct curdlan oligosaccharide production

The production of curdlan oligosaccharides, a multifunctional and valuable carbohydrate, by hydrolyzing polysaccharides is of great interest. The endo-β-1,3-glucanase derived from Trichoderma harzianum was expressed in Pichia pastoris with three commonly used promoters (AOX1, GAP and FLD1). The purified recombinant endo-β-1,3-glucanase expressed by Pichia pastoris with GAP promoter displayed high specific activity at pH 5.5 and 50 °C. Thereafter, a co-culture system of Pichia pastoris GS115 (GAP promoter) and Agrobacterium sp. was constructed in which Agrobacterium sp.-metabolized curdlan can be directly hydrolyzed by Pichia pastoris-secreted endo-β-1,3-glucanase to produce functional curdlan oligosaccharides. The co-culture conditions were optimized and the process was carried out in a 7-L bioreactor. The maximum yield of curdlan oligosaccharides reached 18.77 g/L with 3-10 degrees of polymerization. This study presents a novel and easy curdlan oligosaccharide production strategy that can replace traditional sophisticated production procedures and could potentially be implemented for production of other oligosaccharides.