Biological sources, metabolism, and production of glucosylglycerols, a group of natural glucosides of biotechnological interest

Glucosylglycerol phosphorylase, a potential novel pathway of microbial glucosylglycerol catabolism

Glucosylglycerol (GG) is a natural compatible solute that can be synthesized by many cyanobacteria and a few heterotrophic bacteria under high salinity conditions. In cyanobacteria, GG is synthesized by GG-phosphate synthase and GG-phosphate phosphatase, and a hydrolase GGHA catalyzes its degradation. In heterotrophic bacteria (such as some Marinobacter species), a fused form of GG-phosphate phosphatase and GG-phosphate synthase is present, but the cyanobacteria-like degradation pathway is not available. Instead, a phosphorylase GGP, of which the coding gene is located adjacent to the gene that encodes the GG-synthesizing enzyme, is supposed to perform the GG degradation function. In the present study, a GGP homolog from the salt-tolerant M. salinexigens ZYF650T was characterized. The recombinant GGP catalyzed GG decomposition via a two-step process of phosphorolysis and hydrolysis in vitro and exhibited high substrate specificity toward GG. The activity of GGP was enhanced by inorganic salts at low concentrations but significantly inhibited by increasing salt concentrations. While the investigation on the physiological role of GGP in M. salinexigens ZYF650T was limited due to the failed induction of GG production, the heterologous expression of ggp in the living cells of the GG-producing cyanobacterium Synechocystis sp. PCC 6803 significantly reduced the salt-induced GG accumulation. Together, these data suggested that GGP may represent a novel pathway of microbial GG catabolism. KEY POINTS: • GGP catalyzes GG degradation by a process of phosphorolysis and hydrolysis • GGP-catalyzed GG degradation is different from GGHA-based GG degradation • GGP represents a potential novel pathway of microbial GG catabolism.

Turning waste into treasure: Biosynthesis of value-added 2-O-α-glucosyl glycerol and d-allulose from waste cane molasses through an in vitro synthetic biology platform

The efficient and economical conversion of agricultural waste into glycosides and rare sugars is challenging. Herein, an in vitro synthetic bienzyme system consisting of sucrose phosphorylase and d-allulose 3-epimerase was constructed to produce 2-O-α-glucosyl glycerol and d-allulose from cane molasses. Lactic acid in the cane molasses significantly induced sucrose phosphorylase to hydrolyze sucrose instead of glycosylation. Notably, lactic acid significantly inhibited the catalytic performance of d-allulose 3-epimerase only in the presence of Na+ and K+, with an inhibition rate of 75%. After removing lactic acid and metal ions, 116 g/L 2-O-α-glucosyl glycerol and 51 g/L d-allulose were synthesized from 500 mM sucrose in the treated cane molasses with a sucrose consumption rate of 97%. Our findings offer an economically efficient and environmentally friendly pathway for the industrial production of glycosides and rare sugars from food industry waste.

Arthrospira promotes plant growth and soil properties under high salinity environments

Salt stress detrimentally impacts plant growth, imperiling crop yield and food quality. Ameliorating plant resilience and productivity in saline environments is critical for global food security. Here, we report the positive effect of Arthrospira (Spirulina) on plant growth and salt tolerance in Arabidopsis and sweet sorghum. Arthrospira application greatly promotes seed germination and seedling growth in both species under salt stress conditions in a dosage-dependent manner. Application of 6 mg Arthrospira per plate significantly enhances K+/Na+ equilibrium and reactive oxygen species (ROS) scavenging in Arabidopsis, reducing salt-induced toxicity. The primary root length, survival rate, chlorophyll content, photosynthesis, plant height, biomass and yield were all improved in both species. Concurrently, Arthrospira demonstrated the synthesis of compatible solutes, such as trehalose (Tre) and glucosylglycerol (GG), contributing to heightened stress tolerance when co-cultivated with Arabidopsis on plates. Transcriptome analysis revealed dramatic up-/down- regulation of genes involved in phytohormone signal transduction, chlorophyll and photosynthesis metabolism, and phenylpropanoid metabolism in Arabidopsis. Furthermore, the application of Arthrospira exerted a positive influence on the rhizosphere bacteriome structure in sweet sorghum, crucial for nutrient cycling and soil health enhancement. Our findings uncovered the underlying mechanisms of algae-plants interaction in saline soil, proposing strategies to enhance crop productivity and soil quality, thereby addressing the urgent need for sustainable agriculture practices to mitigate salinity's repercussions amidst climate change challenges.

Research progress and application of enzymatic synthesis of glycosyl compounds

Glucoside compounds are widely found in nature and have garnered significant attention in the medical, cosmetics, and food industries due to their diverse pharmaceutical properties, biological activities, and stable application characteristics. Glycosides are mainly obtained by direct extraction from plants, chemical synthesis, and enzymatic synthesis. Given the challenges associated with plant extraction, such as low conversion rates and the potential for environmental pollution with chemical synthesis, our review focuses on enzymatic synthesis. Here, we reviewed the enzymatic synthesis methods of 2-O-α-D-glucopyranosyl-L-ascorbic acid (AA-2G), 2-O-α-D-glucosyl glycerol (α-GG), arbutin and α-glucosyl hesperidin (Hsp-G), and other glucoside compounds. The types of enzymes selected in the synthesis process are comprehensively analyzed and summarized, as well as a series of enzyme transformation strategies adopted to improve the synthetic yield. KEY POINTS: • Glycosyl compounds have applications in the biomedical and food industries. • Enzymatic synthesis converts substrates into products using enzymes as catalysts. • Substrate bias and specificity are key to improving substrate conversion.

Efficient 2-O-α-D-glucopyranosyl-sn-glycerol production by single whole-cell biotransformation through combined engineering and expression regulation with novel sucrose phosphorylase from Leuconostoc mesenteroides ATCC 8293

2-O-α-D-glucopyranosyl-sn-glycerol (2-αGG) is a high value product with wide applications. Here, an efficient, safe and sustainable bioprocesses for 2-αGG production was designed. A novel sucrose phosphorylase (SPase) was firstly identified from Leuconostoc mesenteroides ATCC 8293. Subsequently, SPase mutations were processed with computer-aided engineering, of which the activity of SPaseK138C was 160% higher than that of the wild-type. Structural analysis revealed that K138C was a key functional residue moderating substrate binding pocket and thus influences catalytic activity. Furthermore, Corynebacterium glutamicum was employed to construct microbial cell factories along with ribosome binding site (RBS) fine-tuning and a two-stage substrate feeding control strategy. The maximum production of 2-αGG by these combined strategies reached 351.8 g·L^-1 with 98% conversion rate from 1.4 M sucrose and 3.5 M glycerol in a 5-L bioreactor. This was one of the best performance reported in single-cell biosynthesis of 2-αGG, which paved effective ways for industrial-scale preparation of 2-αGG.

From lab to application: Cultivating limnetic microalgae in seawater coupled with wastewater for biodiesel production on a pilot scale

The technology of cultivating salt-tolerant limnetic microalgae in seawater reduces the freshwater demand and costs of biodiesel production. However, all current trials still occur on the bench scale, and efforts for pilot-scale operation are urgently needed. This study firstly optimised the diameter of the photobioreactors (PBRs) to 0.2 m, as the single module to produce lipid-rich Golenkinia sp. SDEC-16 because of the better algal growth and light attenuation in the PBRs, and then established a 1000 L algal cultivation system. In a medium of seawater supplemented with monosodium glutamate wastewater at a ratio of 1:1000 (S-MSGW), the biomass productivity was 0.26 g/L/d, which was approaching the 0.30 g/L/d obtained in BG11, and the lipid productivity (98.99 mg/L/d) was doubled in comparison to growth in BG11. C16-C18 accounted for more than 98% of the total fatty acid in S-MSGW, and the biodiesel properties also met the biodiesel standards. The input cost of the biodiesel in this pilot-scale system was estimated to be 2.2 $/kg. When considering the carbon reduction and diversified application of the biomass, Golenkinia sp. would annually capture 186.77 kg/m³ PBR of CO₂, and yield an output-to-input ratio (OIR) of 3.4 in S-MSGW, higher than the 2.8 in BG11.