Kinetic modeling of phosphorylase-catalyzed iterative β-1,4-glycosylation for degree of polymerization-controlled synthesis of soluble cello-oligosaccharides

Bottom-Up Synthesized Glucan Materials: Opportunities from Applied Biocatalysis

Linear d-glucans are natural polysaccharides of simple chemical structure. They are comprised of d-glucosyl units linked by a single type of glycosidic bond. Noncovalent interactions within, and between, the d-glucan chains give rise to a broad variety of macromolecular nanostructures that can assemble into crystalline-organized materials of tunable morphology. Structure design and functionalization of d-glucans for diverse material applications largely relies on top-down processing and chemical derivatization of naturally derived starting materials. The top-down approach encounters critical limitations in efficiency, selectivity, and flexibility. Bottom-up approaches of d-glucan synthesis offer different, and often more precise, ways of polymer structure control and provide means of functional diversification widely inaccessible to top-down routes of polysaccharide material processing. Here the natural and engineered enzymes (glycosyltransferases, glycoside hydrolases and phosphorylases, glycosynthases) for d-glucan polymerization are described and the use of applied biocatalysis for the bottom-up assembly of specific d-glucan structures is shown. Advanced material applications of the resulting polymeric products are further shown and their important role in the development of sustainable macromolecular materials in a bio-based circular economy is discussed.

Pushing the boundaries of phosphorylase cascade reaction for cellobiose production I: Kinetic model development

One-pot cascade reactions of coupled disaccharide phosphorylases enable an efficient transglycosylation via intermediary α-d-glucose 1-phosphate (G1P). Such transformations have promising applications in the production of carbohydrate commodities, including the disaccharide cellobiose for food and feed use. Several studies have shown sucrose and cellobiose phosphorylase for cellobiose synthesis from sucrose, but the boundaries on transformation efficiency that result from kinetic and thermodynamic characteristics of the individual enzyme reactions are not known. Here, we assessed in a step-by-step systematic fashion the practical requirements of a kinetic model to describe cellobiose production at industrially relevant substrate concentrations of up to 600 mM sucrose and glucose each. Mechanistic initial-rate models of the two-substrate reactions of sucrose phosphorylase (sucrose + phosphate → G1P + fructose) and cellobiose phosphorylase (G1P + glucose → cellobiose + phosphate) were needed and additionally required expansion by terms of glucose inhibition, in particular a distinctive two-site glucose substrate inhibition of the cellobiose phosphorylase (from Cellulumonas uda). Combined with mass action terms accounting for the approach to equilibrium, the kinetic model gave an excellent fit and a robust prediction of the full reaction time courses for a wide range of enzyme activities as well as substrate concentrations, including the variable substoichiometric concentration of phosphate. The model thus provides the essential engineering tool to disentangle the highly interrelated factors of conversion efficiency in the coupled enzyme reaction; and it establishes the necessary basis of window of operation calculations for targeted optimizations toward different process tasks.

Continuous process technology for bottom-up synthesis of soluble cello-oligosaccharides by immobilized cells co-expressing three saccharide phosphorylases

BACKGROUND

Continuous processing with enzyme reuse is a well-known engineering strategy to enhance the efficiency of biocatalytic transformations for chemical synthesis. In one-pot multistep reactions, continuous processing offers the additional benefit of ensuring constant product quality via control of the product composition. Bottom-up production of cello-oligosaccharides (COS) involves multistep iterative β-1,4-glycosylation of glucose from sucrose catalyzed by sucrose phosphorylase from Bifidobacterium adeloscentis (BaScP), cellobiose phosphorylase from Cellulomonas uda (CuCbP) and cellodextrin phosphorylase from Clostridium cellulosi (CcCdP). Degree of polymerization (DP) control in the COS product is essential for soluble production and is implemented through balance of the oligosaccharide priming and elongation rates. A whole-cell E. coli catalyst co-expressing the phosphorylases in high yield and in the desired activity ratio, with CdP as the rate-limiting enzyme, was reported previously.

RESULTS

Freeze-thaw permeabilized E. coli cells were immobilized in polyacrylamide (PAM) at 37-111 mg dry cells/g material. PAM particles (0.25-2.00 mm size) were characterized for COS production (~ 70 g/L) in mixed vessel with catalyst recycle and packed-bed reactor set-ups. The catalyst exhibited a dry mass-based overall activity (270 U/g; 37 mg cells/g material) lowered by ~ 40% compared to the corresponding free cells due to individual enzyme activity loss, CbP in particular, caused by the immobilization. Temperature studies revealed an operational optimum at 30 °C for stable continuous reaction (~ 1 month) in the packed bed (volume: 40 mL; height: 7.5 cm). The optimum reflects the limits of PAM catalyst structural and biological stability in combination with the requirement to control COS product solubility in order to prevent clogging of the packed bed. Using an axial flow rate of 0.75 cm- 1, the COS were produced at ~ 5.7 g/day and ≥ 95% substrate conversion (sucrose 300 mM). The product stream showed a stable composition of individual oligosaccharides up to cellohexaose, with cellobiose (48 mol%) and cellotriose (31 mol%) as the major components.

CONCLUSIONS

Continuous process technology for bottom-up biocatalytic production of soluble COS is demonstrated based on PAM immobilized E. coli cells that co-express BaScP, CuCbP and CcCdP in suitable absolute and relative activities.

Engineering cascade biocatalysis in whole cells for bottom-up synthesis of cello-oligosaccharides: flux control over three enzymatic steps enables soluble production

BACKGROUND

Soluble cello-oligosaccharides (COS, β-1,4-D-gluco-oligosaccharides with degree of polymerization DP 2-6) have been receiving increased attention in different industrial sectors, from food and feed to cosmetics. Development of large-scale COS applications requires cost-effective technologies for their production. Cascade biocatalysis by the three enzymes sucrose-, cellobiose- and cellodextrin phosphorylase is promising because it enables bottom-up synthesis of COS from expedient substrates such as sucrose and glucose. A whole-cell-derived catalyst that incorporates the required enzyme activities from suitable co-expression would represent an important step towards making the cascade reaction fit for production. Multi-enzyme co-expression to reach distinct activity ratios is challenging in general, but it requires special emphasis for the synthesis of COS. Only a finely tuned balance between formation and elongation of the oligosaccharide precursor cellobiose results in the desired COS.

RESULTS

Here, we show the integration of cellodextrin phosphorylase into a cellobiose-producing whole-cell catalyst. We arranged the co-expression cassettes such that their expression levels were upregulated. The most effective strategy involved a custom vector design that placed the coding sequences for cellobiose phosphorylase (CbP), cellodextrin phosphorylase (CdP) and sucrose phosphorylase (ScP) in a tricistron in the given order. The expression of the tricistron was controlled by the strong T7_lacO promoter and strong ribosome binding sites (RBS) for each open reading frame. The resulting whole-cell catalyst achieved a recombinant protein yield of 46% of total intracellular protein in an optimal ScP:CbP:CdP activity ratio of 10:2.9:0.6, yielding an overall activity of 315 U/g dry cell mass. We demonstrated that bioconversion catalyzed by a semi-permeabilized whole-cell catalyst achieved an industrial relevant COS product titer of 125 g/L and a space-time yield of 20 g/L/h. With CbP as the cellobiose providing enzyme, flux into higher oligosaccharides (DP ≥ 6) was prevented and no insoluble products were formed after 6 h of conversion.

CONCLUSIONS

A whole-cell catalyst for COS biosynthesis was developed. The coordinated co-expression of the three biosynthesis enzymes balanced the activities of the individual enzymes such that COS production was maximized. With the flux control set to minimize the share of insolubles in the product, the whole-cell synthesis shows a performance with respect to yield, productivity, product concentration and quality that is promising for industrial production.

Precision synthesis of reducing-end thiol-modified cellulose enabled by enzyme selection

Enzyme-catalyzed iterative β-1,4-glycosylation of β-glycosides is promising for bottom-up polymerization of reducing-end-modified cello-oligosaccharide chains. Self-assembly of the chains from solution yields crystalline nanocellulose materials with properties that are tunable by the glycoside group used. Cellulose chains with a reducing-end thiol group are of interest to install a controllable pattern of site-selective modifications into the nanocellulose material. Selection of the polymerizing enzyme (cellodextrin phosphorylase; CdP) was pursued here to enhance the synthetic precision of β-1-thio-glucose conversion to generate pure “1-thio-cellulose” (≥95%) unencumbered by plain (unlabeled) cellulose resulting from enzymatic side reactions. The CdP from Clostridium stercorarium (CsCdP) was 21 times more active on β-1-thio-glucose (0.17 U/mg; 45 °C) than the CdP from Clostridium cellulosi (CcCdP), and it lacked hydrolase activity, which is substantial in CcCdP, against the α-d-glucose 1-phosphate donor substrate. The combination of these enzyme properties indicated that CsCdP is a practical catalyst for 1-thio-cellulose synthesis directly from β-1-thio-glucose (8 h; 25 mol% yield) that does not require a second enzyme (cellobiose phosphorylase), which was essential when using the less selective CcCdP. The 1-thio-cellulose chains had an average degree of polymerization of ∼10 and were assembled into highly crystalline cellulose II crystallinity material.