Preparation and enzymatic behavior of surfactant-enveloped enzymes for glycosynthesis in nonaqueous aprotic media

Progress and challenges in the synthesis of sequence controlled polysaccharides

The sequence, length and substitution of a polysaccharide influence its physical and biological properties. Thus, sequence controlled polysaccharides are important targets to establish structure-properties correlations. Polymerization techniques and enzymatic methods have been optimized to obtain samples with well-defined substitution patterns and narrow molecular weight distribution. Chemical synthesis has granted access to polysaccharides with full control over the length. Here, we review the progress towards the synthesis of well-defined polysaccharides. For each class of polysaccharides, we discuss the available synthetic approaches and their current limitations.

β-Glucan phosphorylases in carbohydrate synthesis

Abstract

β-Glucan phosphorylases are carbohydrate-active enzymes that catalyze the reversible degradation of β-linked glucose polymers, with outstanding potential for the biocatalytic bottom-up synthesis of β-glucans as major bioactive compounds. Their preference for sugar phosphates (rather than nucleotide sugars) as donor substrates further underlines their significance for the carbohydrate industry. Presently, they are classified in the glycoside hydrolase families 94, 149, and 161 (www.cazy.org). Since the discovery of β-1,3-oligoglucan phosphorylase in 1963, several other specificities have been reported that differ in linkage type and/or degree of polymerization. Here, we present an overview of the progress that has been made in our understanding of β-glucan and associated β-glucobiose phosphorylases, with a special focus on their application in the synthesis of carbohydrates and related molecules.

Key points

• Discovery, characteristics, and applications of β-glucan phosphorylases.

• β-Glucan phosphorylases in the production of functional carbohydrates.

Phosphorylase-catalyzed bottom-up synthesis of short-chain soluble cello-oligosaccharides and property-tunable cellulosic materials

Cellulose-based materials are produced industrially in countless varieties via top-down processing of natural lignocellulose substrates. By contrast, cellulosic materials are only rarely prepared via bottom up synthesis and oligomerization-induced self-assembly of cellulose chains. Building up a cellulose chain via precision polymerization is promising, however, for it offers tunability and control of the final chemical structure. Synthetic cellulose derivatives with programmable material properties might thus be obtained. Cellodextrin phosphorylase (CdP; EC 2.4.1.49) catalyzes iterative β-1,4-glycosylation from α-d-glucose 1-phosphate, with the ability to elongate a diversity of acceptor substrates, including cellobiose, d-glucose and a range of synthetic glycosides having non-sugar aglycons. Depending on the reaction conditions leading to different degrees of polymerization (DP), short-chain soluble cello-oligosaccharides (COS) or insoluble cellulosic materials are formed. Here, we review the characteristics of CdP as bio-catalyst for synthetic applications and show advances in the enzymatic production of COS and reducing end-modified, tailored cellulose materials. Recent studies reveal COS as interesting dietary fibers that could provide a selective prebiotic effect. The bottom-up synthesized celluloses involve chains of DP ≥ 9, as precipitated in solution, and they form ~5 nm thick sheet-like crystalline structures of cellulose allomorph II. Solvent conditions and aglycon structures can direct the cellulose chain self-assembly towards a range of material architectures, including hierarchically organized networks of nanoribbons, or nanorods as well as distorted nanosheets. Composite materials are also formed. The resulting materials can be useful as property-tunable hydrogels and feature site-specific introduction of functional and chemically reactive groups. Therefore, COS and cellulose obtained via bottom-up synthesis can expand cellulose applications towards product classes that are difficult to access via top-down processing of natural materials.

Solid-in-oil dispersion: a novel core technology for drug delivery systems

Drug delivery systems using a solid-in-oil (S/O) dispersion as a core technology have advanced significantly over the past ten years. A novel, effective and practical preparation method for a S/O dispersion was originally established in 1997 as a tool for enzymatic catalysis in organic media. This oil-based dispersion containing proteins in non-aqueous media had great potential for applications to other research with one of the most successful being its adaptation as a drug delivery system. The history and features of novel processes for preparing S/O dispersions are presented in this article. In addition, recent research into the use of S/O dispersions for innovative oral and skin drug delivery systems is discussed.

One-step synthesis of cellulose from cellobiose via protic acid-assisted enzymatic dehydration in aprotic organic media

Direct and efficient enzymatic synthesis of long-chain cellulose from cellobiose in its original form was successfully achieved via the combination of a surfactant-enveloped enzyme (SEE) and a protic acid in an aprotic organic solvent, lithium chloride/N,N-dimethylacetamide system. The SEE biocatalyst was prepared by protecting the surface of cellulase with the nonionic surfactant dioleyl-N-D-glucona-L-glutamate for keeping its enzymatic activity in nonaqueous media. Fourier transform infrared and nuclear magnetic resonance analyses elucidated the successful synthesis of cellulose, β-1,4-linked D-glucopyranose polymer, through the reverse hydrolysis of cellobiose. By using protic acid cocatalysts, a degree of polymerization of as-synthesized cellulose reached more than 120, in a ca. 26% conversion, which was 5 times higher than that obtained in an acid-free SEE system. A novel-concept biocatalysis, i.e., a protic acid-assisted SEE-mediated reaction, enables a facile, one-step chain elongation of carbohydrates without any activation via multistep organic chemistry, and can provide potential applications in the functional design of glycomaterials.

Enzymatic synthesis of cellulose II-like substance via cellulolytic enzyme-mediated transglycosylation in an aqueous medium

The enzymatic synthesis of cellulose-like substance via a non-biosynthetic pathway has been achieved by transglycosylation in an aqueous system of the corresponding substrate, cellotriose for cellulolytic enzyme endo-acting endoglucanase I (EG I) from Hypocrea jecorina. A significant amount of water-insoluble product precipitated out from the reaction system. MALDI-TOF mass analysis showed that the resulting precipitate had a degree of polymerization (DP) of up to 16 from cellotriose. Solid-state (13)C NMR spectrum of the resulting water-insoluble product revealed that all carbon resonance lines were assigned to two kinds of anhydroglucose residues in the corresponding structure of cellulose II. X-ray diffraction (XRD) measurement as well as (13)C NMR analysis showed that the crystal structure corresponds to cellulose II with a high degree of crystallinity. We propose the multiple oligomers form highly crystalline cellulose II as a result of self-assembly via oligomer-oligomer interaction when they precipitate.