Recent advances in enzymatic synthesis of β-glucan and cellulose

Abstract

Bottom-up synthesis of β-glucans such as callose, fungal β-(1,3)(1,6)-glucan and cellulose, can create the defined compounds that are needed to perform fundamental studies on glucan properties and develop applications. With the importance of β-glucans and cellulose in high-profile fields such as nutrition, renewables-based biotechnology and materials science, the enzymatic synthesis of such relevant carbohydrates and their derivatives has attracted much attention. Here we review recent developments in enzymatic synthesis of β-glucans and cellulose, with a focus on progress made over the last five years. We cover the different types of biocatalysts employed, their incorporation in cascades, the exploitation of enzyme promiscuity and their engineering, and reaction conditions affecting the production as well as in situ self-assembly of (non)functionalised glucans. The recent achievements in the application of glycosyl transferases and β-1,4- and β-1,3-glucan phosphorylases demonstrate the high potential and versatility of these biocatalysts in glucan synthesis in both industrial and academic contexts.

Keywords: Cellulose; Glucans; Glycoside hydrolases; Glycoside phosphorylases; Glycosyltransferases; Nanostructures.

Graphical abstract

Fig. 1

A) structure of natural glucans. B) Glycosidic bond formation and/or cleavage catalysed by different CAZy families. GS = glycosynthases (Leaving Group: Fluorine or O-para-nitrophenyl); GT = glycosyltransferases (NDP: nucleotide diphosphate); GP = glycoside phosphorylases and GH = glycoside hydrolases. (Part B was adapted from Ref. [23]).

Fig. 2

A & B Freeze fracture replica of CSC (inset) within the plasma membrane of Zinnia elegans, adapted from Ref. [45]. C SEM of K. rhaeticus surrounded by bacterial cellulose, adapted from Ref. [46].

Fig. 3

A) Insoluble cellulose oligomers self-assembled into ordered nanostructures produced by cellodextrin phosphorylase (CDP)-catalysed synthesis from functionalised acceptors with reactive or non-reactive substituents at the anomeric position. B) Post modification example using thiol-functionalised cellulose oligomers in the presence of silver nanoparticles to produce nanocellulose composite (adapted from Ref. [87]).

Fig. 4

Insoluble cellulose oligomers self-assembled into ordered nanostructures produced by cellodextrin phosphorylase (CDP)-catalysed synthesis from selectively labelled acceptors (A), donor bearing site-specific probe (B), and non-functionalised substrates under specific reaction conditions (C).

Fig. 5

Controlled biocatalytic synthesis of soluble cellodextrins (DP ≤ 6, n = 1–4) using cellobiose phosphorylase (CBP) and cellodextrin phosphorylase (CDP) in a linear cascade reaction from α-D-Glc-1P and D-Glc (A), and via a three-enzyme phosphorylase cascade in solution (B) or on solid support (C) from sucrose and D-Glc including sucrose phosphorylase (SCP). (Part C was adapted from Ref. [111]).

Fig. 6

A) One-pot synthesis of d-cellobiose from sucrose catalysed by sucrose phosphorylase (SCP), glucose isomerase (GI) and cellobiose phosphorylase (CBP). B) Enzymatic platform for the synthesis of various disaccharides from starch employing isoamylase (IA), α-glucosidase (αG) and α-glucan phosphorylase (αGP) in parallel, and a disaccharide phosphorylase such as CBP or laminaribiose phosphorylase (LBP). C) d-laminaribiose synthesis from sucrose and D-Glc in one-pot reaction containing individually immobilised SCP and E. gracilis extract with enriched LBP activity. D) Cascade synthesis of d-laminaribiose from starch and glucose using IA, αGP, LBP and 4-α-glucanotransferase (4GT), which was used to recycle maltose into malto-oligosaccharides for continuous production of α-D-Glc-1P.