pubs.acs.org/journal/ascecg Perspective Enzymatic Synthesis of Artificial Polysaccharides Peter J. Smith, Maria Elena Ortiz-Soto, Christian Roth, William J. Barnes, Ju rgen Seibel, Breeanna R. Urbanowicz, and Fabian Pfrengle* Cite This: ACS Sustainable Chem. Eng. 2020, 8, 11853−11871 Read Online ACCESS Metrics & More Article Recommendations ABSTRACT: Polysaccharides are the most important renewable polymers on Earth and hold an enormous potential for the production of ecofriendly functional materials. In addition to being sustainable, they have superior properties to synthetic polymers, particularly in the biomedical field where biocompatibility and biodegradability are vital. Derivatization of polysaccharides obtained from plant biomass paves the path forward for the design and manufacturing of advanced materials with specificproperties adapted to meet definitive needs. However, these advances have been severely limited due to issues with establishing structure− property relationships, which are hampered by the heterogeneity of target polysaccharides and the random distribution of functional groups obtained after their chemical modification. An accurate correlation of structure−property relationships at multiple length scales requires substrates with defined sizes, sequences, and substitution patterns. Such tailor-made polysaccharides may be obtained by implementing a bottom-up approach, starting from monosaccharide or oligosaccharide building blocks followed by their polymerization and substitution through catalysis by different carbohydrate-active enzymes such as glycosynthases, phosphorylases, sucrases, and glycosyltransferases. Recent progress in the enzymatic synthesis of artificial polysaccharides is reviewed, with an emphasis on the potential of the synthesized products, either as new materials or as tools to study structure−property relationships. The obtained information will guide future developments of rationally designed biobased materials for industrial and biomedical applications. KEYWORDS: Carbohydrate-active enzymes, Bioinspired materials, Polymerization, Biomass, Polysaccharide, Glycan ■ INTRODUCTION furnished with heterogeneous substitution patterns, resulting 3 Carbohydrates are essential molecules of life.1 They form the from a less controlled biosynthetic process (Figure 1). In other major component of plant biomass, are responsible for types of types of polysaccharides, the backbone is composed of numerous key biological functions in animals, and mediate more than one type of sugar, including glycosaminoglycans in animal−microbe, plant−microbe, and intermicrobial and intra- the extracellular matrix of animal cells, capsular polysaccharides microbial interactions. Furthermore, carbohydrates are an or lipopolysaccharides in the cell walls of bacteria, and some important source of energy in nearly all organisms. On the plant pectic polysaccharides (Figure 1). The high structural basis of their size, carbohydrates are classified into mono- complexity of polysaccharides makes it difficult to determine saccharides, oligosaccharides, and polysaccharides. Polysacchar- their structures and to identify the molecular pattern responsible ides are linear or branched chains with a degree of polymer- for certain properties or biological functions. ization of typically more than 20 monosaccharide backbone Polysaccharides do not only have essential structural and units. The biosynthesis of polysaccharides is performed by a biological functions, they can also be explored as materials.4 myriad of glycosyltransferases (GTs) that add monosaccharides Being advantageous due to their carbon neutrality, renewability, fi to speci c saccharide acceptors from activated nucleotide sugar and biodegradability, biosourced polymers such as polysacchar- donors.2 During or after assembly of the glycans, further modifications such as sulfation, phosphorylation, acetylation, methylation, and other modifications can be introduced and Received: May 15, 2020 removed. The synthesis of structurally defined polysaccharides Revised: July 3, 2020 becomes possible when, after addition of a monosaccharide Published: July 15, 2020 donor to an acceptor glycan, the product cannot serve as an acceptor substrate again. In plants and fungi, the backbones of the polysaccharides are often homooligomeric, branched, and © 2020 American Chemical Society https://dx.doi.org/10.1021/acssuschemeng.0c03622 11853 ACS Sustainable Chem. Eng. 2020, 8, 11853−11871 ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Perspective exoskeletons of insects and the shells of crustaceans through deacetylation, is frequently used as a food additive and in filtration and packaging materials and has antibacterial proper- ties. Bacterial exopolysaccharides are among other things heavily explored as high-value biomaterials for medical applications.9 Improved functional and biomedical materials based on biomass are being continuously developed. However, their rational design suffers from a lack of control over the length of the polysaccharides combined with modulating the abundance and patterning of the substituents along the polymer backbone. The physicochemical properties of polysaccharides are directly derived from their molecular structures and supramolecular organizations, which are determined by the sequence of their monosaccharide units. Compared to isolation from natural sources, bottom-up synthesis can provide polysaccharides with more defined structures that hold promise as novel materials, or at minimum, model compounds to investigate the influence of structural factors on functional properties.10 Stepwise chemical syntheses benefit from a tremendous amount of control over the individual reactions and can provide glycans of almost any structure. However, although automation can significantly accelerate this process,11 the size of routinely producible glycans by total synthesis remains limited and requires enormous quantities of resources.12 Chemical polymerization of protected and activated saccharide monomers can provide larger glycans in a single reaction step, but the accessible structures are rather primitive and of limited diversity. Furthermore, a final deprotection step is required that may be challenging to perform with very large molecules. Chemical approaches toward polysaccharide synthesis have been reviewed elsewhere and will not be discussed here.13 Another approach to glycan synthesis involves the utilization of enzymes.14 A variety of biocatalysts have been identified that Figure 1. Molecular structures of exemplary polysaccharides from can form glycosidic bonds between saccharides in a highly plants, animals, bacteria, and fungi. NS = N-sulfated, 2S = 2-sulfated, 3S selective and efficient manner without the need for tedious = 3-sulfated, and 6S = 6-sulfated. protection and deprotection steps, as required in chemical synthesis. Enzymatic polymerizations can provide large amounts fi ides have the potential to replace a significant fraction of of well-de ned polysaccharides, as long as the required ffi 15 synthetic polymers derived from fossil resources in the future. substrates and enzymes are available in su cient amounts. Besides being sustainable and ecofriendly, materials from There are three classes of enzymes that can be employed in cell renewable resources often have superior properties to synthetic free systems for polysaccharide synthesis: (i) GTs that transfer polymers, particularly in the biomedical field where biocompat- monosaccharides from activated sugar nucleotides to suitable ibility and biodegradability is vital. In particular, cell wall acceptor substrates, (ii) glycoside hydrolases and glycosyn- polysaccharides from plant biomass represent a diverse and thases, which are engineered glycoside hydrolases that make use abundant source of polymer building blocks whose broad of the reverse reaction of a glycoside hydrolysis, and (iii) exploration for the production of various types of materials has phosphorylases and sucrases that use sugar-1-phosphates or just begun.5 sucrose as glycosyl donors (Figure 2). As the major component of plant biomass, cellulose has been In this perspective, we provide an overview of recent explored for the production of materials to the greatest extent.6 advancements in enzymatic polysaccharide synthesis and give Native cellulose and cellulose fibers are widely used to produce a perspective on the potential of in vitro polysaccharide synthesis tissue and paperboard and textiles and to control rheology in for materials science. food. Derivatized celluloses, such as ethers and esters of cellulose, are used in cosmetics, food, and pharmaceuticals. ■ ENZYMATIC POLYSACCHARIDE SYNTHESIS WITH The hemicelluloses xylan and mannan, which represent the GLYCOSYNTHASES second most abundant polysaccharides in plant biomass, have Concept and Mechanisms. Glycoside hydrolases (GHs) been considered as promising materials for wound dressing, are carbohydrate-active enzymes (CAZymes; www.cazy.org) drug carriers, and edible coatings.7 In contrast to cellulose, they that catalyze the hydrolytic cleavage of glycosidic bonds.16,17 By occur in many structural variations, differing in the amount, degrading polysaccharides, including cellulose, hemicellulose, localization, and constitution of their side chains.8 Starch and and starch, these enzymes are essential for the efficient algal polysaccharides have very good abilities to form valorization of plant biomass.18 They are typically very stable membranes and coatings and have been commercialized in