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Varki A, Cummings RD, Esko JD, et al., editors. Essentials of Glycobiology [Internet]. 4th edition. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 2022. doi: 10.1101/glycobiology.4e.59

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Essentials of Glycobiology [Internet]. 4th edition.

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Chapter 59Glycans in Bioenergy and Materials Science

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Plants provide large amounts of glycans that are used by humans for many purposes. Wood, which is composed predominantly of lignified secondary walls, is used as an energy source, as a building material, and for papermaking. Pectins isolated from the primary cell walls of fruits and polysaccharides isolated from seeds are used as thickeners, stabilizers, and gelling agents in many foods and beverages. Plant cell walls are the major component of forage used as animal feed. These walls, as dietary fiber, also contribute to human health. Recent concerns about the environmental costs of fossil fuel extraction and consumption have led to renewed interest in using plant glycans as feedstocks for energy production, for the generation of polymers with improved or new functionalities, and for the generation of high-value chemical precursors. In this chapter, we briefly describe four broad categories—bioenergy, fine chemicals and chemical feedstocks, polymeric materials, and nanomaterials—in which plant glycans have the potential to replace or to provide alternatives to petroleum-based products.

INTRODUCTION

Plant glycans are used by humans as an energy source, as a building material, and for making numerous bioproducts including paper. Cellulose from diverse plant sources is the primary component of many valuable materials, including textiles and plastics. Pectins are used as thickeners, stabilizers, and gelling agents in many foods and beverages. Plant cell walls are used as animal feed and, as dietary fiber, also contribute to human health. The well-established adverse effects of the extraction and use of fossil fuels on the Earth's climate have led to worldwide efforts to develop plant-derived glycans as a renewable raw material to displace or supplement fossil fuels for energy production, for the generation of polymers with improved or new functionalities, and for the generation of high-value chemical precursors.

GLYCANS AND BIOENERGY

The process of photosynthesis by terrestrial plants has been estimated to assimilate at least 100 billion metric tons of CO2 annually. The chemical energy generated in this manner is stored predominantly in the form of carbohydrates. Some of these carbohydrates are used directly for plant growth and development, whereas others are converted to storage polysaccharides (starch and fructans) that provide plants with a readily available form of energy. A considerable portion of the carbohydrate formed via photosynthesis is used to produce the polysaccharide-rich walls that surround plant cells (Chapter 24). Thus, plant cell walls account for a substantial amount of biological carbon sequestration and are a potentially sustainable and economical source of nonpetroleum-based energy and high-value chemicals.

First-generation bioethanol produced by fermenting the starch present in corn grains currently accounts for virtually all of the liquid transportation fuel generated from plant materials in the United States. The starch is first treated with enzymes that convert it to glucose, which is then fermented to ethanol and carbon dioxide by adding yeast. Yeast can convert 1 kg of glucose to 0.33 gallons (1.25 L) of ethanol and an equivalent amount of carbon dioxide. The United States and Brazil together account for as much as 84% of world ethanol production (https://afdc.energy.gov/). In 2019, 16.9 billion gallons (64 billion liters) of ethanol were produced in the United States according to the U.S. Energy Information Administration (www.eia.gov). Brazil produces approximately 8.57 billion gallons (32 billion liters) of ethanol annually by fermenting the sucrose extracted from sugarcane (https://afdc.energy.gov/). Corn and cane bioethanol are then blended with gasoline in varying amounts or used directly as a transportation fuel.

Concerns about the negative impacts of the large-scale production of corn-based ethanol on food production and the environment has led to renewed interest in generating ethanol and other liquid transportation fuels from sustainable plant lignocellulosic biomass that can be grown on marginal land. This biomass is comprised predominantly of lignified secondary walls (Chapter 24) that are composed of cellulose (40%–50% w/w), hemicellulose (25%–30% w/w), and lignin (15%–25% w/w) and lesser amounts of pectin and protein. Several different plants, including poplar, switchgrass, sorghum, miscanthus, eucalyptus, and sugarcane, are being considered for use as bioenergy crops.

The biomass from energy crops can be converted to liquid fuel by fermentation or gasification. In gasification, the biomass is heated in a low-oxygen environment to generate syngas (hydrogen, carbon monoxide, and carbon dioxide) and heat. The syngas can then be reacted to produce diverse chemicals including alcohols or alkanes via Fischer–Tropsch synthesis that can be further transformed into fuels (mainly diesel oil and jet fuel). Most of the technical challenges to commercial biomass gasification are understood. However, the process has not been widely adopted because of the high capital costs involved.

The generation of liquid fuel from lignocellulose by fermentation currently involves using a cocktail of enzymes to convert the biomass to sugar, which is then fermented to give the desired product. This approach may be superseded by the development of consolidated bioprocessing (CBP) technologies in which the microorganism that deconstructs plant biomass into sugars also converts those same sugars into products such as fuels and chemicals. The fermentation approach is simple in concept but there are many technical challenges that must be solved before it becomes commercially viable. One major obstacle is that the cellulose and hemicellulose in lignocellulosic biomass are not readily accessible to hydrolytic enzymes and thus are not efficiently converted to fermentable sugars. The biomass must be pretreated with dilute acid, ammonia, or steam to decrease this recalcitrance. Cost-effective and environmentally sound pretreatment technologies need to be developed if the commercial production of bioproducts by fermentation is to become a reality. The efficiency of the enzymes used to convert the cellulose and hemicellulose to sugar must also be improved. To this end there is extensive ongoing research to engineer thermophilic microorganisms to more efficiently deconstruct biomass and to convert the released sugars to the desired product, avoiding the necessity of releasing sugars from the biomass with enzyme cocktails before fermentation.

Increased understanding of cell wall structure, together with knowledge of polysaccharide and lignin biosynthesis, is expected to facilitate the engineering of plants to produce biomass that is more amenable to bioprocessing and an improved resource for biofuel, value-added chemicals, and bioproducts. However, the susceptibility of such modified plants to biotic and abiotic stress in the field will need to be addressed. Such concerns, together with those regarding the introduction of genetically modified plants into the environment, may be lessened by identifying natural plant variants that produce lignocellulosic biomass with the desired properties including reduced recalcitrance to saccharification.

Lignin is poised to become a major by-product of commercial biorefineries, as it is not converted to a liquid fuel during fermentation. Early concepts of biorefineries envisioned burning the lignin to generate power. However, there is now a greater emphasis on valorization of the recovered lignin to produce value-added compounds for the chemical industry. Considerable resources are being allocated to research and development worldwide to create a viable and sustainable lignocellulosic advanced biofuels and bioproducts industry. Nevertheless, many technical, environmental, and societal challenges must be solved if this industry is to develop and contribute to a biobased economy and to reduce the demand for fossil fuels.

FINE CHEMICALS AND FEEDSTOCKS

Several of the sugars released from lignocellulosic biomass, including glucose and xylose, are being investigated for use in the production of functional chemical precursors that can be used to make industrially relevant compounds and polymers including plastics. Some examples of functional chemical precursors are alcohols (ethanol, propanol, and butanol), sugar alcohols (xylitol, and sorbitol), furans (furfural, hydroxymethylfurfural), biobased hydrocarbons (isoprene and long-chain hydrocarbons), organic acids (lactic acid, succinic acid, and levulinic acid), and biobased polyurethanes. Current research is focused on optimizing the bioconversion of polysaccharides (yield, rate, separation, titer, and product specificity) by identifying and engineering improved fermentation organisms and fermentation processes and developing enhanced chemical catalysts.

POLYMERIC MATERIALS

Plant-derived cell wall polysaccharides (Chapter 24) including cellulose, xyloglucan, mannan, and xylan (Figure 24.1) are used to produce diverse polymeric materials used by industry. They are both biorenewable and biocompatible, making them advantageous over their petroleum-based counterparts. Cellulose has been extensively modified to develop synthetic cellulose-based polymers. Cellulose films (cellophane) and fibers (rayon) are produced using regenerated cellulose that is itself formed by dissolving natural cellulose (predominantly from wood pulp) in alkali and carbon disulfide and then precipitating the polymer in a process that has been used for at least 125 years (the viscose process).

With society's ongoing need for polymers with new properties and functions, there is increased effort to develop chemical or biocatalytic reaction pathways to modify the structure of a polysaccharide backbone or side chains to enable the production of polysaccharide derivatives with enhanced or new properties. Cellulose is one example of a plant polysaccharide that has been extensively modified to develop new biosourced polymers. Reaction pathways have been developed to generate specific cellulose derivatives by substituting accessible hydroxyl groups with other chemical groups. Such derivatives include cellulose acetate, cellulose acetate propionate, cellulose acetate butyrates, carboxymethyl cellulose, and cellulose butyrate succinate. These products are used in many industrial applications as coatings, inks, binders, and thickening/gelling agents. They are also used in the pharmaceutical industries to produce controlled-release drug tablets and in the cosmetics and food industries as thickening and gelling agents.

Chitin is the second most abundant natural polysaccharide after cellulose. It is present in crustacean shells and insect cuticles and may also be produced by fungi and algae (Chapter 23). Chitin is composed of 1-4-linked β-D-GlcNAc residues. It can be enzymatically or chemically deacetylated to produce chitosan, the cationic and more water-soluble form of the glycan. Large amounts (∼5 million metric tons) of chitin are produced as waste by the seafood processing industry, and thus, there is considerable interest in developing biobased processes to convert this waste into value-added products. Chitosan has reactive amino and hydroxyl groups that can be modified to generate materials with diverse properties and applications.

Hemicellulosic polysaccharides, including xylan and mannan, have a backbone structure similar to cellulose and are abundant in agricultural and forestry sidestreams, including the pulping and the viscose processes. With the complexity and variability of polysaccharide structures there is considerable potential for the development of unique synthetic polysaccharides with new or enhanced functionality. Noncellulosic matrix polysaccharides present an attractive target for enzymatic synthesis and functionalization. They are easily extracted from biomass and, unlike cellulose, are typically soluble in aqueous solutions and are often substituted with both glycosyl and nonglycosyl substituents that can be modified to influence their material properties. To this end, current research aims to further understand and use new reaction pathways that target chemical or enzymatic modifications to functionalize and/or alter specific locations on the polysaccharide and thereby generate regioselective functionalization.

Synthetic or naturally derived oligosaccharides can also be covalently appended to polymer chains built from petroleum-based monomers. This gives rise to glycopolymers with architectures resembling those of glycoproteins or proteoglycans. Such materials have found increasing use as research tools to study the biological functions of glycans and are currently explored as biomaterials for drug delivery or as antifouling and antifreeze agents.

NANOMATERIALS

Nanomaterials from plants and crustacean shells offer new materials for the development of biorenewable and biocompatible products. These nano-sized particles, consisting of bundled polymer chains, have properties and function that are different from the isolated polymer chains from which they are made. Such nanomaterials can be produced from cellulose, hemicellulose, pectin, chitin and chitosan. A more focused description is given below on nanomaterials from cellulose.

Polysaccharides that have little or no branching of their backbone can self-assemble to form ordered structures in which the individual polymer chains stack along the chain axis, thus forming a crystalline structure. Cellulose is one example of a plant polysaccharide that has this type of crystalline structure. During cellulose biosynthesis individual glucan chains assemble to form microfibril structures that contain both crystalline and disordered arrangements (see Figure 59.1A). The high mechanical stiffness and tensile strength along the length of the cellulose microfibrils provide high mechanical strength, high strength-to-weight ratio, and toughness to plant tissues and organs.

FIGURE 59.1.. (A) The stacking of cellulose chains showing that there are regions of “order” and “disorder.

FIGURE 59.1.

(A) The stacking of cellulose chains showing that there are regions of “order” and “disorder.” (B) During one type of cellulose nanomaterial extraction process that uses acid hydrolysis, the disordered regions are preferentially (more...)

The cellulose fibril structures and the crystalline regions can be isolated using specialized chemical–mechanical extraction methods. The resulting nano-sized particles, typically referred to as cellulose nanomaterials (CNs), have properties and functions that are considerably different than individual cellulose chains (CNs; Chapter 58). The CNs’ morphology, properties, and surface chemistry vary depending on the plant source and the conditions used to extract the cellulose. Plant CNs are typically classified as cellulose nanocrystals or cellulose nanofibrils. Cellulose nanocrystals are rod-like particles, which are between 3- and 20-nm-wide and between 50 and 500 nm in length (Figure 59.2A). Cellulose nanofibrils are fibril-like particles, which are between 5- and 100-nm-wide and between 500 nm to several microns in length (Figure 59.2B). Both types of CNs have high stiffness and tensile strength, high surface-area-to-volume ratio, and surfaces that can be readily chemically modified to alter their physicochemical and materials properties. CNs are biorenewable and biocompatible and have minimal environmental, health, and safety risks. Thus, CNs are being used to develop new products including barrier films, separation membranes, antimicrobial films, food coatings, cement/concrete modifiers, rheology modifiers, biomedical applications, template scaffolds for catalytic supports, batteries, supercapacitors, and many others. Research, development, and commercialization in CNs is accelerating and covering an ever-broadening scope, all of which would benefit through advancements in glycoscience and the new characterization and synthesis tools being developed.

FIGURE 59.2.. Transmission electron microscopy images showing two types of cellulose nanomaterials.

FIGURE 59.2.

Transmission electron microscopy images showing two types of cellulose nanomaterials. (A) Cellulose nanocrystals produced by acid hydrolysis. (B) Cellulose nanofibrils produced by mechanical defibrillation of wood pulp.

PERSPECTIVES AND FUTURE CHALLENGES

Considerable resources are being allocated to research and development to create economically viable and sustainable biofuels and bioproducts industries that use plant glycans as feedstocks. In the future, there is likely to be a greater emphasis on using both the glycan and the lignin components of plant biomass to produce liquid transportation fuels and value-added chemicals. Such technologies will require the development of new chemical catalysts and robust enzymes in addition to plants engineered to produce biomass with enhanced processing and value characteristics. Many technical, economic, environmental, and societal challenges must be met if these industries are to develop and contribute to a biobased economy and to reduce the demand for fossil fuels. Despite the commercial and technological success of natural cellulose-based materials, glycans are yet to be systematically explored as building blocks for new materials. The development of controlled polymerization methods for making well-defined glycans, matching those used for producing polymers from petroleum-based building blocks, will need to be developed to fully realize the potential of glycans to produce materials with new properties and functions.

ACKNOWLEDGMENTS

The authors acknowledge helpful comments and suggestions from Martina Delbianco and Markus Pauly.

FURTHER READING

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Copyright © 2022 The Consortium of Glycobiology Editors, La Jolla, California; published by Cold Spring Harbor Laboratory Press; doi:10.1101/glycobiology.4e.59. All rights reserved.

The content of this book is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 Unported license. To view the terms and conditions of this license, visit https://creativecommons.org/licenses/by-nc-nd/4.0/

Bookshelf ID: NBK579971PMID: 35536970DOI: 10.1101/glycobiology.4e.59

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