II.1 Introduction
Les polysaccharides constituent une classe importante de polymères biologiques présents dans presque tous les organismes vivants tels que les algues, les plantes, les micro-organismes et les animaux (Sinha & Kumria, 2001). La plupart de ces polymères sont biologiquement inertes, biodégradables et polyfonctionnels.
Pourtant, leur utilisation sous leur forme native n’est pas toujours commode (solubilité
parfois limitée, dispersion difficile dans différentes matrices, balance hydrophile / hydrophobe inadaptée). Pour ces raisons, différentes méthodes (physiques, chimiques et biologiques) ont été conçues pour modifier leur structure et améliorer leurs propriétés physiques, chimiques et mécaniques. Ces méthodes mettent à contribution les différents groupements fonctionnels
(hydroxyles, amines, carboxyles …) présents dans la structure des polysaccharides.
Ce chapitre est une synthèse bibliographique portant sur quatre polysaccharides
particulièrement abondants dans la nature : la cellulose, l’amidon, la pectine et le chitosane.
Elle se présente sous la forme d'une revue acceptée après modification dans le journal ‘Enzyme
and Microbial Technology’. Après un rappel de la structure, des sources, propriétés et
applications des polysaccharides, le travail de revue se focalise sur les différentes modifications enzymatiques appliquées aux polysaccharides et rapportées dans la littérature. Les principales
voies de transformations décrites sont des réactions d’oxydation, d'acylation/estérification, de
phosphorylation et de copolymérisation. Cette étude bibliographique permettra de faire le point sur les principales avancées scientifiques concernant la fonctionnalisation enzymatique de polysaccharides et de positionner notre travail de fonctionnalisation de la pectine.
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II.2 Enzymatic modification of polysaccharides: mechanisms, properties, and
potential applications. A review
Nadine Karaki1, Abdulhadi Aljawish1, Catherine Humeau2, Lionel Muniglia1, and Jordane
Jasniewski*
1* δaboratoire d’Ingénierie des Biomolécules (δIBio), Université de δorraine, β avenue de la Forêt de Haye, F
-54500 Vandœuvre-lès-Nancy, France
2 Laboratoire Réactions et Génie des Procédés (LRGP), Université de Lorraine, 2 avenue de la Forêt de Haye,
F-54500 Vandœuvre-lès-Nancy, France
*Corresponding author: jordane.jasniewski@univ-lorraine.fr
Article accepté au « Enzyme and Microbial Technology » avec modifications Abstract
Polysaccharides are natural biopolymers found in almost all living organisms. They are used extensively in various industrial applications, such as food, adhesives, pharmaceuticals, and cosmetics. In many cases, their practical use is limited because of their weak solubility in neutral pH and their unsuitable hydrophilic / hydrophobic balance. In this context, chemical or enzymatic modification of their structure appears as a relevant way, to improve their properties, and thus to enlarge the field of their potential applications. Taking into account the reduction of the input energy and the environmental impact, and due to high specificity and selectivity properties, enzymatic bioprocesses have been investigated as attractive alternatives to toxic and non-specific chemical approaches. This review discusses the methods of enzymatic functionalization of four well-known polysaccharides, chitosan, cellulose, pectin and starch. Particular emphasis was placed on the methods, the reaction types such as oxido-reduction, esterification, and copolymerization. The impact of functionalization on the properties and the applications of polysaccharide derivatives was described.
Keywords: Functionalization, enzyme, chitosan, cellulose, pectin, starch.
Highlights:
The main enzymatic pathways for the modification of four well-known polysaccharides
were reported.
The mechanisms of the different enzymatic reactions were described.
Structural modifications and functional properties were addressed.
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I. Introduction
Polysaccharides are an important class of biological polymers joined by glycosidic bonds, universally found in almost all living organisms such as seaweeds (alginate, agar-agar and carrageenan), plants (cellulose, hemicelluloses, pectin and guar gum), microorganisms (dextran and xanthan gum), and animals (hyaluronan, chondroitin, chitin and heparin) [1]. Thus, they constitute interesting renewable sources that can be used for the synthesis of high performance materials [2]. The biological function of polysaccharides is usually either structural or storage-related. For example, starch is a storage polysaccharide in plants, whereas cellulose and chitin are structural polysaccharides respectively in cell walls of plants and exoskeletons of crustaceans [3,4]. Most of these polysaccharides are biologically inert, safe for humans and abundantly available in the natural environment [5]. They possess several others advantages such as low costs, biodegradability in the ecosystem, renewability [6], polyfunctionality, chemical reactivity, chelating and absorptive capacities in the treatment of wastewater [7]. In spite of these properties, the use of these polymers in their native forms is not always convenient (problem of solubility, dispersion in various media, unsuitable hydrophilic / hydrophobic balance). For these reasons, different ways have been tailored to modify their structure, then conferring the necessary physical, chemical and mechanical properties.
Polysaccharides possess many highly reactive groups (acetamido, primary amino, carboxyl and/or hydroxyl groups) that can be involved in various functionalization pathways. In particular, they are characterized by a high hydrophilicity due to the presence of many hydroxyl groups on their composing units [8]. Over the years, many chemical reactions such as oxidation, sulfation, esterification, amidation, have been performed involving the free carboxyl, amino or hydroxyl groups distributed along the polysaccharide backbone [9] aiming to create derivatives with determined/adapted properties [10,11]. These chemical processes usually lead to high conversion yields. Their
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main drawbacks, however, remain the toxicity of the chemical reagents used and the lack of selectivity. Faced with these limitations, an increased emphasis on eco-efficiency and green chemistry has driven a search for renewable and environmentally friendly processes. In the past years, enzymatic bioprocesses have been more and more explored as alternative ways to modify the structure and then the properties of polysaccharides. Their main advantages are a high selectivity and substrate specificity, enabling the synthesis of
products with well-defined and/or stereospecific structures [12–15]. In addition,
enzymatic reactions can be performed under mild conditions and often lead to less or no undesired by-products contrary to chemical processes that require harsh conditions (e.g. strong acids for nitration and sulfonation) and highly reactive compounds (e.g. chloromethyl methyl ether, ethylene oxide, or acid chlorides) [15]. Such procedures have been successfully performed to graft molecules such as fatty acids, proteins, phenols aiming to improve or create biopolymers with interesting mechanical and/or biofunctional properties.
This review aims to present the different strategies of enzymatic modification applied to four of the most abundant polysaccharides, the chitosan from animal and microbial sources and the polysaccharides isolated from vegetal sources in the order of their natural abundance cellulose, pectin and starch (Figure 1).
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Figure 1: Summary of the different enzymatic modifications applied to chitosan, cellulose, pectin and starch, mentioned in the review