Chitin: A Structural Biopolysaccharide with Multiple Applications

Abstract

Abstract Chitin is a naturally occurring fibre‐forming polymer that plays a protective role in many lower eukaryotes similar to that of cellulose in plants. Chemically it is a long‐chain unbranched polysaccharide made of N ‐acetylglucosamine residues linked through β‐1,4 covalent bonds; it is the second most abundant organic compound in nature, after cellulose. Taking into account the role played by chitin in different biological structures (i.e. fungal cell walls, insect peritrophic matrix, insect and crustacean cuticles, eggshells from nematodes, cyst wall of protozoa), its metabolism (biosynthesis and degradation) is essential for different morphogenetic events. Absent in vertebrates and plants, chitin participates in host–parasite interactions and represents a parasite‐specific target for chemotherapeutic attack and also plays a role in host immune responses. Because of its abundance in nature and its properties, biotechnological applications of chitin derivatives, such as chitosan and chito‐oligosaccharides, are currently an expanding area in biomedicine, pharmaceutical and food technology and agro‐biosciences. Key Concepts: Chitin is a long‐chain unbranched polysaccharide made of β‐1,4‐linked anhydro‐2‐acetamido‐2‐deoxy‐ d ‐glucose (GlcNAc) which forms crystalline fibrillar structures following association of adjacent chains through hydrogen bonds between the N–H and the CO groups. Nascent chitin is a growing chitin chain which is being synthesised by the chitin synthase and it represents a good substrate for chitinolytic enzymes. Microfibrillar chitin is a crystalline structure formed by chains of the polysaccharide which associate through hydrogen bonds between adjacent chains and it is responsible for the physico‐chemical properties of the polymer. The fungal cell wall is a supramolecular network outside the plasma membrane, formed by structural polysaccharides, including chitin, and proteins and glycoproteins, that protects the fungal cell and determines morphology, similarly to cuticle (exosqueleton) in insects. Insect cuticle (exosqueleton) is an extracellular matrix covering the epidermis and trachea, composed mainly by chitin (and proteins), which protects the animal and confers morphology. The peritrophic membrane (matrix) is an extracellular layer that covers the midgut in most arthropods and it is made of chitin, proteins and proteoglycans, and provides protection to the underlaying digestive cells. Chitin biosynthesis is a strongly regulated process, both spatially and temporally, as chitin deposition is essential for fungal growth and development (moulting) in arthropods. Chitin synthases are membrane bound enzymes that incorporate the substrate (UDP‐ N ‐acetylglucosamine) from the cytosol to the nascent chitin chain that is extruded outside the membrane. Chitinases are chitin‐hydrolysing enzymes that play important roles in the physiology of chitin‐containing eukaryotes, and chitinolytic bacteria are active in a scavenging role by degrading massive amounts of chitin in marine and soil biomass, avoiding its accumulation and favouring the utilisation of chitin as a renewable source. The absence of chitin in vertebrates and plants makes the chitin metabolism a potentially useful parasite‐specific target for chemotherapeutic attack. In mammals, chitin regulates immune responses playing a role in inflammation, and allergic diseases. In plants, chitin elicits defence responses, and in leguminous plants, chitin oligosaccharides produced by rhizobia promote plant nodulation. Owing to their abundance in nature and properties, chitin, chitosan and their derivatives have gained potential interest for a wide range of areas, including biopharmaceutical and biomedical applications.