ABSTRACT
Ronald P. de Vries,a,b Marina Nadal,b Joost van den Brink,a D. Alexandra Vivas-Duarte,b and Henrik StålbrandcaFungal Physiology, CBS-KNAW Fungal Biodiversity Centre, Uppsalalaan 8, 3584 CT Utrecht, The NetherlandsbMicrobiology, Utrecht University, Padualaan 8, 3584 CH Utrecht, The NetherlandscDepartment of Biochemistry, Lund university, PO Box 124, SE-221 00 Lund, Sweden r.devries@cbs.knaw.nl
1 INTRODUCTION19.Plant polysaccharides are the most abundant carbon source on earth for microorganisms. These polysaccharides consist of many different building blocks (sugars and other compounds) and linkages. As most microbial enzymes involved in degradation of polysaccharides are highly specific for specific linkages and components, it is clear that a broad range of enzymes is required to efficiently convert plant polysaccharides into usable monomeric carbon sources. These enzymes have also become essential components of many industrial processes in different areas such as food and feed, pulp and paper, beverages, detergents, and biofuel. This commercial value has provided a great impulse to research into these enzymes
and their corresponding genes, in particular in microorganisms that are broadly applied in these industrial applications. Among these organisms, filamentous fungi, in particular Aspergillus species (e.g. Aspergillus niger, Aspergillus oryzae) and Trichoderma reesei (a.k.a. Hypocrea jecorina), have become the major players due to their ability to secrete large quantities of a broad range of plant polysaccharide degrading enzymes. In this chapter we will review the current knowledge of fungal plant polysaccharide degrading enzymes, with a special emphasis on Aspergillus. Due to the enormous number of reports on this topic, we would like to stress that this chapter aims to provide an overview of the enzymes involved rather than a complete listing of all the currently available scientific literature on this topic. In the chapter we will regularly refer to glycoside hydrolase (GH), polysaccharide lyase (PL) and carbohydrate esterase (CE) families. These designations are part of a classification system that is presented in a public database at https://www.cazy.org (Coutinho and Henrissat, 1999). In this system, enzymes are classified into families based on signatures (modules) in their amino acid sequence as it was found that these modules are conserved in enzymes with a similar function. Some of these families contain only a single enzymatic activity, while other families contain multiple activities. In the later cases, further classification into subfamilies can be applied to predict the enzymatic function of a protein. Many, but far from all, enzymes that degrade plant polysaccharides are modular, carrying carbohydrate-binding modules (CBMs) that often are crucial for enzyme performance (Boraston et al., 2004). In this review, however, the focus is on the main catalytic activities provided by catalytic modules alone or naturally fused to CBMs or other modules.
