Fungal enzyme sets for plant polysaccharide degradation

Abstract

Enzymatic degradation of plant polysaccharides has many industrial applications, such as within the paper, food, and feed industry and for sustainable production of fuels and chemicals. Cellulose, hemicelluloses, and pectins are the main components of plant cell wall polysaccharides. These polysaccharides are often tightly packed, contain many different sugar residues, and are branched with a diversity of structures. To enable efficient degradation of these polysaccharides, fungi produce an extensive set of carbohydrate-active enzymes. The variety of the enzyme set differs between fungi and often corresponds to the requirements of its habitat. Carbohydrate-active enzymes can be organized in different families based on the amino acid sequence of the structurally related catalytic modules. Fungal enzymes involved in plant polysaccharide degradation are assigned to at least 35 glycoside hydrolase families, three carbohydrate esterase families and six polysaccharide lyase families. This mini-review will discuss the enzymes needed for complete degradation of plant polysaccharides and will give an overview of the latest developments concerning fungal carbohydrate-active enzymes and their corresponding families.

Fig. 1

Schematic structure of cellulose with cellulolytic enzymes. BGL β-glucosidase, CBH cellobiohydrolase, EGL β-1,4-endoglucanase

Fig. 2

a–c Schematic structure of three hemicelluloses, xylan, galacto(gluco)mannan, and xyloglucan, with hemicellulolytic enzymes. ABF α-arabinofuranosidase, AFC α-fucosidase, AGL α-1,4-galactosidase, AGU α-glucuronidase, AXE acetyl (xylan) esterase, AXH arabinoxylan α-arabinofuranohydrolase, AXL α-xylosidase, BXL β-1,4-xylosidase, FAE feruloyl esterase, LAC β-1,4-galactosidase, MAN β-1,4-endomannanase, MND β-1,4-mannosidase, XEG xyloglucan-active β-1,4-endoglucanase, XLN β-1,4-endoxylanase

Fig. 3

a–c Schematic structures of three pectins, rhamnogalacturonan I, homogalacturonan, xylogalacturonan, with pectinolytic enzymes. ABF α-arabinofuranosidase, ABN endoarabinanase, ABX exoarabinanase, BXL β-1,4-xylosidase, FAE feruloyl esterase, GAL β-1,4-endogalactanase, LAC β-galactosidase, PEL pectin lyase, PLY pectate lyase, PGA endopoly-galacturonase, PGX exo-polygalacturonase, PME pectin methyl esterase, RGAE rhamnogalacturonan acetyl esterase, RGL rhamnogalacturonan lyase, RHG endorham-nogalacturonase, RGX exorham-nogalacturonase, XGH endoxylo-galacturonase, XGX exoxylogalacturonase. α-Rhamnosidase (RHA), unsaturated rhamnogalacturonase (URH), and unsaturated glucuronyl hydrolase (UGH) are not depicted in this figure