Yeasts and their Enzyme Systems Degrading Cellulose, Hemicelluloses and Pectin
Peter Biely, Lubomír Kremnický
Institute of Chemistry, Slovak Academy of Sciences, 842 38 Bratislava, Slovakia
Received September 10, 1997
Accepted November 11, 1998
yeasts, enzymes, plant polysaccharides, xylan, mannan, pectin, inducers
In contrast to most fungi, yeasts are generally unable to utilize insoluble, crystalline cellulose as a carbon source. Some yeasts are capable of utilizing amorphous, matrix polysaccharides of plant cell walls. Xylan, the main hardwood hemicellulose and the second most abundant softwood hemicellulose, serves as a carbon source for strains in the following genera: Aureobasidium, Cryptococcus, Pichia and Candida. Colour variants of Aureobasidium pullulans belong to the best producers of microbial xylanases. Galactoglucomannan is the major softwood hemicellulose and various β-l,4-mannans occur in plant seeds. Our recent screening for the production of galactomannan-depolymerizing enzymes showed that the ability occurs within 5 different yeast genera, with highest frequency among Aureobasidium pullulans and Stephanoascus. The deterioration of softwood by Aureobasidium pullulans might be associated with its ability to colonize wood due to the production of both xylanolytic and mannanolytic enzyme systems. A new screening for pectin-depolymerizing yeasts showed that the production of pectin-depolymerizing enzymes is scatterred within 13 genera, most frequently in Aureobasidium, Cryptococcus, Kluyveromyces, Rhodosporidium, Trichosporon and Ustilago. The enzyme systems are generally inducible with fragments of the corresponding polysaccharides. With disaccharides (xylobiose, mannobiose) serving as inducers, the endoglycanase is secreted out of the cells into the surrounding medium and the corresponding glycosidase remains localized intracellularly. The oligosaccharides formed from the polysaccharides extracellularly are then transported into the cells by an inducible membrane-bound permease, an active transport system. If monosaccharides are the compounds triggering the enzyme synthesis (e.g. xylose, galacturonic acid), the corresponding exoacting enzymes, e.g. glycosidases cleaving oligosaccharides liberated from the polymer by endoglycanases, are secreted into the medium.