Fungal Cell Factories - Food and Industrial Bioproducts and Bioprocessing - page 206

8.3.2 Enzymes
Saprophytic fungi are efficient producers of a myriad of extracellular enzymes. More than
2000 fungal enzymes have been isolated and characterized, but only a small number are
commercially exploited (Saxena et al ., 2005). In the food industry, fruit juices and wines are
clarified using Aspergillus and Rhizopus pectinases; milk is coagulated with Mucor rennet
for making cheese; and high fructose corn syrup is converted via an enzymatic route using
Aspergillus and Mucor species-derived glucose isomerase. Industrial applications for fungal
amylases and phytases include the removal of sizing from fabrics and as a nutritive supplement
in animal feeds for liberation of phosphate from plant material, respectively. Microbial
cellulases are used in the biorefinery to hydrolyze plant biomass to mixed sugars that are
converted to chemical intermediates and biofuels. Genome sequence analyses indicate that
filamentous fungi have undiscovered and unexploited enzymatic capacities. Thus, the
number of commercial applications that employ fungal-derived enzymes is likely to expand.
Research efforts are aimed at developing biofuels and bioproducts using lignocellulosic
biomass from agricultural crop residue and grasses as a sustainable source of mixed sugars
for fermentation. Fungal-derived cellulolytic enzymes degrade plant cell wall material,
a key step in the conversion of cellulosic material into glucose, which is then fermented to
bioethanol by yeast. The mesophilic ascomycete fungus Trichoderma reesei is an attractive
candidate for inexpensive cellulase and hemicellulase production because of its capacity to
secrete abundant amounts of extracellular enzyme, the availability of genetic tools and its
simple fermentation. This organism is one of the most prolific generators of cellulolytic
enzymes and presents a paradigm for the efficient degradation of plant cell wall
polysaccharides. T. reesei , the anamorph of Hypocrea jecorina , was originally isolated from
rotting cotton fabric in the Solomon Islands during World War II (Reese, 1976; Reese and
Levinson, 1952 ; Reese et al ., 1950). Mutant lines derived from this original isolate, selected
for their potent secretory system and high enzyme expression levels, are the industrial
workhorse organisms for cellulase and hemicellulase production (Mandels et al ., 1971 ;
Montenecourt and Eveleigh, 1977). Derivatives of these strains are unofficially reported to
produce over 100 g/l of secreted protein. In addition to the conversion of plant biomass into
sugars and bioethanol, these plant cell wall degrading enzymes have wide ranging
commercial applications in the food, pulp and paper and textile industries (Kubicek et al .,
2009 ; Schuster and Schmoll, 2010 ).
Presently, T. reesei strain development to boost cellulase production is a key focus of
industrial research. In addition to the classical mutagenesis approaches, a wide array of
genetic engineering tools, DNA-mediated transformation protocols and gene knock-out
techniques (Nevalainen et al ., 1994) has been developed to improve T. reesei 's enzyme
production capacity. The recent sequencing of the 34 million base pairs of the T. reesei
genome (Martinez et al ., 2008) opens other molecular genetic avenues, such as manipulating
inducer forming pathways, signaling cascades or transcriptional activation to enhance
cellulase expression. The recent discovery that T. reesei has a mating type-dependent sexual
cycle will also yield fast and efficient strain improvement and boost research toward
economically feasible biofuel production (Seidl et al ., 2009 ).
8.3.3 Lovastatin
Filamentous fungi generate a chemically diverse array of bioactive, low molecular weight
compounds, termed secondary metabolites. Secondary metabolites are dispensable for