Enzymes - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 370

nearly 5.9 108 kg/year of glucose is produced by the enzymatic hydrolysis of starch. The

enzyme pullulase also hydrolyzes

-l,6 glycosidic linkages in starch selectively.
Cellulases are used in the hydrolysis of cellulose and are produced by some Trichoderma
species, such as Trichoderma viride or Trichoderma reesei; by some molds, such as A. niger
and Thermomonospora; and by some Clostridium species. Cellulase is an enzyme complex
and its formation is induced by cellulose. Trichoderma cellulose hydrolyzes crystalline cellu-
lose, but Aspergillus cellulase does not. Cellulose is first hydrolyzed to cellobiose by cellulase,
and cellobiose is further hydrolyzed to glucose by

a

-glucosidase. Both of these enzymes are
inhibited by their end products, cellobiose and glucose. Cellulases are used in cereal process-
ing, alcohol fermentation from biomass, brewing, and waste treatment.
Hemicellulases hydrolyze hemicellulose and are produced by some molds, such as white
rot fungi and A. niger. Hemicellulases are used in combination with other enzymes in baking
doughs, brewing mashes, alcohol fermentation from biomass, and waste treatment.
Lactases are used to hydrolyze lactose in whey to glucose and galactose and are produced
by yeast and some Aspergillus species. Lactases are used in the fermentation of cheese whey
to ethanol.
Other microbial

b

-1,4 glucanases produced by Bacillus amyloliquefaciens, A. niger, and
Penicillium emersonii are used in brewing mashes containing barley or malt. These enzymes
improve filtration efficiency and extract yield.
Penicillin acylase is used by the antibiotic industry to convert penicillin G to 6-aminope-
nicillanic acid (6-APA), which is a precursor for semi-synthetic penicillin derivatives.
Among other important industrial applications of enzymes are the conversion of fumarate
to L -aspartate by aspartase. In industry, this conversion is realized in a packed column of
immobilized dead Escherichia coli cells with active aspartase enzyme. Fumarate solution is
passed through the column, and aspartate is obtained in the effluent stream. Aspartate
is further coupled with L -phenylalanine to produce aspartame, which is a low-calorie sweet-
ener known as “Nutrasweet .”
The conversion of glucose to fructose by immobilized glucose isomerase is an important
industrial process. Fructose is nearly 1.7 times sweeter than glucose and is used as a sweet-
ener in soft drinks. Glucose isomerase is an intracellular enzyme and is produced by different
organisms, such as F. arborescens, Bacillus licheniformis, and some Streptomyces and Arthrobacter
species. Immobilized inactive whole cells with glucose isomerase activity are used in
a packed column for fructose formation from glucose. Cobalt (Co 2 þ ) and magnesium
(Mg 2 þ ) ions (4

b

10 4 M) enhance enzyme activity. Different immobilization methods are
used by different companies. One uses flocculated whole cells of F. arborescens treated with
glutaraldehyde in the form of dry spherical particles. Entrapment of whole cells in gelatin
treated with glutaraldehyde, the use of glutaraldehyde-treated lysed cells in the form of
dry particles, and immobilization of the enzyme on inorganic support particles such as silica
and alumina are methods used by other companies.
DL -Acylaminoacids are converted to a mixture of L - and D -aminoacids by immobilized
aminoacylase. L -Aminoacids are separated from D -acylaminoacid, which is recycled back
to the column. L -Aminoacids have important applications in food technology and medicine.
Enzymes are commonly used in medicine for diagnosis, therapy, and treatment purposes.
Trypsin can be used as an antiinflammatory agent; lysozyme, which hydrolyzes the cell wall
of gram-positive bacteria, is used as an antibacterial agent; streptokinase is used as an