Biomedical Engineering Reference
In-Depth Information
al. 2002). It has been well documented that the use of thermostable enzymes
offers many advantages including reduced risk of microbial contamination
during enzyme-catalyzed reactions, higher reaction rates and greater
product yields (Haki and Rakshit 2003). This has prompted the search for
thermophilic sources of enzymes. To date, relatively little is known about
β-galactosidases from fi lamentous fungi, especially thermophilic fungi. In
addition, few β-galactosidase genes have been cloned from fi lamentous
fungi.
β
-GALACTOSIDASE
β-Galactosidase enzymes are members of the superfamilies 1, 2, 35
and 42 of carbohydrate active enzymes. β-Galactosidase catalyzes the
hydrolysis of β-1,4-galactosyl linkages, removing β-linked galactose
residues from a range of substrates including plant derived oligo- and
polysaccharides, galactose-containing glycoproteins and lactose. However,
some β-galactosidase enzymes may remove β-1,6, β-1,3 and or β-1,2-linked
galactosyl groups, e.g., β-galactosidase plays a role in the hydrolysis of
xyloglucan, a hemicellulose that consists of a glucan backbone carrying
1,6-α-D-xylose moieties on three consecutive glucose residues, with
the second and third xylose residues being substituted by β-1,2-linked
D-galactose units (Lima et al. 2003); (Wolf-Dieter 2002). The galactose
containing plant cell wall glycoproteins, AGPs and extensins, also are
substrates for β-galactosidase, containing β-1,3 and β-1,6-linked galactose
oligosaccharides.
β-Galactosidases are distributed widely in nature, being produced
by both prokaryotic and eukaryotic organisms. The role of microbial
β-galactosidase most likely involves the removal of β-linked galactose
residues from plant-derived oligo- and polysaccharides, for example, the
hemicellulose xylan and pectin. Typical substrates for β-galactosidase are
di- and tri-galactose oligosaccharides; oligosaccharides released from β-D-
galactose-containing plant polysaccharides and lactose (Gal-β-1,4-Glc).
Galactose containing glycans on glycoproteins are also potential substrates
for β-galactosidase. For example β-galactosidase can cleave β-galactose
residues present in Gal-NAc-Ser or -Thr O-linked oligosaccharides
(
O
-glycans). These
O
-glycans have been found on soluble, secreted and
membrane bound glycoproteins in a variety of eukaryotes (Brokhausen
1999). However, β-galactosidases also cleave synthetic substrates, such as
4-nitrophenol-β-D-galactose (4NP-β-Gal) and 2-nitrophenol-β-D-galactose
(2NP-β-Gal), hydrolysis of which can readily be detected. Consequently,
synthetic substrates are widely used to measure β-galactosidase activity.
