Biomedical Engineering Reference
In-Depth Information
is composed of two 120 kDa subunits and one 97 kDa subunit (Diaz
1996). As a general rule, fungal β-galactosidases are active in acidic
environments, but temperature optima vary, often refl ecting whether the
source of the enzyme is mesophilic or thermophilic. The thermophilic
fungus,
Rhizomucor
sp. produces a 250 kDa dimeric galactosidase with
pI of 4.2. This enzyme has maximal activity at pH 4.5 and 60°C. The
K
m
for lactose was 50 mM and 1.32 mM for 2NPG (Shaikh et al. 1999).
Thermomyces lanuginosus
, another thermophilic fungal species, produces
a dimeric β-galactosidase (each subunit being 75-80 kDa), with a pI of
4.4-4.5. Optimal activity was between pH 6.7-7.2 and values of K
m
11.3 mM
and 18.2 mM were obtained for 2NPG and lactose, respectively (Fischer
1995).
Fusarium oxysporum
var
lini
secretes a hexameric galactosidase with
subunits of M
r
36.3 kDa. Maximal activity for this enzyme was observed at
pH 5.0 and 55°C; the K
m
for 2NPG was 6.76 mM (Brandao 1987). A 105 kDa
β-galactosidase was characterised from
Aspergillus oryzae
; optimal activity
was observed at 46°C and pH 4.5. The K
m
for lactose was 18 mM while that
for 2NPG was 0.72 mM (Tanaka 1975).
A. niger
produces multiple forms
of β-galactosidase, i.e. 124 kDa, 150 kDa and 173 kDa and the observed
multiplicity has been shown to be mainly due to differential glycosylation.
The isoelectric points of all isoforms were at an approximate pH of 4.6,
while optimal activity was determined between pH 2.5-4.0. The K
m
for
lactose ranged from 85 mM to 125 mM but studies with 2NPG yielded a
single K
m
value of 2.4 mM for all isoenzymes (Widmer and Leuba 1979).
A 126 kDa β-galactosidase with pI of 4.2 was purifi ed from
A. fonsecaeus
.
Maximal activity for this enzyme was between pH 2.6-4.5, while the K
m
value for lactose was 61.3 mM and that for 2NPG was 1.78 mM (Gonzalez
1991).
A. nidulans
produces a 450 kDa β-galactosidase composed of two
types of monomers with M
r
values of 120 kDa and 97 kDa. Maximal
activity of both isoenzymes was observed at pH 7.5 and 30°C (Diaz 1996).
β
-GALACTOSIDASE GENE
A large number of the genes encoding enzymes responsible for cellulose
degradation have been isolated and well characterized. However, there is
considerably less information available for the main chain and accessory
enzymes involved in the degradation of hemicelluloses and pectin. With
the recent advent of whole genome sequencing projects, hemicellulase
and pectinase gene sequence data is increasing. Nonetheless, while the
number of identifi able hemicellulose- and pectin- degrading genes is
increasing (predominately from bioinformatics analysis) relatively few of
these genes have been isolated and characterized to date. β-Galactosidase
genes are found throughout nature and examples in the databases to date
include genes from
Arabidopsis thaliana
(GenBank AC074360),
B. circulans
