What are the three types of alpha-
glucosidase and what is the function
of this enzyme function?
WHAT IS POMPE’S DISEASE?
Pompe’s disease also referred to as
Glycogen Storage Disease Type II or aicd
maltase deficiency, is an autosomal
recessive disorder of glycogen metabolism
caused by a deficiency of the lysosomal
enzyme acid glucosidase.
People affected with this disease are unable
to degrade glycogen stored in the lysosome
and thus leading to the accumulation of
glycogen in lysosomal storage vacuoles.
HISTORY OF THE DISEASE
Existence of the disease was first
described in 1932 by Dr JC Pompe.
The disease is a rare neuromuscular
genetic disorder that occurs in babies,
children and adults who inherit a
defective gene from each of their
Pompe’s disease is the most devastating
glycogen storage disease. The disease has
been divided into three forms defined by
age of onset and progression of symptoms.
The three forms include infantile onset,
juvenile onset and adult onset.
In the infantile form of the disease, patients
display cardiac impairment, which is fatal
before two years of life.
Patients with juvenile or adult forms
can present diaphragm involvement
leading to respiratory failure.
The adult onset symptoms involve
generalized muscle weakness and
wasting of respiratory muscles in the
trunk, lower limbs, and diaphragm.
WHAT MAKES UP GLYCOGEN
Glycogen is mostly found in the liver
and skeletal muscles.
It is a polymer of 120,000 glucose
residues and is a primary
carbohydrate storage form in
WHAT MAKES UP
The polymer is composed of units of
glucose linked α-1-4 with branches
occurring at a-1-6, approximately
every 8-12 residues.
The end of the molecule containing a
free carbon number one on glucose is
called a reducing end. The other
ends are all called non-reducing ends
SIZE OF GLYCOGEN
Glycogen molecules are very large in
Therefore inability to degrade them
results to a large accumulation of
normal structure in the lysosomes of
all cells. The excess storage of
glycogen in the vacuoles is the
consequence of defects in the
STRUCTURE OF GLYOGEN
The acid a-glucosidase normally designated
as GAA gene resides on chromosome
17q25, spanning 20 kb and composed of 20
Glycogen storage disease type II has been
shown to be caused by missense, nonsense
and splice-site mutations, partial deletions
and insertions. Some mutations are specific
to certain ethnic groups.
FORMS OF α-GLUCOSIDASE
There are three common allelic forms
of acid a-glucosidase that segregate
in the general population.
These forms are designated GAA1,
GAA2 and GAA4. The normal
function of acid a-glucosidase is to
hydrolyze both a-1,4- and a-1,6-
glucosidic linkages at acid Ph.
ACTIVITY OF α-GLUCOSIDASE
The activity of the enzyme leads to
the complete hydrolysis of glycogen
which is its natural substrate.
As would be expected from this
activity, deficiency in acid a-
glucosidase leads to the accumulation
of structurally normal glycogen in
numerous tissues, most notably in
cardiac and skeletal muscle.
METABOLISM OF GLYCOGEN
Activity of α-1,4-glucosidase in
The lysosomal α-1,4-glucosidase was found
to be active at pH 4. However, its activity
is not present in the liver, heart and
skeletal muscles of children with pompe’s
Although the lysosomal α-1,4-glucosidase is
often referred to as maltase, the enzyme is
known to have a broader specificity in that
it acts also on the outer chains of glycogen.
Studies have shown that α-1,4-
glucosidase is able to catalyze the
total degradation of glycogen to
Also, its activity towards glycogen can
be greatly stimulated by monovalent
and divalent cations, and that the
degree of stimulation is dependent
The effect is so pronounced at pH 4
that the possibility might be
considered that intralysosomal
glycogen catabolism in vivo could be
regulated within the organelle.
Glucosidase has been purified from
rat liver enzyme and was found to
posses the ability to catalyze various
However, there is no evidence as to
whether these transglucosylaton reactions
should be regarded as having any
physiological importance within the
It clearly shows that α-1,4-glucosidase is
present in lysosome from rat liver but its
ability to debranch glycogen granules is
Kinetics of α-glucosidase
Studies done on the kinetics of α-
glucosidase shows differences in its
response to cation concentration and pH.
This suggested that the enzyme have more
than one catalytically active binding site as
well as at least one separate inhibitory site.
One of the two catalytic sites could bind
maltose, as well as other maltosidically
linked oligosaccharides of low molecular
STRUCTURE OF MALTOSE(a
Kinetics of α-glucosidase(cont.)
The same binding site also have affinity for
the α-1,6-glucosidasidically linked
isomaltase since it is a competitive inhibitor
of maltose hydrolysis.
The other catalytic binding sites might bind
polysaccharide substrate such as glycogen
and would be the locus of α-1,4-glucosidase
action on such molecules.
Catalytic Site of Lysosomal a-
The primary structure of lysosomal α-
glucosidase was studied through
molecular cloning and analysis of
cDNA and genomic sequences.
It was found that the cDNA codes for
a protein of 952 amino acids with an
apparent molecular mass of 110 kDa.
Catalytic Site of Lysosomal a-
The residues that were found in the
catalytic site of this enzyme were
located in a peptide extending from
Asp-513 to Asn-520 in mammalian
These amino acid substitutions were
made by site-directed mutagenesis.
Hermans, M. P. Monique; Kroos, A. Marian; van Beeurnens; Oostra, A. Ben;
ReuserSll, J.J. Arnold. Human Lysosomal a-Glucosidase: CHARACTERIZATION OF THE CATALYTIC SITE. Jbc. 1991, 266, 13507-13512.
Oligonucleotides used for
D518 5' ATGTGGATTGACATGAACGAG 3'
G518 5' ATGTGGATTGGCATGAACGAG 3'
N518 5' ATGTGGATTAACATGAACGAG 3'
E518 5' ATGTGGATTGAAATGAACGAG 3'
R516 5' GACGGCATGCGGATTGACATG 3'
E513 5' GCCCTTCGAAGGCATGTGGAT 3'
Hermans, M. P. Monique; Kroos, A. Marian; van Beeurnens;
Oostra, A. Ben; ReuserSll, J.J. Arnold. Human Lysosomal a-
Glucosidase: CHARACTERIZATION OF THE CATALYTIC SITE. Jbc.
1991, 266, 13507-13512.
MUTATION AT THE CATALYTIC SITE
Substitutions were made at positions
Asp-513, Trp-516, and Asp-518 to
obtain direct information on the role
of potentially important residues in
the catalytic site region.
MUTATION AT THE CATALYTIC
The substitution of Asp-513 by Glu-
513 had the most dramatic effect as
it causes the blockage of both the
posttranslational modification and the
intracellular transport of α-
MUTATION AT THE CATALYTIC
The substitution of Trp-516 by Arg-
516 leads to loss of catalytic function.
Tryptophan has a non-polar side
chain and practically no acidic or
basic properties. Thus the inactivation
is more likely due to an altered
Regulation of GAA
The regulation of GAA may be quite
complex according to analysis of results
obtained from resent experiments.
Therefore this lysosomal enzyme may have
still unrecognized physiological roles in
The discovery of the GAA gene however,
has led to rapid progress in understanding
the biological mechanisms and properties of
An enzyme replacement called
myozyme has been shown to be a
safe and effective treatment for
Researchers are still working on
having a better understanding of the
mechanism behind the uptake and
degradation of glycogen by GAA .