Download Lactase

An enzyme is a protein that catalyses a chemical reaction.
Enzymes are able to speed up the reaction by lowering the activation energy
needed for the reaction to occur.
Lactose itself is a
disaccharide, a
compound sugar made
up from two simple
sugars…
Galactose and Glucose.
Lactase, also known as beta-galactosidase, is the enzyme that
catalyzes the hydrolysis of lactose.
•In the normal human, lactase in a
pivotal enzyme that can be found on the
brush border of the small intestine.
•Lactase is needed to break down
lactose into glucose and galactose.
Glucose is the usable form of sugar
required by the body The galactose is
converted into even more glucose to be
used by the body.
•Lactose is a sugar is found in many
food products, especially dairy products
such as milk, cheese, and ice cream.
•When there is a lack of
the enzyme lactase in
the small intestine, a
condition known as
lactose intolerance
results.
•Without a sufficient
amount of lactase, a
patient will not be able
to break down all of the
lactose he or she
ingests. As a result,
digestive difficulties
become evident.
Symptoms tend to vary in severity depending
on the amount of lactase a person produces as
well as their age, ethnicity, and digestive rate,
but common symptoms include…
•Intestinal Cramping
• Gas
• Diarrhea
• Nausea
• Bloating
The Lactose Tolerance Test
The Hydrogen Breath Test
•After fasting, the patient must drink a
liquid that contains lactose.
•Normally there is very little
hydrogen in a person’s breath
•The patient’s blood sugar levels are
then monitored.
• Yet, undigested lactose can be
fermented by bacteria to produce
hydrogen gas.
•The blood sugar levels indicate how
well the lactose is broken down by
lactase into glucose (measured by the
blood sugar) and galactose.
•The gas is absorbed by the
intestines and carried through the
bloodstream to the lungs where it
will be exhaled.
Stool Acidity Test
•When a infant is lactase deficient, the undigested lactose
becomes fermented by bacteria in the colon, creating lactic acid
and other fatty acids that can be measured in the child’s stool.
•Also present in the stool sample in these cases is glucose since
the absorbed lactose can be present in the stool sample.
• The glucose would be present in the stool sample as part of
the lactoes if the lactase was not able to break it down in the
intestine.
•If the lactose was broken down, the glucose would have been
used by the body and not excreted as part of the lactose.
Primary lactase
deficiency
•This deficiency is a result
of the humans producing
less and less lactase after
the age of two years.
• There is a genetic
component in many of
these cases, and a blood
test can be used to identify
the presence of the
problematic gene.
Secondary lactase
deficiency
•This deficiency is a result of
injury to the intestine as occurs
in patients with intestinal
diseases such as Crohn’s
disease and Celiac Disease.
• When these diseases are
under control, the lactase
comes back in normal
functioning and digests the
lactose.
•When the body fails to produce enough lactase in
the intestines to break down the lactose ingested
with a person’s meals, the person can avoid
unpleasant symptoms by avoiding products
containing lactose.
• Yet, products containing lactose are also the
products high in calcium. Denying one’s body of the
calcium they need would be detrimental to the
person’s health.
• Therefore, artificial forms of lactase can be taken
in pill/ capsule form or added directly to the food.
These medications contain the lactase enzyme so
that lactose can be digested in these patients.
•Lactase is needed in order
for many biological processes to
occur.
• First lactase breaks down the
lactose into glucose and galactose.
•The glucose is then used for
glycolysis.
• During glycolysis, glucose is
oxidized to two molecules of pyruvic acid,
yielding 2 ATP of energy.
• Glycolysis occurs in the cytoplasm of cells,
not in organelles, and occurs in all kinds of
living organisms.
•The enzyme lactase is not only found in
humans. It is involved in many biological
processes in other living things as well.
•For example, lactase can be found in
Escherichia coli where it is a product of
the Z gene of the lac operon of E. coli.
•Here lactase hydrolyzes the disaccharide
lactose to galactose and glucose in addition to
converting the lactose into another
disaccharide, allolactose.
•Allolactose is the natural inducer
for the lac operon.
•The initial binding of a lactose
substrate to the lactase enzyme is
observed based on the stacking on
tryptophan (Trp999).
•The stacking of networks of specific
interaction made by galactsyl hydroxyls
occurs on the Trp 999 residue before
catalysis can occur.
•Once the substrate is aligned in the
correct position, then a glutamic acid
(Glu 461) can act as an acid and
donate a proton to a glycosidic
oxygen.
•The glycosidic oxyge is involved in
the formation of an intermediate with
another glutamic acid (Glu 537).
•Then a base enters in the reaction
to assist in the release of the
intermediate. This base is also the
Glu 461 which can now act as a
base since it has lost protons in the
pervious step.
•Acting now as a base, Glu 461
accepts from the acceptor molecule,
which is an alcohol, R’OH in this
case.
•The formation of the covalent
intermediate acts to neutralize the
charge on the nucleophile Glu537.
•Because of this close proximity of
Glu 537 and Glu 461, the
neutralization of the Glu537
decreases the pKa of Glu 461.
• Glu 461 also interacts with a
magnesium ion that it believed to
assist in this lowering of pKa.
•With this decrease in pKa, the
Glu461 is more likely to be
deprotonated.
• Since it is has lost protons when it
was deprotonated, the Glu461 can
act as a base in the next step
accepting the protons it originally lost
protons to replace the ones it lost.
•Now in the second half of the
reaction, there is a galactosyl
transfer from Glu 537 to an
acceptor molecule.
• As a result of this transfer,
Glu 537 becomes more
negatively charged.
• This negative charge
formation is facilitated by
partial proton donation of a
tyrosine (Tyr 503).
•At this point the lactose hydrolyzes and
breaks off the active site revealing glucose
and galactose.
• In place of the substrate, a water molecule
hydrogen bonds to Glu 461 and to the ring of
the galactsyl moiety.
• Sometimes the glucose molecule does not
diffuse away because it interacts with
asparagine (Asn 102) and histidine (His 418)
since it is covalently bonded deep into the
active site.
• As a result, the hydroxyl with the longest
connection to the pyranose ring, in this cases
allolactose, has the best chance of releasing
this intermediate that was formed.
In this process of transglycosylation, which is
the transfer of a glycosidically bound sugar to
another hydroxyl group, allolactose, or
galaactsyl 1-6 glucose, is the preferred
product.
•As a result of the
hydrolysis, the enzyme
lactase is able to break
down its substrate
lactose into glucose
and galactose.
• In addition lactase
converted the lactose
into another
disaccharide,
allolactose.
•Now, the glucose is
available for use by the
cell.
Sites
Juers, Douglas H. , Tom D. Heightman, Andrea Vasella, John D. McCarter, Lloyd Mackenzie, Stephen G. Withers,
and Brian W. Matthews. “A Structural View of the Action of Escherichia coli (lacZ) –Galactosidase.” Institute of
Molecular Biology, Howard Hughes Medical Institute and Department of Physics, University of Oregon, Eugene,
Oregon 97403-1229, Laboratorium für Organische Chemie, ETH-Zentrum, Universitätstrasse 16, CH-8092 Zürich,
Switzerland, and Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada
V6T 1Z1, 2001.
“Lactase.” Science Projects Online, 2001. http://www.science-projects.com/Lactase.htm
“Lactase – Beta-galactosidase” China GreatVista Chemicals, 2005.
http://www.greatvistachemicals.com/biochemicals/lactase.html
“Lactose Intolerance - NIH Publication No. 06–2751 National Institute of Health – National Institute of Diabetes
and Digestive and Kidney Diseases, March 2006. http://digestive.niddk.nih.gov/ddiseases/pubs/lactoseintolerance/
McKusick, Victor A. “Lactase – LCT” OMIM - Online Mendelian Inheritance in Man. John Hopkins University,
2006.
Montalto, Massimo, Valentina, Curigliano, Luca Santoro, Monica Vastola, Giovanni Cammarota, Raffaele Manna,
Antonio Gasbarrini, Giovanni Gasbarrini. “Management and Treatment of Lactose Malabsorption” World Journal
of Gastroenterology. Department of Internal Medicine, Catholic University, Rome, Italy, 2005.
“A Primer on Photosynthesis and the Functioning of Cells.” Introduction to Global Change. University of
Michigan. http://www.globalchange.umich.edu/globalchange1/current/lectures/kling/energyflow/PSN_primer.html
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