2nd Unit / Ch 12 & 7 Carbs & Metabolism of Monosaccharides and Disaccharides Flashcards
Frutose metabolism requres 2 enzymes
Entry of fructose into cells is not insulin dependent
(unlike that of glucose into certain tissues,
and, in
contrast to glucose, fructose does not promote the secretion of insulin.
In the formation of joining Monosaccharides:
They form Lactose, Surose, and Maltose.
What enzyme is and substrate pattern is used?
They are linked by glycosidic bonds, formed by Glycosyltransferases, which uses nucleotide sugers such as UDP-glucose as substrates.
there is little monosaccharide present in diets of mixed
animal and plant origin, the enzymes are primarily endoglycosidases
that hydrolyze polysaccharides and oliosaccharides, and disaccharidases
that hydrolyse tri- and disaccharides into their reducing sugar
components,
which we absorb as….
monosaccharides, glucose, galactose and fructose,
which are absorbed by cells of the small intestine.
Absorption of monosaccharides by intestinal mucosal cells:
Galactose and glucose are transported into mucosal cells by an active, energy-requiring process that requires a concurrent uptake of sodium ions. The transport protein is sodium dependent glucose transporter 1
(SGLT-1 co-transport)
But Fructose..
Fructose
uptake requires a sodium-independent monosaccharide transporter
(GLUT-5) for its absorption.
After all are in th emucosal cells, All three monosaccharides are transported
from the intestinal mucosal cell into the portal circulation by
yet another transporter, GLUT-2.
Abnormal degradation of disaccharides:
Is when undigestive carbs goto Large intestine, this active osmotically active material causes…
Water to be drawn from the mucosa into the large intestine, causing osmotic diarrhea. This is reinforced by the bacterial fermentation of the remaining carbohydrate to two- and three-carbon compounds (which are also osmotically active) plus large volumes of CO2 and H2 gas, causing abdominal cramps, diarrhea, and flatulence.
Diagnosis of of defiency of enzymes:
Can be done by Measurement of hydrogen gas in the breath is a reliable
test for determining the amount of
ingested carbohydrate not
absorbed by the body, but which is metabolized instead by the
intestinal flora.
Unlike other Forms of Glucose, Fructose is unique in that it..
Does not require Insulin to absorbed from the blodstream.
It also does not promote secretion of insulin.
Fructose must be phosphorylated to be absorbed. That can be done by Hexokinase or Fructokinase (liver). The method is…
Hexokinase needs a high amount because of its low affinity for it. So Fructose kinase is preffered and phosphorylated it to Fructose-1-phosphate using ATP as a donor of a phosphate.
The major Dietary Carbohydrates are
Amylose and Amylopectin, which are components of Starch
- Maltose is produced as a by product of the digestion of
- Milk suger is digested as…
- Table suger is absorbed as….
- starch….
- Lactose
- Sucrose
Cleavage of fructose 1-phosphate
Fructose 1-phosphate is not phosphorylated to fructose 1,6-bis -
phosphateas isfructose 6-phosphate.
It has another route…
it is cleaved by
- aldolase B (*also called fructose 1-phosphate aldolase) to
- *dihydroxy acetone phosphate (DHAP)** and glyceraldehyde.
Cleavage of fructose 1-phosphate
After DHAP and glyceraldehyde have been made…..
DHAP can directly enter glycolysis or gluconeogenesis,
whereas glyceraldehyde can be metabolized by a number of pathways,
Kinetics of fructose metabolism
The rate of fructose metabolism is more rapid than that of glucose
because
the trioses formed from fructose 1-phosphate bypass phospho
fructokinase-1—the major rate-limiting step in glycolysis.
Fructose D Metabolism disorders: Can be harmful or not,
fructokinase is made specific to the Liver, if deficienct will cause
Essential fructosesuria:
Benign asymptomic condition (becuz no toxic metabolite synthesis)
-small amounts of frutose can be phosphorylated to Frutose-6-Phosphate.
SYMPTOMS : fructose shows up in urine.
Disorders of fructose metabolism like Essential Fructosuria is not harmful but..
a severe disturbance of liver and kidney metabolism
as a result of aldolase B deficiency
(hereditary fructose intolerance, HFI),
The first symptoms of HFI appear when a baby is weaned
from milk, and begins to be fed food containing sucrose
or fructose. Fructose 1-phosphate accumulates, resulting in a drop
in the level of inorganic phosphate (Pi) and, therefore, of ATP. As
ATP falls, AMP rises. In the absence of Pi, AMP is degraded, causing
hyperuricemia and lactic acidosis.
Conversion of mannose to fructose 6-phosphate
Mannose, the C-2 epimer of glucose is an important component of glycoproteins .
- Hexokinase* phosphorylates
- *mannose**, producing mannose 6-phosphate, which, in turn,
is
(reversibly) isomerized to fructose 6-phosphate by phospho -
mannose isomerase.
Synthesis of sorbitol: Aldose reductase reduces glucose, producing
sorbitol.
there is a second enzyme, sorbitol dehydrogenase, which can oxidize the sorbitol to produce fructose.
This is For…
the seminal vesicles is for the benefit of sperm cells, which use
fructose as a major carbohydrate energy source.
from sorbitol to fructose in the liver provides a mechanism.
The teacher called it Polyol Pathway
any available sorbitol is converted into a substrate that can
enter glycolysis or gluconeogenesis.
The effect of hyperglycemia on sorbitol metabolism:
Elevated intracellular glucose concentrations and an
adequate supply of NADPH cause aldose reductase to produce a
significant increase in the amount of sorbitol.
exacerbated when sorbitol
dehydrogenase is low or absent.
As a result, sorbitol accumulates in these
cells, causing strong osmotic effects and, therefore, cell swelling as
a result of water retention.
cataract formation, peripheral neuropathy, and
microvascular problems leading to nephropathy and retinopathy.
After a meal, glucose rapidly enters Hepatacytes wihtout theneed of GLUT.
Then in the liver…
In response to high insulin, glucose enters and
is phosphorylated to be converted to glycogen.
GALACTOSE METABOLISM
The major dietary source of galactose is lactose (galactosyl β-1,4-glucose)
obtained from milk and milk products.
Some galactose can also be obtained by lysosomal
degradation of complex carbohydrates, such as glyco proteins and
glycolipids, which are important membrane components.
GALACTOSE METABOLISM STEPS
A. Phosphorylation of galactose
Like fructose, galactose must be phosphorylated before it can be further
metabolized.
* galactokinase*, which produces galactose 1-phosphate
As with other kinases, ATP is the phosphate donor.
GALACTOSE METABOLISM STEPS
B. Formation of UDP-galactose
Galactose 1-phosphate cannot enter the glycolytic pathway unless it
is first converted to UDP-galactose.
This occurs in
an exchange reaction, in which UDP-glucose reacts with galactose
1-phosphate, producingUDP-galactoseandglucose 1-phosphate.
The enzyme that catalyzes this reaction is
galactose-1-phosphate uridyltransferase
(GALT).
GALACTOSE METABOLISM
C. Use of UDP-galactose as a carbon source for glycolysis or
gluco neogenesis.
For UDP-galactose to enter the mainstream of glucose metabolism,
it must first be converted to its C-4 epimer, UDP-glucose, by UDP hexose
4-epimerase.
then….
This “new” UDP-glucose (produced from the
original UDP-galactose) can then participate in many biosynthetic reactions, as well as being used in the GALT reaction.
