2nd Unit / Ch 12 & 7 Carbs & Metabolism of Monosaccharides and Disaccharides

Question 1

Frutose metabolism requres 2 enzymes

Entry of fructose into cells is not insulin dependent
(unlike that of glucose into certain tissues,

Answer 1

and, in
contrast to glucose, fructose does not promote the secretion of insulin.

Question 2

In the formation of joining Monosaccharides:

They form Lactose, Surose, and Maltose.

What enzyme is and substrate pattern is used?

Answer 2

They are linked by glycosidic bonds, formed by Glycosyltransferases, which uses nucleotide sugers such as UDP-glucose as substrates.

Question 3

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….

Answer 3

monosaccharides, glucose, galactose and fructose,
which are absorbed by cells of the small intestine.

Question 4

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..

Answer 4

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.

Question 5

Abnormal degradation of disaccharides:

Is when undigestive carbs goto Large intestine, this active osmotically active material causes…

Answer 5

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.

Question 6

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

Answer 6

ingested carbohydrate not
absorbed by the body, but which is metabolized instead by the
intestinal flora.

Question 7

Unlike other Forms of Glucose, Fructose is unique in that it..

Answer 7

Does not require Insulin to absorbed from the blodstream.

It also does not promote secretion of insulin.

Question 8

Fructose must be phosphorylated to be absorbed. That can be done by Hexokinase or Fructokinase (liver). The method is…

Answer 8

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.

Question 9

The major Dietary Carbohydrates are

Answer 9

Amylose and Amylopectin, which are components of Starch

Question 10

1. Maltose is produced as a by product of the digestion of
2. Milk suger is digested as…
3. Table suger is absorbed as….

Answer 10

1. starch….
2. Lactose
3. Sucrose

Question 11

Cleavage of fructose 1-phosphate

Fructose 1-phosphate is not phosphorylated to fructose 1,6-bis -
phosphateas isfructose 6-phosphate.

It has another route…

Answer 11

it is cleaved by

• aldolase B (*also called fructose 1-phosphate aldolase) to
• *dihydroxy acetone phosphate (DHAP)** and glyceraldehyde.

Question 12

Cleavage of fructose 1-phosphate

After DHAP and glyceraldehyde have been made…..

Answer 12

DHAP can directly enter glycolysis or gluconeogenesis,
whereas glyceraldehyde can be metabolized by a number of pathways,

Question 13

Kinetics of fructose metabolism

The rate of fructose metabolism is more rapid than that of glucose
because

Answer 13

the trioses formed from fructose 1-phosphate bypass phospho
fructokinase-1—the major rate-limiting step in glycolysis.

Question 14

Fructose D Metabolism disorders: Can be harmful or not,

fructokinase is made specific to the Liver, if deficienct will cause

Answer 14

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.

Question 15

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),

Answer 15

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.

Question 16

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,

Answer 16

is
(reversibly) isomerized to fructose 6-phosphate by phospho -
mannose isomerase.

Question 17

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…

Answer 17

the seminal vesicles is for the benefit of sperm cells, which use
fructose as a major carbohydrate energy source.

Question 18

from sorbitol to fructose in the liver provides a mechanism.

The teacher called it Polyol Pathway

Answer 18

any available sorbitol is converted into a substrate that can
enter glycolysis or gluconeogenesis.

Question 19

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.

Answer 19

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.

Question 20

After a meal, glucose rapidly enters Hepatacytes wihtout theneed of GLUT.

Then in the liver…

Answer 20

In response to high insulin, glucose enters and

is phosphorylated to be converted to glycogen.

Question 21

GALACTOSE METABOLISM

The major dietary source of galactose is lactose (galactosyl β-1,4-glucose)
obtained from milk and milk products.

Answer 21

Some galactose can also be obtained by lysosomal
degradation of complex carbohydrates, such as glyco proteins and
glycolipids, which are important membrane components.

Question 22

GALACTOSE METABOLISM STEPS

A. Phosphorylation of galactose

Answer 22

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.

Question 23

GALACTOSE METABOLISM STEPS

B. Formation of UDP-galactose

Galactose 1-phosphate cannot enter the glycolytic pathway unless it
is first converted to UDP-galactose.

Answer 23

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).

Question 24

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….

Answer 24

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.

Question 25

GALACTOSE METABOLISM

D. Role of UDP-galactose in biosynthetic reactions.

UDP-galactose can serve as the donor of galactose units in a number
of synthetic pathways, including synthesis of lactose, glycoproteins, glycolipids and glycos aminoglycans

Answer 25

If galactose is not provided by
the diet (for example, when it cannot be released from lactose as a
result of a lack of β-galactosidase in people who are lactose-intolerant),
all tissue requirements for UDP-galactose can be met by the
action of UDP-hexose 4-epimerase on UDP-glucose, which is efficiently
produced from glucose 1-phosphate.

Question 26

E. Disorders of galactose metabolism

Galactose 1-phosphate uridyltransferase (GALT) is deficient in individuals with classic galactosemia (see
In this disorder, galactose 1-phosphate and, therefore,
galactose accumulate in cells. This causes..

Answer 26

The accumulated galactose
is shunted into side pathways such as that of galactitol production.

vomiting, diarrhea, and jaundice.

Question 27

E. Disorders of galactose metabolism

Galactose 1-phosphate uridyltransferase (GALT) deficiency.

TERMED CLASSOIC GALACTOSERUIA

1. Prenatal diagnosis is possible by..
2. Therapy:

Answer 27

1. chorionic villus sampling. Newborn screening is available
2. Rapid diagnosis and removal of
   galactose (and therefore lactose) from the diet.
   • Despite adequate treatment, at risk for developmental
   delays and, in females, premature ovarian failure.

Question 28

IV. LACTOSE SYNTHESIS

Lactose is a disaccharide that consists of a molecule of β-galactose
attached by a β(1→4) linkage to glucose.

Lactose, known as the “milk sugar,” is produced
by the mammary glands of most mammals.

Answer 28

Lactose is
synthesized in the Golgi by lactose synthase (UDP- galactose:glucose
galactosyltransferase), which transfers galactose from UDP-galactose
to glucose, releasing UDP

Question 29

E. Disorders of galactose metabolism

Galactose Kinase Deficinecy leads to

Non-classical glactosemia

Answer 29

A deficiency in
galactokinase results in a less severe disorder of galactosemia
metabolism, although cataracts are common