Download 4-Carbohydrate metabolism

Carbohydrate metabolism
 Carbohydrate metabolism means the various biochemical processes
responsible for the
formation, breakdown and interconversion of
carbohydrates in living organisms.
 All carbohydrates share a general formula of approximately C nH2nOn; glucose
is C6H12O6. Monosaccharides may be chemically bonded together to form
disaccharides such as sucrose and longer polysaccharides such as starch and
cellulose.
 Carbohydrates are a superior short-term energy reserve for organisms,
because they are much simpler to metabolize than fats or proteins.
 Carbohydrates are typically stored as long polymers of glucose molecules
with Glycosidic bonds for structural support (e.g. chitin, cellulose) or energy
storage (e.g. glycogen, starch).
 However, the strong affinity of carbohydrates for water makes storage of
large quantities of carbohydrates inefficient due to the large molecular weight
of the solvated water-carbohydrate complex.
 In some organisms, excess carbohydrates are catabolised to form AcetylCoA, where they enter the fatty acid synthesis pathway.
 Fatty acids, triglycerides, and other lipids are commonly used for long-term
energy storage.
Dietary Carbohydrates:
 Monosaccharides:
glucose, fructose and galactose in fruits and honey & obtained by hydrolysis of
oligo- & polysacs.
 Disaccharides:
sucrose, lactose, maltose (by hydrolysis of starch).
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 Polysaccharides:
starch (in potatoes, rice, corn and wheat)
Cellulose (in cell wall of plants) not digested by humans due to absence of
cellulase
Catabolism
 Carbohydrate catabolism is the breakdown of carbohydrates into smaller
units. Oligo/polysaccharides are cleaved first to smaller monosaccharides by
enzymes called Glycoside hydrolases.
Digestion of Carbohydrates::
1- In the mouth:
Salivary amylase hydrolyzes starch into dextrin +maltose
2- In the stomach:
Due to drop of pH, salivary amylase acts for a very short time
3- In the small intestines:
Pancreatic and intestinal enzymes hydrolyze the oligo- and polysaccharides as
follows:
Pancreatic amylase
Starch
maltose + isomaltose
Maltase
Maltose
2 glucose
Lactase
Lactose
glucose + galactose
Sucrase
Sucrose
glucose + fructose
Absorption of monosaccharides:
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 Particularly important dietary carbohydrates include starch and disaccharides
such as lactose and sucrose. None of these molecules can be absorbed for the
simple reason that they cannot cross cell membranes, unlike the situation for
monosaccharides, there are no transporters to carry them across.
 Absorption of glucose and other monosaccharides occurs in the small
intestine only by co-transport with sodium, across the epithelium and into
blood.
Fate of absorbed monosaccharides:
In the liver, fructose and galactose are converted to glucose.
Fate of glucose:
A. Uptake by different tissues.
B. Utilization by the tissues: in the form of:
1. Oxidation to produce energy:
- Major pathways (glycolysis & Krebs' cycle).
- Minor pathways (hexose monophosphate pathway & uronic acid pathway)
2. Conversion to other substances:
Carbohydrates: ribose (RNA,DNA), galactose (in milk), fructose (semen)
C. Storage of excess glucose:
As glycogen in liver and muscles, when these reserves are filled it is converted to
TAG & deposited in adipose tissue.
D. Excretion in urine
If blood glucose exceeds renal threshold (180 mg/dL), it will be excreted in urine.
 Oxidation of one gram of carbohydrate yields approximately 4 kcal of
energy. Energy obtained from carbohydrate metabolism is usually stored in
the form of ATP.
 The monosaccharide units can then enter into monosaccharide catabolism.
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Krebs' Cycle (Citric Acid Cycle) (Tricarboxylic Acid Cycle)"TCA"
 Site: mitochondria of every cell
 Series of biochemical reactions that are responsible for complete oxidation of
CHO, fats and Ptns to form : CO2 + H2O + Energy
 During this process the following is produced:
3x2=6 NADH+H+
1x2=2 FADH2
1x2=2 ATP
Regulation of Carbohydrate Metabolism:
The liver, pancreas, and other endocrine glands are all involved in controlling
the blood glucose concentrations.
During a brief fast, glucose is supplied to the extra cellular fluid from the
liver through glycogenolysis.
When fasting period is longer than 1 day, glucose is synthesized by
gluconeogenesis.
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Insulin:

Produced and secreted by β-cell of islets of Langerhans.

Hypoglycemic agent
 Increases glycogenesis and glycolysis: Glucose  glycogen  pyruvate 
acetyl CoA.

Increases lipogenesis.

Decreases glycogenolysis.
 Promotes the use of glucose by the muscles.
Glucagon:

Produced and secreted by α-cell of islets of Langerhans.

Hyperglycemic agent.
 Increases glycogenolysis: Glycogen  glucose.

Increases gluconeogenesis: Fatty acids  acetyl CoA  ketone.

Proteins  amino acids.
Human disease of carbohydrate metabolism are:

Diabetes mellitus

Lactose intolerance

Fructose intolerance

Galactosemia

Glycogen storage disease
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