Chapter 1

Introduction In Carbohydrate Metabolism

CHO, (CnH2nOn)
•Carbohydrates are called carbohydrates because they are
essentially hydrates of carbon (i.e. they are composed of
carbon and water and have a composition of Cn(H2O)n.
•The major nutritional role of carbohydrates is to provide
energy, and digestible carbohydrates provide 4 kilocalories
per gram.
• No single carbohydrate is essential, but carbohydrates do
participate in many required functions in the body.
To show how carbohydrate feeding is necessary to our life: •
Ex.:Human brain can't store glucose, so it need daily high
concentrations of glucose about 120 grams of glucose per day to obtain
(480 cal) to work efficiently

Metabolism
Involves the chemical changes that various food stuffs undergo inside the body,
and is formed of a sequence of enzymatic reactions
Metabolism is classified into aspects: anabolism and catabolism
Ⅰ-Anabolism:-
It includes all the biosynthetic pathways which are concerned with the
synthesis of
macromolecules eg.(glycogen, proteins and lipids). Anabolic reactions require
energy which is supplied mainly by adenosine triphosphate (ATP)
Ⅱ-catabolism:-
It includes all the degradative processes whereby complex molecules are
broken into simpler compounds
carbohydrates proteins lipids
stage one
Monosaccharide Amino Acids Glycerol + Fatty
Acids
stage two
Acetyl - COA or Intermediates of Citric Acid Cycle
stage three
CO2+ H2O+ Free Energy (ATP)
 Digestion of carbohydrates:-
•Digestion is a hydrolytic process where food
molecules are broken down into simple chemical units
that can be absorbed by the human or animal body In
man, digestion takes place in the digestive tract –
while absorption occurs primarily in the small
intestine.
1-In the mouth :-
Digestion of starch (the principle CHO ingested by man begins in the
mouth by the action of salivary Amylase enzyme (or ptyaline) giving
a mixture of dextrin's and maltose

Amylase
Starch +H20 Dextrin +Maltose

Amylase
Dextrin +H2O Maltose+Iso maltose +Maltotriose
OR
Starch
Amylodextrin +Maltose
Salivary Amylase (ptyalin)
Erythrodextrin +Maltose
Salivary Amylase
Achrodextrin +Maltose
 Salivary Amylase:
Functions until the food reaches the stomach where it is
quickly inactivated by the acidity of the stomach.
Very little digestion occurs in the stomach through acid
hydrolysis.

2-In the Stomach:-

It continues to hydrolyzed Starch only for a few minutes,
the pH being unfavorable for the action of the enzyme
3-In the Small intestine:-
a- Another amylase called pancreatic amylase (or Amylopsin)
converts the remaining dextrins into maltose ( its pH is
alkaline (7.9-8.2))

Dextrin's and Pancreatic amylase

remaining of starch Maltose units

Maltose units are then cleaved by maltase enzyme into glucose

units
 b-Intestinal disaccharidases :-
They complete the action of other enzymes with the production of
monosaccharides as follows :
Maltase
Maltose 2 Glucose
Maltase is secreted from the walls of small intestine

Iso maltase
Iso maltose 2 Glucose

Lactase
Lactose Galactose+ Glucose

Sucrase
Sucrose . Glucose + Fructose
•That means that sucrose and lactose are not digested until
reaching the small intestine.
•The overall digestion of disaccharides and polysaccharides
produces three units (Glucose – fructose – Galactose).
• These are then absorbed through the walls of the small intestine
(microvilli) into the blood stream by active transport. (i.e must
be driven by energy).
•Monosaccharide's that are absorbed differ in their rate of
passage across membranes:
Galactose > Glucose > Fructose
•Following absorption – monosaccharide's are carried to the liver
through the portal vein where Galactose and fructose are
enzymatically converted into Glucose .
• Conversion of Galactose into Glucose:
•Galactose is phosphorylated by ATP using the enzyme
galactokinase to give galactose -1- ph .
•Galactose -1- ph. reacts with UDP-glucose (UDP-GLU.) to form
UDP-galactose (UDP-Gal.) by the enzyme: galactose -1-ph.uridyl
transferase, where galactose can take the place of glucose in UDP.
And glucose -1- phosphate is produced.
•Glucose -1- ph. could be then hydrolyzed to glucose.
Conversion of galactose into glucose :-
B) Conversion of Fructose into Glucose:
•Fructose in the diet, after being absorbed and reached
the liver is converted into fructose-6-ph by fructokinase
enzyme.
•By action of phosphohexose isomerase enzyme-fructose
-6-ph is converted into Glucose-6-ph.
Glucose -6- ph by phosphatase enzyme is converted into
Glucose
 Notes :
Glucose therefore is the only monosaccharide serving as the major
energy source in human
The processes of digestion and absorption are usually excluded from the
definition because the food stuffs are not physiologically inside the body
until they have passed across the epithelial lining of the intestinal
mucosa.
Carbohydrates represent the chief fuel of the animal kingdom.
It must be burned or oxidized in order to release its stored energy.

Oxidation results in the conversion of CHO into CO2+ water.

C6H12O6+ 6O2 6CO2 + 6H2O + 686.000 claories.
Organisms conserve almost half of the 686.000 calories by a series of
reactions liberating small amounts of utilizable energy stored as ATP.
The remainder of energy is used to heat the body and to maintain its
temperature.
:. Bioenergetics system requires:
•A source of energy, in the form of oxidizable food stuff.
•A mechanism for the degradation of food stuffs to liberating energy,
in the form of a system of enzymes .
•A mechanism for the collection and storage of free energy liberated
in the form of (high –energy phosphate bond).
•A mechanism for utilization of the stored free energy, converting it
to work which may be.
• Mechanical work ex. (Muscle contraction).
• Osmotic work ex. (secretion or absorption).
• Chemical work ex. (synthetic reactions).
•Or: Electrical work ex. (transmission of nerve impulses)
 Degradation of food stuffs (energy gain)

1. No- free energy is 2. some free energy is 3. most of the free

obtained: obtained energy :
From the breakdown of From break down of Obtained from the break-
big. Organic molecules of glucose, glycerol, fatty down of acetyl coA via
food staffs into smaller acids and amino acids TCA cycle to CO2
molecules into acetyl coA. &H2O
 Metabolism of Glucose starts by Glycolysis
Glycolysis
Embden-Meyerhof-parnas pathway
It is a series of biochemical reactions by which glucose is converted to pyruvate i
n
aerobic condition or converted to lactate in anaerobic condition.
site and steps :-
The enzymes of glycolysis are present in the cytosol of all cells
The steps of glycolysis can be classified into two phases:-
phase one:-
In this phase glucose is converted into two molecules of glyceraldehyde-3-phosph
ate.
phase two:-
In this phase the two molecules of glyceraldehydes-3-phosphate are converted
Into two molecules of pyruvate (aerobic) or Lactate ( anaerobic)
 The metabolic pathway of cellular respiration:-
Cellular respiration is an example of a metabolic pathways
A Series of chemical reactions in cells - building or degradation processes
All of the reactions involved in cellular respiration can be grouped into
three main
stages .
Glycolysis.
The Krebs cycle
Electron transport chain.

The overall Equation for Cellular Respiration

A Common fuel molecule for Cellular respiration is glucose this is the
overall equation for what happens to glucose during the cellular
respiration:-

C6H12O6 + 602 6C02+6H20+ATP

Glycolysis

Note:-
There are 2 molecules GAP (Glyceraldehydes -3-phosphate) per each glucose
molecule
Importance of glycolysis:-
Glycolysis both occurs under aerobic and anerobic conditions.
1-Under aerobic condition:-
Glucose 2pyruvate + 8 ATP
Glucose is converted into 2 molecules of pyruvate .
Pyrurate is transported into the mitochondria and converted into
two active acetate that are oxidized by Citric acid cycle.
So the total smrount of ATP formed by glycolysis under aerobic
condition equals 10 molecules
2 molecules of ATP are utilized by hexokinase and PFK-1
( phosphofructokinase-1).
The net gain = 8 molecules of ATP (8 moles of ATP/ one mole of
glucose
2-Under anerobic condition :-
Glucose 2 Lactate +2 ATP
This occurs in muscles during Severe exercise and in
(red cells due to absence of mitochondria)
Regulation of Glycolysis:-
I- Regulation of individual key enzymes:-
The rate of the glycolytic pathway is regulated by the activity
of the three irreversible enzymes the activity of the three
irreversible enzymes e (Key enzymes), hexokinase
(glucoKinase), PFK-1 and pyruvate kinase .
Generally, the activity of these enzymes increases during
carbohydrate feeding and
decreases during fasting or carbohydrate deprivation
1- Hexokinase and Glucokinase:-
The first step in metabolism of glucose and other sugars, after
entering a cell is phosphorylation.
Phosphorylation prevents the leakage of sugar molecules from the
cell, since phosphorylated sugars do not move across cell
membranes.
2- Phosphofructokinase1 (PFKI):-
It is inhibited by glucagon, ATP and citrate. It is activated by insulin
, AMP.
3-Pyruvate Kinase (PK):-
It is inhibited by glucagon and ATP. It is activated by fructose l,6
bisphosphate and insulin.
 Inhibitors of glycolysis :-
Iodoacetate :-produces inhibition of glyceraldehyd.3- phos
phate dehydrogenase ( inhibitor of OH group )
G-3-ph l,3bisphosphoglycerat
e
Fluoride:-produce inhibition of the enolase enzyme by removal
of Mg+2 as Mg fluoride
This property is used to Keep blood Samples for of glucose
estimation enolase
2 phosphoenolate phosphoenol pyruvate
Importance of Cori cycle :-
1- It prevents loss of Lactate as waste products in urine.
2- It helps to maintain blood glucose level.
3- It supplies red cells and contracting muscles with
glucose for neutralization and ATP production.
4-It spares energy in red cells and contracting muscles
Conversion of Pyruvate into Acety -
COA is a major primer for initiation of Kreb's cycle ( TCA Cycle)
by help of enzyme complex Known by pyruvate dehydrogenase
complex
Location: of TCA cycle is Mitochondria
Citric Acid cycle = TCA Cycle= Kreb's cycle
TCA= Tricarboxylic acid cycle
∴Acetate in the form of acetyl COA is derived from pyruvate and other metabolites
and is oxidized to C02 in the citric acid cycle
This reaction is irreversible, which means acetyl - COA Cannot be converted
backward to pyruvate

∴Citric acid cycle or Kreb's cycle is a series of biochemical reactions that are
responsible for Complete oxidation of organic substrates (Carbohydrates, fats and
proteins) to form CO2, H2O and energy.
site and steps:-
The enzymes of the cycle are present mainly in the mitochondrial matrix except
succinate dehydrogenase which is tightly bound to the inner mitochondrial membrane
Regulation of Citric acid cycle:-
1-Pyruvatede dehydrogenase:-
Inhibited by acetyl COA and NADH.
2-Citrate synthase:-
Inhibited by its substrate oxaloacetate.
3-Isocitrate dehydrogenase:-
Activated allosterically by ADP
Inhibited allosterically by NADH
4-α Ketoglutarate dehydrogenase:-
Inhibited allosterically by products = succinyl-COA
and NADH
 Electron Transport chain (ETC)
Respiratory chain
Oxidation of carbohydrates, fats, and proteins in the animal body
produces reduced coenzymes (mainly NADH, H+and FADH2)
Definition: is formed of a Series of elelectron carriers, which
catalyze the transfere of electrons from reduced Coenzymes to
oxygen to form H2O part of energy released is utilized for synthesis of
high energy phosphate bonds
(Conversion of ADP+Pi→ATP)
Components of ETC :-
The components of the ETC are located in the inner
mitochondrial membrane it is
formed of four complexes , coenzymes Q and cytochrome C
0xidation of NADH by the respiratory chain produces 3 ATP
because hydrogen ions or electrons pass through the three
coupling sites
Oxidation of FADH2 by the respiratory chain. produces 2
ATP because hydrogen ions
or electrons pass through two coupling sites only.
Inhibitors of ETC :-
H2S , CO, and cyanide inhibit complexes of cytochrome.1
Note:-
The 1st step in glycolysis catalyzed by hexokinase ,in which glucose
and other sugars after entering the cell is phosphorylated.
Phosphorylation prevents the leakage of sugar molecules from the
cell.
This step requires Mg+2 and ATP.
Mg+2 interacts with negatively charged phosphate oxygen atoms of
ATP ,providing charge compensation and promoting a favorable
conformation of ATP at active site of hexokinase enzyme and
liberating of phosphate group to make phosphorylation for sugar and
convert ATP into ADP
Structure of ATP :-
Glucose may undergo one of the following fates
1-Oxidation :
Oxidation of glucose may follow one of 3 pathways
a-Embden –Meyerhof –pathway or glycolysis ,then followed by oxidation of pyruvate
(This is the major pathway )
b-The pentose –phosphate pathway.
c-Uronic acid pathway.
2-Conversion to substances of biological importance :-
a-Ribose and deoxyribose
(In formation of DNA, RNA )
b-Fructose :
Secreted in semen
c-Galactose :
Biosynthesis of lactose , glycolipids ,and glycosaminoglycans.
d- Glucouronic acid :
Used in detoxication and biosynthesis of glycosaminoglycans .
e- Aminosugars :-
Used in biosynthesis of glycosaminoglycans and gangliosides.
3-Storage : in the form of
•Glycogen : ( Name of process is glycogenesis )
•Triacylglycerols : ( Name of process is lipogenesis )
Conversion of glucose into triglycerides
4- Excretion in the urine :
We can see glucose in urine only if the plasma glucose level exceeds a
certain value ( (renal threshould ) normally =180 mg /dl ) it becomes
excreted by kidney in urine