CARBOHYDRATE DIGESTION
& METABOLISM
Definition
• Metabolism (from Greek: μεταβολή "metabolē",
"change" or Greek: μεταβολισμός metabolismos,
"outthrow") is the set of chemical reactions that
happen in the cells of living organisms to sustain life.
• These processes allow organisms to grow and
reproduce, maintain their structures, and respond to
their environments. Metabolism is usually divided into
two categories.
•
Catabolism breaks down organic matter, for
example to harvest energy in cellular respiration.
•
Anabolism uses energy to construct components of
cells such as proteins and nucleic acids.
Chemical Reaction
• The chemical reactions of metabolism are
organized into metabolic pathways, in which one
chemical is transformed through a series of steps
into another chemical, by a sequence of enzymes.
• Enzymes are crucial to metabolism because they
allow organisms to drive desirable reactions that
require energy and will not occur by themselves,
by coupling them to spontaneous reactions that
release energy.
• As enzymes act as catalysts they allow these
reactions to proceed quickly and efficiently.
• Enzymes also allow the regulation of metabolic
pathways in response to changes in the cell's
environment or signals from other cells.
Selective reaction
• The metabolism of an organism
determines which substances it will find
nutritious and which it will find poisonous.
• For example, some prokaryotes use
hydrogen sulfide as a nutrient, yet this gas
is poisonous to animals.
• The speed of metabolism, the metabolic
rate, influences how much food an
organism will require, and also affects how
it is able to obtain that food.
CARBOHYDRATE DIGESTION
• AMYLUM digestion by amylase enzyme
Disaccharides digestion
► Glucose
is the most important carbohydrate
► Glucose is the major metabolic fuel of mammals,
except ruminants
► Monosaccharide from diet :
- Glucose
- Fructose
- Galactose
► Fructose and Galactose
glucose at the liver
Galactose Metabolism
Fructose Metabolism
 Blood glucose
carbohydrate metabolism
exist are :
1. Glycolisis
2. Glycogenesis
3. HMP Shunt
4. Oxidation of Pyruvate
5. Kreb’s Cycle
6. Change to lipids
 Fasting
blood glucose
carbohydrate
metabolism :
1. Glycogenolisis
2. Gluconeogenesis
GLYCOLISIS
 Glycolisis
oxidation of glucose
energy
 It can function either aerobically or anaerobically
pyruvate
lactate
 Occurs in the cytosol of all cell
 AEROBICALLY GLYCOLYSIS :
Pyruvate
Mitochondria
oxidized to
Asetil CoA
Kreb’s Cycle
CO2 + H2O + ATP
Glycolisis
 Most of the reaction of glycolysis are reversible,
except of three reaction :
1. Glucose
Glucose-6-phosphate,
catalyzed by Hexokinase / Glucokinase
 Hexokinase :
- Inhibited allosterically by its product
glucose-6-p
- Has a high affinity for its substrate
glucose
- available at all cell, except liver and islet cell
 Glucokinase :
- available at liver and islet cell
- in the liver
to remove glucose from the
blood after meal
2. Fructose-6-P
Fructose-1,6-biP
- catalyzed by Phosphofructokinase enzyme
- Irreversible
- Rate limiting enzyme in glycolysis
3. Phosphoenolpyruvate
Enol Pyruvate
- Catalyzed by Pyruvate kinase enzyme
 Oxidation of 1 mol glucose
8 mol ATP and
2 mol Pyruvate
 ANAEROBICALLY GLYCOLYSIS :
- The reoxidation of NADH through the
respiratory chain to oxygen is prevented
- Pyruvate is reduced by the NADH to lactate, by
Lactate dehidrogenase enzyme
Lactate dehydrogenase
 Pyruvate + NADH + H+
Lactate + NAD+
- Oxidation 1 mol glucose via anaerobically
glycolysis
2 mol ATP
 ANAEROBICALLY GLYCOLYSIS :
Respiratory chain is absence
Reoxidation of NADH
chain is inhibited
NAD+ via Respiratory
Reoxidation of NADH via lactate formation
allows glycolysis to proceed in the absence of
oxygen by regenerating sufficient NAD+
GLYCOLYSIS IN ERYTHROCYTE
• Erythrocyte lack mitochondria
•
•
•
respiratory
chain and Kreb’s cycle are absence
Always terminates in lactate
In mammals
the reaction catalyzed by
phosphoglycerate kinase may be bypassed by a
process that catalyzed Biphosphoglycerate mutase
Its does serve to provide 2,3-biphosphoglycerate
bind to hemoglobin
decreasing its affinity
for oxygen
oxygen readily available to
tissues
GLYCOLYSIS IN ERYTHROCYTE
OXIDATION OF PYRUVATE
• Occur in mitochondria
• Oxidation of 1 mol Pyruvate
CoA + 3 mol ATP
• CH3COCOOH + HSCoA + NAD+
(Pyruvate)
1 mol AsetylCH3CO-SCoA + NADH
(Asetyl-CoA)
• Catalyzed by Pyruvate dehydrogenase enzyme
• This enzyme need CoA as coenzyme
• In Thiamin deficiency, oxydation of pyruvate is
impaired
lactic and pyruvic acid
OXIDATION OF PYRUVATE
GLYCOGENESIS
• Synthesis of Glycogen from glucose
• Occurs mainly in muscle and liver cell
• The reaction :
• Glucose
Glucose-6-P
Hexokinase / Glucokinase
• Glucose-6-P
Glucose-1-P
Phosphoglucomutase
• Glucose-1-P + UTP
UDPG + Pyrophosphate
UDPG Pyrophosphorylase
GLYCOGENESIS
• Glycogen synthase catalyzes the formation of α•
1,4-glucosidic linkage in glycogen
Branching enzyme catalyzes the formation of α1,6-glucosidic linkage in glycogen
• Finally
the branches grow by further
additions of 1 → 4-gucosyl units and further
branching (like tree!)
SYNTHESIS OF GLYCOGEN
SYNTHESIS OF GLYCOGEN
GLYCOGENESIS AND GLYCOGENOLYSIS PATHWAY
Glycogenesis
Glycogenolysis
GLYCOGENOLYSIS
• The breakdown of glycogen
• Glycogen phosphorilase catalyzes cleavage of
•
•
•
the 1→4 linkages of glycogen to yield glucose-1phosphate
α(1→4)→α(1→4) glucan transferase transfer a
trisaccharides unit from one branch to the other
Debranching enzyme hydrolysis of the 1→6
linkages
The combined action of these enzyme leads to
the complete breakdown of glycogen.
GLYCOGENOLYSIS
Phosphoglucomutase
• Glucose-1-P
Glucose-6-P
Glucose-6-phosphatase
• Glucose-6-P
Glucose
• Glucose-6-phosphatase enzyme
•
•
a spesific
enzyme in liver and kidney, but not in muscle
Glycogenolysis in liver yielding glucose
export to blood
to increase the blood glucose concentration
In muscle
glucose-6-P
glycolysis
GLUCONEOGENESIS
Pathways that responsible for converting
noncarbohydrate precursors to glucose or glycogen
 In mammals
occurs in liver and kidney
 Major substrate :
1. Lactic acid
from muscle, erythrocyte
2. Glycerol
from TG hydrolysis
3.Glucogenic amino acid
4. Propionic acid
in ruminant




Gluconeogenesis meets the needs of the body for
glucose when carbohydrate is not available from the
diet or from glycogenolysis
A supply of glucose is necessary especially for nervous
system and erythrocytes.
The enzymes :
1. Pyruvate carboxylase
2. Phosphoenolpyruvate karboxikinase
3. Fructose 1,6-biphosphatase
4. Glucose-6-phosphatase
GLUCONEOGENESIS
GLUCONEOGENESIS FROM AMINO
ACID
GLUCONEOGENESIS FROM PROPIONIC ACID
CORY CYCLE
HMP SHUNT/HEXOSE MONO PHOSPHATE
SHUNT = PENTOSE PHOSPHATE PATHWAY
• An alternative route for the metabolism of
glucose
• It does not generate ATP but has two major
function :
1. The formation of NADPH synthesis of fatty
acid and steroids
2. The synthesis of ribose nucleotide and
nucleic acid formation
HMP SHUNT
• Active in : liver, adipose tissue, adrenal cortex,
thyroid, erythrocytes, testis and lactating mammary
gland
• Its activity is low in muscle
• In erythrocytes :
• HMP Shunt provides NADPH for the reduction of
oxidized glutathione by glutathione reductase
reduced glutathione removes H2O2 glutathione peroxidase
HMP SHUNT
Glutathione reductase
• G-S-S-G
2-G-SH
(oxidized glutathione)
(reduced glutathione)
Glutathione peroxidase
• 2-G-SH + H2O2
G-S-S-G + 2H2O
• This reaction is important accumulation of H2O2
may decrease the life span of the erythrocyte
damage to the membrane cell
hemolysis
HMP SHUNT
BLOOD GLUCOSE
• Blood glucose is derived from the :
1. Diet the digestible dietary carbohydrate yield glucose blood
2. Gluconeogenesis
3. Glycogenolysis in liver
• Insulin play a central role in regulating blood glucose
blood glucose
• Glucagon
blood glucose
• Growth hormone inhibit insulin activity
• Epinefrine stress
blood glucose