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Chapter 5
Carbohydrate Metabolism
Lecture 1
Lecture 2
Lecture 3
Lecture 4
Lecture 5
Lecture 6
Function of carbohydrate
The classification of carbohydrates
Glycolysis
The fates of pyruvate
Gluconeogenesis
The pentose phosphate pathway of
glucose oxidation
Lecture 7 Citric acid cycle
Lecture 1 Function of carbohydrate
Chapter 8 Carbohydrates
What
are Carbohydrates (saccharides) ?
Carbohydrate is an organic compound with the
general formula Cm(H2O)n, which consists of carbon,
hydrogen and oxygen, the last two in the 2:1 atom ratio.
Carbohydrates are hydrates of carbon, hence their name.
Carbohydrate, (C-H2O)n = “Carbon + Water”
Where are Carbohydrates?
Function of Carbohydrates ?
Upland cotton produces the most widely used
natural fibers, cellulose (polysaccharide)
Cellulose:
 the most abundant organic
compounds in the biosphere
 1015 kg of cellulose is
synthesized and degraded
on earth each year
Carbohydrates in food are important
sources of energy.
Energy sources and structural elements
Energy sources
Cell wall
Energy sources and structural elements
Bioactive substance——Glycoproteins
and Glycolipids
Function of Carbohydrates
 Source of energy
 Structure (cotton fibers: cellulose )
 Building blocks
 Cellular recognition
Lecture 2 The classification of carbohydrates
（Saccharides ）
1 Monosaccharide
• The generic name of the simplest carbohydrates.
• Monosaccharides can not be hydrolysised to give
smaller carbohydrates.
Monosaccharides can be classified
according to the number of carbons:





Triose
Tetrose
Pentose
Hexose
Heptose
3 carbons
4 carbons
5 carbons
6 carbons
7 carbons
Important hexose :
Glucose , fructose , galactose, mannose

D-Ribose
Pentose：
2 Oligosaccharide
Any molecule that contains a small number (2 to
about 20) of monosaccharide residues
connected by glycosidic linkages.
Oligosaccharide
Maltose
1
4

Sucrose
It is formed by plants
but not by animals.
1
2
1
2

Lactose
Lactose occurs naturally in milk but rarely in plants.
Cellobiose
3 Polysaccharide


An alternative name for glycan ;
Any linear or branched polymer consisting of
monosaccharide residues. Important polysaccharides
include glycogen, starch and cellulose .
Starch

Energy store of plants

Amylose
It can be made of several thousand glucose units.
In amylose, the 1st carbon on one glucose
molecule is linked to the 4th carbon on the next
glucose molecule (α(1→4) bonds).
Amylose is soluble in water.

Amylopectin
Glucose units are linked in a linear way with α(1→4)
glycosidic bonds. Branching takes place with α(1→6)
bonds occurring every 24 to 30 glucose units.
It is not soluble in water.
Glycogen
Glycogen is found mainly in the
liver and skeletal muscle.
Muscle:
energy (ATP)
production.
Liver: balance
blood glucose
levels.
Cellulose

A linear, unbranched β l-4 glucan molecular
Lecture 3 Glycolysis
Chapter 11 Glycolysis
In glycolysis, a molecule of glucose is broken down in a
series of enzyme-catalyzed reactions to yield two
molecules of the three-carbon compound pyruvate .
During the sequential reactions of glycolysis, some of
the free energy released from glucose is conserved in
the form of ATP and NADH.
 First stage of carbohydrate metabolism.
 Glycolysis was first discovered by Gustav Embden and
Otto Meyerhof and Parnus. Glycolysis is named as
EMP
 Simple sugars are broken down to pyruvate
 No oxygen needed.
 All life uses this process.
 There are 10 steps in glycolysis.
1 The Reaction of the glycolysis (Two Phases)


Preparatory phase（step①-⑤）：
Phosphorylation of glucose and its
conversion to glyceraldehyde 3phosphate
Payoff phase（step⑥-⑩）：
Oxidative conversion of glyceraldehyde 3phosphate to pyruvate and the coupled
formation of ATP and NADH
(1) Phosphorylation of Glucose
or glucokinase
Transfer of a phosphoryl group from ATP to glucose
(2) Isomerization :Conversion of Glucose 6Phosphate to Fructose 6-Phosphate
(3) Phosphorylation of Fructose 6-Phosphate to Fructose
1,6-Bisphosphate
(4) Cleavage of Fructose 1,6-Bisphosphate
（5）Interconversion of the Triose Phosphates
(6) Oxidation of Glyceraldehyde 3-Phosphate to
1,3-Bisphosphoglycerate
Oxidation and phosphorylation, yielding a high-energy
mixed-acid anhydride
(7) Phosphoryl Transfer from 1,3-bisphosphoglycerate
to ADP
Transfer of a high-energy phosphoryl group to ADP, yielding ATP
(8) Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate
(9) Dehydration to an energy-rich enol ester
PEP
(10) Transfer of the Phosphoryl Group from
Phosphoenolpyruvate to ADP
Transfer of a high-energy phosphoryl group to ADP, yielding ATP

For each molecule of glucose that passes
through the preparatory phase (a), two
molecules of glyceraldehyde 3-phosphate are
formed; both pass through the payoff phase
(b), Pyruvate is the end product of the
second phase of glycolysis.

For each glucose molecule, two ATP are
consumed in the preparatory phase and four
ATP are produced in the payoff phase, giving
a net yield of two ATP per molecule of
glucose converted to pyruvate.


The glycolytic breakdown of glucose is the
sole source of metabolic energy in some
mammalian tissues and cell types
(erythrocytes, renal medulla, brain, and
sperm, for example).
Many anaerobic microorganisms are
entirely dependent on glycolysis.
ATP and NADH Formation Coupled to Glycolysis

During glycolysis some of the energy of the
glucose molecule is conserved in ATP, while
much remains in the product, pyruvate. The
overall equation for glycolysis is
Regulation of glycolysis
Three glycolytic reactions(the reactions catalyzed
by hexokinase, PFK-1, and pyruvate kinase) are
irreversible.
1. Phosphofructokinase （PFK）
Inhibited by：ATP，Citrate
Activated by：AMP， 2,6-bisphosphate（F-2,6-BP）
2 Hexokinase
Inhibited by：G-6-P
3 pyruvate kinase
Inhibited by：ATP
Activated by：FBP
Lecture 4 The fates of pyruvate
Three catabolic routes.
1. oxidized to acetyl-CoA
2. Ethanol fermentation
3. Lactic acid fermentation
Lactic acid fermentation
Under anaerobic conditions, reduction of pyruvate
provides a means of reoxidizing the NADH produced
in the glyceraldehyde-3-phosphate dehydrogenase
reaction of glycolysis converted to lactate.
Lactic acid fermentation
Ethanol fermentation
The reduction of acetaldehyde to ethanol by NADH,
under anaerobic conditions
Ethanol fermentation
Aerobic pathways for pyruvate
The five coenzymes participating in this reaction: TPP, Lipoic
Acid , CoA-SH, NAD+, FAD,
The Pyruvate Dehydrogenase Complex Consists
of Three Distinct Enzymes



Pyruvate dehydrogenase (E1)
Dihydrolipoyl transacetylase (E2)
Dihydrolipoyl dehydrogenase(E3)
FIGURE 16–5 The pyruvate dehydrogenase complex.
FIGURE 16–6 Oxidative decarboxylation of pyruvate to acetyl-CoA by the PDH complex.