Glycogenesis

ANN MARY MATHEW
BGS051706
MSc GENOMIC SCIENCE

14/09/2017 BIOCHEMISTRY UNIT 3 CARBOHYDRATES 2
GLYCOGENESIS
WHAT IS GLYCOGEN ???
Glycogen is a homo-
polymer made up of
repeated units of α D
glucose and each
molecule is linked to each
other by 1→4 glycosidic
bond which is a link
connecting the 1st C atom
of the active glucose
residue to the 6th C atom
of the approaching
glucose molecule.

• Once there is a chain
consisting of 8 to 10
glycosidic residues in the
glycogen fragment,
branching begins by 1 – 6
linkages.
• Liver glycogen is synthesized
in the well fed states.
• Muscle glycogen is
synthesized when the muscle
glucose get depleted in
intense physical exercise.

 Glycogenesis is the biosynthesis of glycogen, the major storage form of
carbohydrates in animals similar to starch in plants.
 Sites of storage – liver and muscles.
 Sub cellular site – cytosol.

 Stores excess carbohydrates we consume, in the form of glycogen which
could be broken down to glucose when needed (glycogenolysis)
 Muscle glycogen provides a readily glucose for glycolysis within the
muscle itself
 Liver glycogen functions to store and export glucose to maintain blood
glucose between meals.
GLYCOGENESIS

GLYCOGENESIS PATHWAY

STEPS
• Glucose phosphorylation
• Glu 6 P to Glu 1 P conversion
• UDP Glucose – synthesis of the carrier molecule
• Glycogen primer
• Elongation of glycogen chain
• Branching in glycogen

Glucose phosphorylation
Glu 6 P to Glu 1 P conversion
Reaction catalyzed by hexokinase in muscles and glucokinase in liver
Glucose 6 phosphate is converted to glucose 1 phosphate in a
reaction catalyzed by the enzyme phosphoglucomutase
Glucose + ATP Glucose 6 phosphate +ADP + Pi
Glu 6 P + Enz-P Glu 1.6 bis P + Enz Glu1 P + Enz-P

UDP Glucose – Synthesis of the carrier molecule
ENZYME – UDP glucose pyrophosphorylase
acts as a vehicle
that carries the
glucose molecule
which is to be added
to the budding
glycogen molecule

Glycogen Primer
• A small fragment of pre-existing glycogen must act as a ‘primer ‘ to
initiate glycogen synthesis.
• In glycogen depleted condition, a protein primer called glycogenin
acts as the flooring to which the glucose molecules from UDP glucose are
added like blocks
• The hydroxyl (OH) of the amino acid tyrosine of glycogenin is the site
at which the initial glucose unit is attached
• The enzyme glycogen initiator synthase transfers the first molecule of
glucose to glycogenin.

• During initial additions of glucose glycogenin acts as auto
catalyst and forms the glycogen fragment on which further
glucose residues are added by 1-4 linkage by the enzyme
glycogen synthase
• Then glycogenin itself takes up few glucose residues to
form a fragment of primer which serves as an acceptor for the
rest of the glucose molecules

Elongation of Glycogen Chain
Link is formed between the 1st C atom of the standing glucose residue on
the end point of the fragment and 4th C of the glucose residue that is being
added to fragment → 1-4 glycosidic bond
Enzyme catalyzing this step is glycogen synthase

Branching in glycogen
• Glycogen is a branched
tree-like structure.
• Branching enzyme – amylo-
(1 →4)→(1→6) transglucosylase
• α-(1→6) linkages, which
occurs every 8-12 residues
• Transfers 6-7 residue
segment from the nonreducing
end
• Newly created branch
further glucose units are added
by α(1→4)linkage by glycogen
synthase

 The branching result in more number of end points for UDP glucose to
add further glucose residues to it. Thus branching enzyme results in
extensively branched large glycogen molecule.
 Defective branching enzyme Anderson disease
 2 ATP used in this process .
1 for the phosphorylation of glucose to glucose 6 phosphate
another for the conversion of UDP to UTP
Glycogen synthesis is strictly monitored to regulate the blood
glucose level. It is activated in well fed state and suppressed in
fasting.

 When two metabolic pathways runs in opposite direction at
the same time
 No overall net effect, no net production of ATP.
 In fact, ATP is used up in generating heat.
 Used in hibernation animals to produce heat, in brown
adipose tissues of young animals & to regulate metabolic
pathways

Under physiological conditions , the reaction catalyzed by PFK:
F6P + ATP → FBP +ADP + Pi
Is highly exergonic. Consequently the back reaction has a negligible rate
compared to the forward reaction. Fructose 1,6bisphosphatase (FBPase),
however which is present in many mammalian tissues (and which is essential
enzyme in gluconeogenesis) catalyzes the exergonic hydrolysis of FBP
FBP + H2O → F6P +Pi
Note that the combined reactions catalyzed by PFK and FBPase result in net ATP
hydrolysis:
ATP + H2O ADP +Pi
Such a set of opposing reactions is known as a substrate cycle because it cycles
a substrate to an intermediate and back again. When this set of reactions was
discovered, it was referred to as a futile cycle since its net result seemed to be
the useless consumption of ATP.

Substrate Cycling Is Related to Thermogenesis and Obesity
Many animals, including adult humans, are thought to generate
much of their body heat, particularly when it is cold, through
substrate cycling in muscle and liver, a process known as
nonshivering thermogenesis.
Substrate cycling is stimulated by thyroid hormones. Chronically
obese individuals tend to have lower than normal metabolic rates,
which is probably due, in part, to a reduced rate of nonshivering
thermogenesis.
Such individuals therefore tend to be cold sensitive.
Indeed, whereas normal individuals increase their rate of thyroid
hormone activation on exposure to cold, genetically obese animals
and obese humans fail to do so.

 Dark reactions
 C3 cycle
 Calvin – Benson – Bassham (CBB) cycle
 Reductive pentose phosphate cycle

CALVIN
Named after American biochemist Melvin Calvin.
This pathway was elucidated between 1946 and 1953 by
Melvin Calvin, James Bassham, and Andrew Benson.
They use radioactive carbon-14 to trace the path of carbon
atoms in carbon fixation in cultures of algal cells (Chlorella
pyrenoidosa)
Most commonly used pathway by most plants.
Calvin cycle is used by plants that are called C3 because of
the 3-Carbon molecules that are made.

CALVIN CYCLE
Light- independent reaction dark reaction
First stable compound – 3 phosphoglyceric acid C3 cycle
 the entire pathway , which involves the carboxylation of a pentose, the
formation of carbohydrate products , and the regeneration of the pentose
reductive pentose phosphate cycle
Occurs in stroma of chloroplast
Plants exhibit C3 cycle C3 Plants( Eg. Most crop plants, cereals,
tobacco, beans etc.)
The Calvin cycle uses products from the light reactions + CO2 to make
sugars and other compounds.

What are the products of the light reactions ????
Where does CO2 comes from ??
ATP & NADPH
Atmosphere through stomata

Three major steps :

Carbon fixation
Begins with 5 carbon sugar call ribulose bisphosphate (RuBP)
CO2 combines with RuBP to form 2 molecules of 3-phosphoglycerate (3PGA
or PGA).
First CO2 converts to a very unstable 6 carbon intermediate that is
immediately converted to 2 molecules of 3PGA or PGA.
Carboxylation catalyzed by an enzyme called ribulose 1,5- bisphosphate
carboxylase
3RuBP + 3CO2 6 glycerate 3 phosphate

Rubisco – Ribulose 1,5 bisphosphate carboxylase oxygenase
 Plants need large amounts of rubisco for Calvin cycle
• Half the protein in a leaf
• Most abundant enzyme in the biosphere
• It can be isolated in large amounts
 Almost all plants contain it
 Rubisco have a site for CO2 and O2
 If CO2 is bind to it,
RuBP + CO2 6C intermediate 2 PGA
 O2 act as inhibitor
 Rate limiting step of Calvin cycle

Reduction Phase ( 2 steps )
NADP+
PGA kinase ADP +Pi GAP dehydrogenase
PGA 1,3BPGA GAP + Pi
ATP NADPH
It is the only reductive phase Phosphate releasing phase
6PGA + 6 ATP +6 NADPH 6 GAP + 6 ADP + 6 Pi + 6 NADP+

Regeneration Phase

5GAP phosphorylated by ATP to recreate 3RuBP to restart the cycle.
For every 2 GAP, 1 molecule glucose is removed from cycle, so need 6
CO2 to create one glucose.
6 GAP + 3 ATP 3 RuBP + GAP +3 ADP + 3Pi
Product synthesis phase
The overall stoichiometry of the Calvin cycle is :
3 CO2 + 9 ATP + 6 NADPH → GAP + 9 ADP + 9 Pi + 6 NADP +

from two cycles 2 GAP is formed GAP + GAP fructose 6
phosphate
glucose 6 phosphate
glucose 1 phosphate
Starch/Sucrose
/Cellulose
Product synthesis phase
Precursor of
higher order
carbohydrates
Stroma of chloroplast
Long term storage
molecule
Leaves seeds and roots
G1P+ATP→ADP-Glucose
↓
α- amylose
↓
starch
cytosol
Sugar for non
photosynthesizing cells
UDPG+F6P→sucrose6P
↓
sucrose
Long chain of
β(1→4)linked glucose
From UDPG
Synthesized by
multisubunit enzyme
complexes in the plasma
membrane
The net reaction of C3 dark fixation of carbon dioxide is
6CO2+18ATP+12NADPH C6H12O6+18ADP+18Pi+12NADP+

Hatch-Slack-Kortschak Pathway
C4 Cycle

Certain species of plants, such as sugarcane, corn, and most important
weeds, have a metabolic cycle that concentrates CO2 in their
photosynthetic cells, thereby almost totally preventing
photorespiration.
C4 CYCLE
 The pathway was elucidated in
the 1960s by Marshall Hatch and
his colleague Roger Slack on
studying the unusual patterns in
localization of carbon-14 in
contrast to Calvin’s observations
on sugarcane leaves.

First stable compound – oxaloacetate C4 Cycle
Plants exhibit C4 cycle C4 Plants
vital adaptation
of tropical plants
C4 plants have a characteristic leaf anatomy Kranz anatomy
 occurs in bundle sheath and/or mesophyll cells
(Eg. Sugarcane, sorghum, millets,
some dicots such as Amaranthus and
Euphorbia hirta)

Kranz Anatomy
Kranz = crown, ring or halo
The vascular elments in C4 leaves are surrounded
by a layer of bundle sheath cells which in turn, are
surrounded by one or more layers of mesophyll
cells that largely fill the leaf.
Dimorphic chloroplasts
• Bundle sheath chloroplasts large,
agranal, centripetally arranged
• Mesophyll chloroplasts small
and granal

C4 Pathway

Advantages of C4 Cycle
Serves as an adaptation to overcome the losses and disadvantages of
photorespiration.
Feeds the Calvin cycle with CO2 and thus functions as a carbon dioxide
pump to turn the Calvin cycle round and round. This, in turn increase the
rate of photosynthesis.
It requires 30 ATP molecules for the synthesis of one molecule of
glucose, against 18 ATP in C3 pathway
C4 plants can absorb CO2 even in much lower concentration and it can
perform a high rate of photosynthesis even when the stomata are nearly
closed
They requires more light energy to fix CO2 to maintain high rate of
photosynthesis under water shortage
Better adapted to tropical and desert areas where sun light is more
intense

Difference Between C3 and C4 Plants
C3 Pathway C4 Pathway
The CO2 molecule acceptor is RuBP The CO2 molecule acceptor is phosphoenol
pyruvate
The first stable compound is a 3C compound
called PGA
First stable product is a 4C compound called
OAA
Photorespiration rate is high and leads to loss
of fixed CO2. it decreases CO2 fixation rate
Photorespiration is negligible and almost
absent. Hence it increases carbon fixation
rate.
Single CO2 fixation Two CO2 fixations
CO2 fixation is slow and efficient CO2 fixation is fast and more efficient
Fixation of one molecule of CO2 requires
3ATP and 2 NADPH
Fixation of one molecule of CO2 requires
5ATP and 3 NADPH
Only granal chloroplasts are involved Granal and agranal chloroplasts are involved
Cannot operate under very low CO2
concentrations
Can operate under very low CO2
concentration
Optimum temperature is 20-25 ⁰C Optimum temperature is 30 -45⁰C

C3 Plants C4 Plants
The leaves do not possess kranz anatomy The leaves have kranz anatomy
Chloroplasts do not have peripheral
reticulum
Chloroplasts have peripheral reticulum
Chloroplasts are of one type
(monomorphic)
There are two types of chloroplasts
(dimorphic)
In higher plantsoperating C3 cycle, all the
chloroplasts are granal
There are two types of chloroplasts, granal
in mesophyll cells and agranal in bundle
sheath cells.
Mesophyll cells perform complete
photosynthesis
Mesophyll cells perform only initial fixation
Perform photosythesis only when stomata
are open
Perform photosynthesis even when stomata
are closed ( from CO2 produced in
respiration).

CAM Pthway
• Crassulacean acid metabolism
• Water conserving pathway
• Hot dry climates
• Stomata closed during night & open at night
• opposite of ordinary plants
• Plants belonging to the family Crussulaceae.. Pine apples &
carrots….etc..

• During the daylight
Stomata are closed
CO2 is released from
compounds and enters
Calvin cycle
• During the night
Stomata are open
Take CO2 and fix into
carbon compounds

CAM Plants
Succulent plants with xerophytic characters
• thick fleshy leaves
• thick cuticle
• sunken stomata
Stomata scotoactive : open at night
Scotoactive mechanism of stomata helps CAM plants to conserve water
CAM pathway is an adaptation for a xerophytic mode of life and it adds to the
survival value of xerophytic plants

Glycogenesis

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