Anaerobic and aerobic oxidation of
glucose. Studing of biosynthesis and
catabolism of glycogen.
Gluconeogenesis.
DIGESTION OF CARBOHYDRATES
Glycogen, starch and disaccharides (sucrose,
lactose and maltose) are hydrolyzed to
monosaccharide units in the gastrointestinal tract.
The process of digestion starts in the mouth by the
salivary enzyme –amilase.
The time for digestion in mouth is limited.
Salivary -amilase is inhibited in stomach due to the
action of hydrochloric acid.
Another -amilase is produced in pancreas and is
available in the intestine.
-amilase hydrolyzes the -1-4-glycosidic
bonds randomly to produce smaller subunits like
maltose, dextrines and unbranched
oligosaccharides.
-amilase
The intestinal juice contains enzymes hydrolyzing
disaccharides into monosaccharides (they are produced
in the intestinal wall)
Sucrase hydrolyses sucrose into glucose and fructose
Glucose
sucrase
Fructose
Sucrose
Galactose
lactase
Glucose
Lactase hydrolyses
lactose into glucose
and galactose
Lactose
Glucose
maltase
Maltase hydrolyses
maltose into two
glucose molecules
Maltose
Glucose
ABSORPTION OF CARBOHYDRATES
Only monosaccharides are absorbed
The rate of absorption: galactose > glucose > fructose
Glucose and galactose from the intestine into endothelial
cells are absorbed by secondary active transport
Na+
Protein
Glucose
Protein
The fate of glucose molecule in the cell
Glucose
Glycogenogenesis
(synthesis of
glycogen) is
activated in well
fed, resting state
Glucose-6phosphate
Pentose phosphate
pathway supplies
the NADPH for lipid
synthesis and
pentoses for nucleic
acid synthesis
Ribose,
NADPH
Glycogen
Pyruvate
Glycolysis
is activated if
energy is required
Glycolysis is the earliest discovered and most important
process of carbohydrates metabolism.
Glycolysis – metabolic pathway in which glucose is
transformed to pyruvate with production of a small
amount of energy in the form of ATP or NADH.
Glycolysis is an anaerobic process (it does not require
oxygen).
Glycolysis pathway is used by anaerobic as well as aerobic
organisms.
In glycolysis one molecule of glucose is converted into two
molecules of pyruvate.
In eukaryotic cells, glycolysis takes place in the cytosol.
Pyruvate can be further metabolized to:
(1) Lactate or ethanol (anaerobic conditions)
(2) Acetyl CoA (aerobic conditions)
• Acetyl CoA is further oxidized to CO2 and H2O via
the citric acid cycle
• Much more ATP is generated from the citric acid
cycle than from glycolysis
Acetyl CoA
• Catabolism of glucose in
aerobic conditions via
glycolysis and the citric
acid cycle
Stage 1, which is the conversion of glucose into fructose
1,6-bisphosphate, consists of three steps: a phosphorylation,
an isomerization, and a second phosphorylation reaction.
The strategy
of these
initial steps
in glycolysis
is to trap
the glucose
in the cell
and form a
compound
that can be
readily
cleaved into
phosphorylated
threecarbon units.
Stage 2 is the cleavage of the fructose
1,6-bisphosphate into two three-carbon fragments
dihydroxyacetone phosphate and glyceraldehyde 3phosphate.
Dihydroxyacetone phosphate and glyceraldehyde 3phosphate are readily interconvertible.
In stage 3,
ATP is
harvested
when the
threecarbon
fragments
are
oxidized to
pyruvate.
Glycolysis Has 10 Enzyme-Catalyzed Steps
• Each chemical reaction prepares a substrate for the next
step in the process
1. Hexokinase
• Transfers the g-phosphoryl of ATP to glucose C-6 oxygen to
generate glucose 6-phosphate (G6P)
• Four kinases in glycolysis: steps 1,3,7, and 10
• All four kinases require Mg2+ and have a similar mechanism
2. Glucose 6-Phosphate Isomerase
• Converts glucose 6-phosphate (G6P) (an aldose) to
fructose 6-phosphate (F6P) (a ketose)
• Enzyme preferentially binds the a-anomer of G6P
(converts to open chain form in the active site)
• Enzyme is highly stereospecific for G6P and F6P
• Isomerase reaction is near-equilibrium in cells
3. Phosphofructokinase-1 (PFK-1)
• Catalyzes transfer of a phosphoryl group from ATP to the
C-1 hydroxyl group of F6P to form fructose 1,6bisphosphate (F1,6BP)
• PFK-1 is metabolically irreversible and a critical regulatory
point for glycolysis in most cells
• A second phosphofructokinase (PFK-2) synthesizes
fructose 2,6-bisphosphate (F2,6BP)
4. Aldolase
• Aldolase cleaves the hexose F1,6BP into two triose
phosphates: glyceraldehyde 3-phosphate (GAP) and
dihydroxyacetone phosphate (DHAP)
• Reaction is near-equilibrium, not a control point
5. Triose Phosphate Isomerase (TPI)
• Conversion of DHAP into GAP
• Reaction is very fast, only the D-isomer of GAP is formed
• Reaction is reversible. At equilibrium, 96% of the triose
phosphate is DHAP. However, the reaction proceeds readily
from DHAP to GAP because the subsequent reactions of
glycolysis remove this product.
6. Glyceraldehyde 3-Phosphate
Dehydrogenase (GAPDH)
• Conversion of GAP to 1,3-bisphosphoglycerate (1,3BPG)
• Molecule of NAD+ is reduced to NADH
• Energy from oxidation of GAP is conserved in acidanhydride linkage of 1,3BPG
• Next step of glycolysis uses the high-energy phosphate
of 1,3BPG to form ATP from ADP
7. Phosphoglycerate Kinase (PGK)
• Transfer of phosphoryl group from the energy-rich mixed
anhydride 1,3BPG to ADP yields ATP and
3-phosphoglycerate (3PG)
• Substrate-level phosphorylation - Steps 6 and 7 couple
oxidation of an aldehyde to a carboxylic acid with the
phosphorylation of ADP to ATP
8. Phosphoglycerate Mutase
• Catalyzes transfer of a phosphoryl group from one part
of a substrate molecule to another
• Reaction occurs without input of ATP energy
9. Enolase: 2PG to PEP
• 2-Phosphoglycerate (2PG) is dehydrated to
phosphoenolpyruvate (PEP)
• Elimination of water from C-2 and C-3 yields the enolphosphate PEP
• PEP has a very high phosphoryl group transfer potential
because it exists in its unstable enol form
10. Pyruvate Kinase (PK)
PEP + ADP  Pyruvate + ATP
• Catalyzes a substrate-level
phosphorylation
• Metabolically irreversible
reaction
• Regulation both by
allosteric modulators and
by covalent modification
• Pyruvate kinase gene can be
regulated by various
hormones and nutrients
The Fate of Pyruvate
The sequence of reactions from glucose to pyruvate is
similar in most organisms and most types of cells.
The fate of pyruvate is variable.
Three reactions of pyruvate are of prime importance:
1. Aerobic conditions:
oxidation to acetyl CoA
which enters the citric acid
cycle for further oxidation
2. Anaerobic conditions
(muscles, red blood cells):
conversion to lactate
3. Anaerobic conditions
(microorganisms, yeast):
conversion to ethanol
Diverse
Fates of
Pyruvate
Metabolism of Pyruvate to Acetyl CoA
In aerobic conditions pyruvate is converted to acetyl coenzyme A (acetyl CoA).
Acetyl CoA enters citric acid cycle where degrades to CO2 and H2O and the
energy released during such oxidation is utilized in NADH and FADH2.
Pyruvate is converted to acetyl CoA in the matrix of mitochondria.
The overall reaction: Pyruvate + NAD+ + CoA  acetyl CoA + CO2 + NADH
Reaction is catalyzed by the pyruvate dehydrogenase complex (three
enzymes and five coenzymes).
If pyruvate is converted to acetyl CoA, NADH formed in the oxidation of
glyceraldehyde 3-phosphate ultimately transfers its electrons to O2 through
the electron-transport chain in mitochondria.
Regulation of Glycolysis
The rate glycolysis is regulated to meet two major cellular needs:
(1) the production of ATP, and
(2) the provision of building blocks for synthetic reactions.
There are three control sites in glycolysis - the reactions catalyzed by
hexokinase,
phosphofructokinase 1, and
pyruvate kinase
These reactions are irreversible.
Their activities are regulated
by the reversible binding of allosteric effectors
by covalent modification
by the regulation of transcription (change of the enzymes amounts).
The time required for allosteric control, regulation by phosphorylation,
and transcriptional control is typically in milliseconds, seconds, and
hours, respectively.
Inhibition
1) PFK-1 is
inhibited by ATP
and citrate
2) Pyruvate
kinase is
inhibited by ATP
and alanine
3) Hexokinase is
inhibited by
excess glucose
6-phosphate
Regulation of
Glycolysis
Stimulation
1) AMP and fructose 2,6bisphosphate (F2,6BP) relieve
the inhibition of PFK-1 by ATP
2) F1,6BP stimulate the activity
of pyruvate kinase
Alanine
Regulation of Hexose Transporters
Several glucose transporters (GluT) mediate the thermodynamically downhill
movement of glucose across the plasma membranes of animal cells.
GluT is a family of 5 hexose transporters.
Each member of this protein family consists of a single polypeptide chain
forming 12 transmembrane segments.
GLUT1 and GLUT3,
present in erythrocytes,
endothelial, neuronal
and some others
mammalian cells, are
responsible for basal
glucose uptake. Their Km
value for glucose is about
1 mM.
GLUT1 and GLUT3
continually transport
glucose into cells at an
essentially constant rate.
The Pasteur Effect
Under anaerobic conditions
the conversion of glucose to
pyruvate is much higher than
under aerobic conditions
(yeast cells produce more
ethanol and muscle cells
accumulate lactate)
The Pasteur Effect is the
slowing of glycolysis in the
presence of oxygen.
• More ATP is produced under aerobic conditions than
under anaerobic conditions, therefore less glucose is
consumed aerobically.
The fate of glucose molecule in the cell
Synthesis of
glycogen
Glucose
Pentose phosphate
pathway
Glucose-6phosphate
Glycogen
Ribose,
NADPH
Degradation of
glycogen
Gluconeogenesis
Glycolysis
Pyruvate
The Role of Pentose Phosphate
Pathway (phosphogluconate pathway)
(1) Synthesis of NADPH (for reductive reactions in
biosynthesis of fatty acids and steroids)
(2) Synthesis of Ribose 5-phosphate (for the
biosynthesis of ribonucleotides (RNA, DNA) and several
cofactors)
(3) Pentose phosphate pathway also provides a means for
the metabolism of “unusual sugars”, 4, 5 and 7 carbons.
Pentose phosphate pathway does not function in the
production of high energy compounds like ATP.
GLYCOGEN SYNTHESIS AND DEGRADATION
In the well-fed state the glucose after absorption is taken
by liver and deposited as a glycogen
Glycogen is a very large, branched polymer of glucose
residues that can be broken down to yields glucose
molecules when energy is needed
Most glucose residues in glycogen are linked by a-1,4-glycosidic bonds, branches are created by a-1,6-glycosidic bonds
Glycogen serves as a buffer to maintain blood-glucose level.
Stable blood glucose level is especially important for brain
where it is the only fuel.
The glucose from glycogen is readily mobilized and is
therefore a good source of energy for sudden, strenuous
activity.
Liver (10 % of weight)
and skeletal muscles
(2 %) – two major
sites of glycogen
storage
Glycogen is stored in
cytosolic granules in
muscle and liver cells
of vertebrates
Glucose-6-phosphate is
the central metabolite in
the synthesis and
decomposition of
glycogen.
In the well-fed state
glucose is converted to
glucose-6-phosphate,
which is the precursor
for the glycogen
synthesis.
The glucose-6-phosphate
derived from the
breakdown of glycogen
has three fates: (1)
glycolysis; (2) pentosephosphate pathway; (3)
convertion to free
glucose for transport to
another organs.
DEGRADATION OF GLYCOGEN
Glycogenolysis - degradation of glycogen
The reaction to release glucose from polysaccharide is not
simple hydrolysis as with dietary polysaccharides but
cleavage by inorganic phosphate – phosphorolytic
cleavage
Phosphorolytic cleavage or phosphorolysis is catalyzed
by enzyme glycogen phosphorylase
There are two ends on the molecules of starch or glycogen:
a nonreducing end (the end glucose has free hydroxyl
group on C4) and a reducing end (the end glucose has an
anomeric carbon center (free hydroxyl group on C1)
Glycogen phosphorylase removes glucose residues
from the nonreducing ends of glycogen
Acts only on a-1-4 linkages of glycogen polymer
Product is a-D-glucose 1-phosphate (G1P)
Cleavage of a glucose
residue from the
nonreducing end of
glycogen
Structure of glycogen phosphorylase (GP)
• GP is a dimer of identical
subunits (97kD each)
• Catalytic sites are in clefts
between the two domains of
each subunit
• Binding sites for glycogen,
allosteric effectors and a
phosphorylation site
• Two forms of GP
Phosphorylase a (phosphorylated) active form
Phosphorylase b (dephosphorylated) less active
• GP catalyzes the
sequential removal of
glucose residues from the
nonreducing ends of
glycogen
• GP stops 4 residues from
an a 1-6 branch point
• Tranferase shifts a block
of three residues from
one outer branch to the
other
• A glycogen-debranching
enzyme or 1,6glucosidase hydrolyzes
the 1-6-glycosidic bond
• The products are a free
glucose-1-phosphate
molecule and an elongated
unbranched chain
Metabolism of Glucose 1-Phosphate (G1P)
• Phosphoglucomutase catalyzes the conversion
of G1P to glucose 6-phosphate (G6P)
Glycogen Synthesis
• Synthesis and degradation
of glycogen require
separate enzymatic steps
• Cellular glucose converted
to G6P by hexokinase
• Three separate enzymatic
steps are required to
incorporate one G6P into
glycogen
• Glycogen synthase is the
major regulatory step
Glucose 1-Phosphate formation
• Phosphoglucomutase catalyzes the conversion
of glucose 6-phosphate (G6P) to glucose 1phosphate (G1P).
UDP-glucose is activated
form of glucose.
UDP-glucose is
synthesized from glucose1-phosphate and uridine
triphosphate (UTP) in a
reaction catalized by
UDP-glucose
pyrophosphorylase
Glycogen synthase adds glucose to
the nonreducing end of glycogen
A branching enzyme forms -1,6-linkages
Glycogen synthase
catalyzes only -1,4linkages.
The branching enzyme
is required to form
-1,6-linkages.
Branching is important
because it increases
the solubility of
glycogen.
Branching creates a
large number of
terminal residues, the
sites of action of
glycogen phosphorylase
and synthase.
Regulation of Glycogen Metabolism
• Muscle glycogen is fuel for muscle contraction
• Liver glycogen is mostly converted to glucose
for bloodstream transport to other tissues
• Both mobilization and synthesis of glycogen
are regulated by hormones
• Insulin, glucagon and epinephrine regulate
mammalian glycogen metabolism
Hormones Regulate Glycogen Metabolism
Insulin
• Insulin is produced by b-cells of the pancreas
(high levels are associated with the fed state)
• Insulin increases rate of glucose transport
into muscle, adipose tissue via GluT4
transporter
• Insulin stimulates glycogen synthesis in the
liver via the second messenger
phosphatidylinositol
3,4,5-triphosphate (PIP3)
Glucagon
• Secreted by the a cells of the pancreas in
response to low blood glucose (elevated
glucagon is associated with the fasted state)
• Stimulates glycogen degradation to restore
blood glucose to steady-state levels
• Only liver cells are rich in glucagon receptors
and therefore respond to this hormone
Epinephrine (Adrenalin)
• Released from the adrenal glands in response
to sudden energy requirement (“fight or
flight”)
• Stimulates the breakdown of glycogen to G1P
(which is converted to G6P)
• Increased G6P levels increase both the rate
of glycolysis in muscle and glucose release to
the bloodstream from the liver and muscles
• Both liver and muscle cells have receptors to
epinephrine
Gluconeogenesis – synthesis of glucose from
noncarbohydrate precursors
• Liver and kidney are major sites of glucose synthesis
• Main precursors: lactate, pyruvate, glycerol and some
amino acids
• Under fasting conditions, gluconeogenesis supplies
almost all of the body’s glucose
• Gluconeogenesis – universal pathway. It present in
animals, microorganisms, plants and fungi
• Plants synthesize glucose from CO2 using the energy of
sun, microorganisms – from acetate and propionate
The Net Reaction of Gluconeogenesis
2 Pyruvate + 2 NADH + 4 ATP + 2 GTP + 6 H2O 
Glucose + 2 NAD+ + 4 ADP + 2 GDP + 6 Pi + 2H+
G°' = -9 kcal mol-1
Six nucleotide triphosphate molecules are hydrolyzed
to synthesize glucose from pyruvate in gluconeogenesis,
whereas only two molecules of ATP are generated in
glycolysis in the conversion of glucose into pyruvate.
The extra cost of gluconeogenesis is four high
phosphoryl-transfer potential molecules per molecule of
glucose synthesized from pyruvate.
Subcellular Locations of
Gluconeogenic Enzymes
• Gluconeogenesis enzymes are cytosolic
except:
(1) Glucose 6-phosphatase (endoplasmic
reticulum)
(2) Pyruvate carboxylase (mitochondria)
(3) Phosphoenolpyruvate carboxykinase
(cytosol and/or mitochondria)
Regulation of Gluconeogenesis
Gluconeogenesis and glycolysis are reciprocally regulated
- within a cell one pathway is relatively inactive while the
other is highly active.
The amounts and activities of the distinctive enzymes of
each pathway are controlled.
The rate of glycolysis is determined by the concentration
of glucose.
The rate of gluconeogenesis is determined by the
concentrations of precursors of glucose.
AMP stimulates phosphofructokinase, whereas ATP and
citrate inhibit it. Fructose 1,6bisphosphatase is inhibited by
AMP and activated by citrate.
Fructose 2,6-bisphosphate
strongly stimulates phosphofructokinase 1 and inhibits
fructose 1,6-bisphosphatase.
During starvation, gluconeogenesis predominates because
the level of F-2,6-BP is very low.
High levels of ATP and alanine,
which signal that the energy
charge is high and that building
blocks are abundant, inhibit the
pyruvate kinase.
ADP inhibits phosphoenol-pyruvate carboxykinase.
Pyruvate carboxylase is
activated by acetyl CoA and Gluconeogenesis is favored when the cell is rich
inhibited by ADP.
in biosynthetic precursors and ATP.
Regulation of the Enzymes Amount by Hormones
Hormones affect gene expression primarily by changing the
rate of transcription.
Insulin, which rises subsequent to eating, stimulates the
expression of phosphofructokinase and pyruvate kinase.
Glucagon, which rises during starvation, inhibits the
expression of these enzymes and stimulates the production
of phosphoenolpyruvate carboxykinase and fructose 1,6bisphosphatase.
Transcriptional control in eukaryotes is much slower than
allosteric control; it takes hours or days in contrast with
seconds to minutes.
Precursors for Gluconeogenesis
• Any metabolite that can be converted to
pyruvate or oxaloacetate can be a glucose
precursor
• Major gluconeogenic precursors in mammals:
(1) Lactate
(2) Most amino acids (especially alanine),
(3) Glycerol (from triacylglycerol hydrolysis)
The Cori Cycle
Liver lactate dehydrogenase converts lactate to pyruvate (a substrate
for gluconeogensis)
Glucose produced by liver is delivered to peripheral tissues via the
bloodstream
Contracting
skeletal muscle
supplies
lactate to the
liver, which
uses it to
synthesize
glucose.
These
reactions
constitute the
Cori cycle