Download Fructose 6-Phosphate

Scientific investigations
into fermentation of
grape sugar were
pioneering studies
of glycolysis
GLYCOLYSIS
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 Ethanol
Ethanol is formed from pyruvate in yeast and several other
microorganisms in anaerobic conditions.
Two reactions required:
The first step is the decarboxylation of pyruvate to
acetaldehyde.
Enzyme - pyruvate decarboxylase.
Coenzyme - thiamine pyrophosphate (derivative of the vitamin
thiamine B1)
The second step is the reduction of acetaldehyde to ethanol.
Enzyme - alcohol dehydrogenase (active site contains a zinc).
Coenzyme – NADH.
The conversion of glucose into ethanol is an example of alcoholic
fermentation.
The net result of alcoholic fermentation is:
Glucose+2Pi + 2ADP + 2H+  2 ethanol + 2CO2 + 2ATP + 2H2O
The ethanol formed in alcoholic fermentation provides a key
ingredient for brewing and winemaking.
There is no net NADH formation in the conversion of glucose
into ethanol.
NADH generated by the oxidation of glyceraldehyde 3-phosphate
is consumed in the reduction of acetaldehyde to ethanol.
Metabolism of Pyruvate to Lactate
Lactate is formed from pyruvate in an animal organism
and in a variety of microorganisms in anaerobic
conditions.
The conversion of glucose into lactate is called lactic
acid fermentation.
Enzyme - lactate dehydrogenase.
Coenzyme – NADH.
• Muscles of higher organisms and humans lack pyruvate
decarboxylase and cannot produce ethanol from pyruvate
• Muscle contain lactate dehydrogenase. During intense
activity when the amount of oxygen is limiting the lactic acid
can be accumulated in muscles (lactic acidosis).
• Lactate formed in skeletal muscles during exercise is
transported to the liver.
• Liver lactate dehydrogenase can reconvert lactate to
pyruvate.
Overall reaction in the conversion of glucose into lactate:
Glucose + 2 Pi + 2 ADP  2 lactate + 2 ATP + 2 H2O
As in alcoholic fermentation, there is no net NADH
formation.
NADH formed in the oxidation of glyceraldehyde 3phosphate is consumed in the reduction 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.
Other Sugars Can Enter Glycolysis
• Glucose is the main metabolic
fuel in most organisms
• Other sugars convert to
glycolytic intermediates
• Fructose and sucrose (contains
fructose) are major sweeteners
in many foods and beverages
• Galactose from milk lactose (a
disaccharide)
• Mannose from dietary
polysaccharides, glycoproteins
The Entry of Fructose into Glycolysis
Much of the ingested fructose is metabolized by the liver, using
the fructose 1-phosphate pathway.
The first step is the phosphorylation of fructose to fructose 1phosphate by fructokinase.
Fructose 1-phosphate is then
split into glyceraldehyde and
dihydroxyacetone phosphate, an
intermediate in glycolysis, by a
specific fructose 1 -phosphate
aldolase.
Glyceraldehyde is then
phosphorylated to glyceraldehyde
3-phosphate, a glycolytic
intermediate, by triose kinase.
Fructose Is Converted to Glyceraldehyde 3-Phosphate
Fructose can be phosphorylated to fructose 6-phosphate by
hexokinase.
However, the affinity of hexokinase for glucose is 20 times
as great as it is for fructose.
Little fructose 6-phosphate is formed in the liver because
glucose is so much more abundant in this organ.
Glucose, as the preferred fuel, is also trapped in the muscle
by the hexokinase reaction.
Because liver and muscle phosphorylate glucose rather than
fructose, adipose tissue is exposed to more fructose than
glucose.
Hence, the formation of fructose 6-phosphate in the adipose
tissue is not competitively inhibited to a biologically significant
extent, and most of the fructose in adipose tissue is
metabolized through fructose 6-phosphate.
The Entry of Galactose into Glycolysis
Galactose is converted into glucose 6-phosphate in four
steps.
The first reaction is the phosphorylation of galactose to
galactose 1-phosphate by galactokinase.
Galactose 1-phosphate react with uridine
diphosphate glucose (UDP-glucose).
UDP-galactose and glucose 1-phosphate are
formed.
Enzyme - galactose 1-phosphate uridyl
transferase.
The galactose moiety of UDP-galactose is then
epimerized to glucose.
The configuration of the hydroxyl group at carbon 4
is inverted by UDP-galactose 4-epimerase.
Glucose 1-phosphate, formed from galactose, is
isomerized to glucose 6-phosphate by
phosphoglucomutase.
The Entry of Mannose into Glycolysis
Mannose is converted to Fructose 6-Phosphate in two
steps.
Hexokinase catalyzes the convertion of mannose into
mannose 6-phosphate.
Isomerase converts mannose 6-phosphate into fructose
6-phosphate (metabolite of glycolysis).
Intolerance to Milk
Many people are unable to metabolize the
milk sugar lactose and experience gastrointestinal disturbances if they drink milk.
Lactose intolerance, or hypolactasia, is caused by a deficiency of the
enzyme lactase, which cleaves lactose into glucose and galactose.
Microorganisms in the colon ferment undigested lactose to lactic acid
generating methane (CH4) and hydrogen gas (H2). The gas produced
creates the uncomfortable feeling of gut distention and the annoying
problem of flatulence.
The lactic acid is osmotically active and draws water into the intestine,
as does any undigested lactose, resulting in diarrhea.
The gas and diarrhea hinder the absorption of other nutrients (fats
and proteins).
Treatment:
- to avoid the products containing lactose;
- the enzyme lactase can be ingested.
Galactosemia
The disruption of galactose metabolism is referred to as
galactosemia.
Classic galactosemia is an inherited deficiency in galactose
1-phosphate uridyl transferase activity.
Symptoms:
- vomiting, diarrhea after consuming milk,
- enlargement of the liver, jaundice,
sometimes cirrhosis,
- cataracts,
- lethargy and retarded mental
development,
- markedly elevated blood-galactose level
- galactose is found in the urine.
The absence of the transferase in red blood cells is a
definitive diagnostic criterion.
The most common treatment is to remove galactose (and
lactose) from the diet.
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.
Phosphofructokinase 1 Is the Key Enzyme in
the Control of Glycolysis
Phosphofructokinase 1 is the most important control element
in the mammalian glycolytic pathway.
Phosphofructokinase 1
in the liver is a tetramer
of four identical
subunits.
The positions of the
catalytic and allosteric
sites are identical.
High levels of ATP allosterically inhibit the
phosphofructokinase 1 in the liver lowering its affinity for
fructose 6-phosphate.
AMP reverses the inhibitory action of ATP, and so the
activity of the enzyme increases when the ATP/AMP ratio
is lowered (glycolysis is stimulated as the energy charge
falls).
A fall in pH also inhibits phosphofructokinase 1 activity.
The inhibition of phosphofructokinase by H+ prevents
excessive formation of lactic acid and a precipitous drop in
blood pH (acidosis).
Phosphofructokinase 1 is inhibited by citrate, an early
intermediate in the citric acid cycle.
A high level of citrate means that biosynthetic precursors are
abundant and additional glucose should not be degraded for this
purpose.
Fructose 2,6-bisphosphate (F-2,6-BP) is a
potent activator of phosphofructokinase 1.
F-2,6-BP activates phosphofructokinase I by
increasing its affinity for fructose 6-phosphate
and diminishing the inhibitory effect of ATP.
Fructose 2,6-bisphosphate is formed in a reaction catalyzed by
phosphofructokinase 2 (PFK2), a different enzyme from
phosphofructokinase 1.
Fructose 2,6-bisphosphate
is hydrolyzed to fructose 6phosphate by a specific
phosphatase, fructose
bisphosphatase 2 (FBPase2).
Both PFK2 and FBPase2 are
present in a single
polypeptide chain
(bifunctional enzyme).
Regulation of Glycolysis by Fructose 2,6-bisphosphate
 When blood glucose
level is low the glucagon
is synthesized by
pancreas
 Glucagon binds to cell
receptors, stimulates
the protein kinase A
activity
 Protein kinase A
phosphorylates the
PFK-2 inhibiting its
kinase activity and
stimulating its
phosphatase activity
As result the amount
of F-2,6-BP is decreased and glycolysis is
slowed.
Regulation of Hexokinase
Hexokinase is inhibited by its
product, glucose 6-phosphate
(G-6-P).
High concentrations of G-6-P signal that the cell no longer
requires glucose for energy, for glycogen, or as a source of
biosynthetic precursors.
Glucose 6-phosphate levels increase when glycolysis is inhibited at
sites further along in the pathway.
Glucose 6-phosphate inhibits hexokinase isozymes I, II and III.
Glucokinase (isozyme IV) is not inhibited by glucose 6-phosphate.
The role of glucokinase is to provide glucose 6-phosphate for the
synthesis of glycogen.
Regulation of Pyruvate Kinase (PK)
Several isozymic
forms of pyruvate
kinase are present in
mammals (the L type
predominates in liver,
and the M type in
muscle and brain).
Fructose 1,6-bisphosphate allosterically activates pyruvate kinase.
ATP allosterically inhibits pyruvate kinase to slow glycolysis
when the energy charge is high.
Finally, alanine (synthesized in one step from pyruvate) also
allosterically inhibits the pyruvate kinases (signal that
building blocks are abundant).
The isozymic forms of pyruvate kinase differ in their
susceptibility to covalent modification.
The catalytic properties of the L (liver) form—but not of the
M (brain) form controlled by reversible phosphorylation.
When the
blood-glucose
level is low, the
glucagon leads
to the
phosphorylation of
pyruvate
kinase, which
diminishes its
activity.
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.
GLUT2, present in liver and pancreatic -cells has a very
high Km value for glucose (15-20 mM).
Glucose enters these tissues at a biologically significant
rate only when there is much glucose in the blood.
GLUT4, which has a Km value of 5 mM, transports glucose
into muscle and fat cells.
The presence of insulin leads to a rapid increase in the
number of GLUT4 transporters in the plasma membrane.
Insulin promotes the uptake of glucose by muscle and fat.
The amount of this transporter present in muscle
membranes increases in response to endurance exercise
training.
GLUT5, present in the small intestine, functions primarily
as a fructose transporter.
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.