Transcript Hormonal regulation and pathologies of carbohydrate metabolism

Hormonal regulation and pathologies
of carbohydrate metabolism.
Diabetes mellitus.
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.
Regulation of Glycolysis
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.
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.
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
Effects of hormones on glycogen metabolism
Reciprocal Regulation of Glycogen
Phosphorylase and Glycogen Synthase
• Glycogen phosphorylase (GP) and glycogen
synthase (GS) control glycogen metabolism in
liver and muscle cells
• GP and GS are reciprocally regulated both
covalently and allosterically (when one is active
the other is inactive)
• Covalent regulation by phosphorylation (-P) and
dephosphorylation (-OH)
• Allosteric regulation by glucose-6-phosphate
(G6P)
Activation of GP and inactivation of GS by
Epinephrine and Glucagone
Activation of GS and inactivation of GP by Insulin