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Coordinated regulation of
glycolysis/gluconeogenesis
Regulation of glycolysis
Hexokinase
Phosphofructokinase-1
Pyruvate kinase
Hexokinase
• There are four isozymes (I, II, III and IV) of
hexokinase encoded by four different genes.
• Hexokinase I and II are allosterically
inhibited by their product, glucose 6phosphate. Hexokinase IV is not inhibited
by G-6-P.
Hexokinase
• Hexokinase I and II are
the predominant forms
existing in muscle.
Hexokinase IV is the
predominant form in liver.
• Hexokinase I and II will
be half-saturated at about
0.1mM, but hexokinase IV
will not be half-saturated
until 10mM.
Hexokinase
• Hexokinase has
different functions in
liver and muscle.
• Muscle consumes
glucose, using it for
energy production.
• Liver maintains blood
glucose homeostasis
by removing or
producing glucose.
Muscle hexokinase
• Because blood glucose
concentration is about
4 to 5 mM, hexokinase
in the muscle (which
will be half saturated
at 0.1mM) is always
working at or near its
maximal rate.
Liver hexokinase
• However, liver
hexokinase (halfsaturated at 10mM)
will not ever reach its
maximal rate even
after meal.
Liver hexokinase
• Liver hexokinase (glucokinase) is also regulated
by a regulatory protein. When glucokinase is
bound by it (which is enhanced by F-6-P), it
become nuclear-localized and inactive.
Liver hexokinase
• After meal, glucose enter hepatocytes by GLUT2
transporter and is converted to G-6-P. G-6-P
competes with F-6-P for glucokinase, which
relieves its inhibition by regulatory protein.
Phosphofructokinase-1
• PFK-1 catalyze the
committing step of
glycolysis.
• This enzyme is
regulated by ATP,
AMP, ADP, citrate and
fructose 2,6bisphosphate.
ATP regulate the affinity of PFK-1
towards its substrate F-6-P
• Not only as a substrate,
ATP is also one of the
end product of the
glycolytic pathway.
• ATP inhibit PFK-1 by
binding to an allosteric
site and lowering the
affinity of the enzyme
for F-6-P.
Other molecules regulate PFK-1
• ADP and AMP relieve the inhibition by ATP.
• Citrate increases the inhibitory effect of ATP.
• F-2,6-BP is the strongest activator of PFK-1.
Pyruvate kinase
• Pyruvate kinase has at least three isozymes
and one of them is liver-specific.
• The liver pyruvate kinase is being regulated
differently than other tissue type.
Regulation of pyruvate kinase
cAMP dependent
Regulation of gluconeogenesis
Pyruvate carboxylase
FBPase-1
Pyruvate carboxylase
• Pyruvate carboxylase is being
positively regulated by acetylCoA.
• The accumulation of acetylCoA signals that cell’s energy
demands are met.
• Acetyl-CoA also indirectly
inhibit pyruvate
dehydrogenase complex.
How acetyl-CoA regulate PDC
E2
• Acetyl-CoA indirectly inhibit PDC by stimulating
a protein kinase that inactivates the dehydrogenase.
FBPase-1 is inhibited by AMP
F-2,6-BP: a potent regulator
F-2,6-BP reciprocally regulate
PFK-1 and FBPase-1
F-2,6-BP activates PFK-1
F-2,6-BP inhibit FBPase-1
The synthesis and breakdown of
F-2,6-BP
• F-2,6-BP is synthesized by PFK-2 and brokedown
by FBPase-2, which is a single, bifunctional
protein. When it is phosphorylated, it is FBPase-2.
The bifunctional protein
PFK-2/FBPase-2
Insulin and glucagon levels affect the
balance between PFK-2/FBPase-2
A PP2A activated by xylulose 5phosphate also activate FBPase-2
PP2A
Regulation of glycogen
metabolism
Glycogen phosphorylase
Glycogen synthase
Muscle glycogen
phosphorylase
• Muscle
glycogen
phosphorylase
has two forms:
the active a
form and the
less active b
form. The
active form is
phosphorylated.
Muscle glycogen
phosphorylase
• Glucagon and
epinephrine
stimulate the
kinase that
phosphorylate
phosphorylase b,
therefore active
the whole
glycogen
breakdown
process.
How glucagon/epinephrine
activate phosphorylase b
kinase
• When
epinephrine/glucagon
is secreted, it started
the whole enzyme
cascade by activate a
GTP-binding protein.
• Enzyme cascade
allows for large
amplification of the
initial signal.
Muscle glycogen
phosphorylase
• At resting stage,
PP1
(phosphorylase a
phosphatase) will
dephosphorylate
phosphorylase a,
which will make it
returning to the
less active form
(phosphorylase b).
Liver glycogen phosphorylase
• The dephosphorylated form (b) of liver
glycogen phosphorylase is essentially
inactive. Phosphorylation activates it, but
when blood glucose is high, glucose will
bind to the inhibitory allosteric site, induces
a conformational change that will expose its
phosphorylated Ser for PP1 to
dephosphorylate (inactivate) this enzyme.
Glycogen synthase
• The activate form of
glycogen synthase is
not phosphorylated.
• To inactivate glycogen
synthase, it must be
phosphorylated by
casein kinase II (CKII)
first, then glycogen
synthase kinase 3
(GSK3) will add
phosphoryl groups to
three Ser residues near
the carboxyl terminus
of this protein.
GSK3 inactivate glycogen
synthase by phosphorylation
Glycogen synthase
• The activation of
glycogen synthase
requires PP1.
• Glucose 6-phosphate
will bind to the
allosteric site of
glycogen synthase b,
making the enzyme
a better substrate for
PP1.
GSK3 can be inactivated by
phosphorylation
• Insulin triggers activation
of a protein kinase B to
phosphorylate GSK3 at a
Ser residue near the amino
terminus, converting that
region of the protein to a
pseudosubstrate,
preventing GSK3 from
binding the real substrate
(glycogen synthase).
Insulin enhance
glycogen
synthesis by
inhibiting the
kinase that
inactivate
glycogen synthase
Phosphoprotein phosphatase 1
(PP1)
• PP1 can remove phosphoryl group from
phosphorylase kinase, glycogen
phosphorylase (inactivation), and glycogen
synthase (activation) in response to
glucagon/epinephrine.
• By activating PP1 and inactivating GSK3,
insulin stimulates glycogen synthesis.
PP1 binds to glycogen-targeting
protein (GM) and also other proteins
Muscle has a different glucose
transporter
• GLUT2 is not
present in
myocyte. Instead,
GLUT4 is
present in
myocyte and its
expression is
regulated by
insulin.
Insulin regulated the
externalization/internalization of GLUT4