Kinetic model of the human hepatocyte
glycolysis, gluconeogenesis and
glycogen metabolism
Matthias König
Charite Berlin
Group meeting
Berlin [11/01/10]
Outline
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Introduction
–
Glucose homeostasis & role of liver
–
Regulatory mechanisms
–
Diabetes
–
Model hypotheses
Kinetic Model
–
Overview model
–
Insulin and glucagon dose-response curves
–
Signal function 
–
Interconvertible enzymes
Simulations & Results
Glucose homeostasis
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Plasma glucose controlled in small range despite large changes
in glucose supply (meals) and glucose usage (muscle activity)
–
~ 5mM (max < 9mM: postprandial, min > 3mM: prolonged fasting / intensive muscle
activity )
–
Other metabolites like glycerol, free fatty acids or ketone bodies
much larger fluctuations
Glucose homeostasis
●
Plasma glucose controlled in small range despite large changes
in glucose supply (meals) and glucose usage (muscle activity)
–
~ 5mM (max < 9mM: postprandial, min > 3mM: prolonged fasting / intensive muscle
activity )
–
Other metabolites like glycerol, free fatty acids or ketone bodies
much larger fluctuations
I. Avoid hypoglycaemic conditions
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Glucose supply for tissues and cells (brain, erythrocytes)
I. Avoid hyperglycaemic conditions
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Toxicity of glucose (protein modifications, glucose induced
oxidative effects)
Glycogen has buffer role (storage of glucose)
Central role of liver in homeostasis
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glucose production under hypoglycaemic conditions (hepatic
glucose production HGP)
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glycogenolysis and gluconeogenesis
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liver central organ of gluconeogenesis (kidney only under
prolonged fasting of importance)
glucose uptake under hyperglycaemic conditions (hepatic
glucose utilization HGU)
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glycogen storage and glycolysis
–
bulk of glucose is postprandial removed in extra-hepatic tissue
(especially muscle)
Gluconeogenesis
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Synthesis of glucose from precursors
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Most important precursor lactate (glutamine, alanine and glycerol
also important)
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Lactate important in Cori cycle
Regulation mechanisms
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Two different modes depending on blood glucose (and
hormones)
The switch between HGU and HGP is regulated by different
mechanisms on different time scales
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Fast allosteric regulation
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Regulation by interconvertible enzymes in the range of minutes
(hormonal signals)
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Slow regulation by changes in gene expression of key enzymes
Regulation mainly by reciprocal regulation of enzymes
characteristic for HGP and HGU
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HGU (GK, PFK1, PK, GS)
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HGP (G6PASE, FBP1, PC, PEPCK)
Allosteric regulations
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Fast regulation of key enzymes
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Especially important are fru26bp and glucose
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fru26bp potent allosteric activator of PFK1 and competitive inhibitor
of FBP1 (control of flux trough glycolysis and gluconeogenesis)
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glucose has positive feedback mechanism on GK by decreasing
binding of GKRP to GK; glucose potent inhibitor of GP [p]
Regulation by insulin and glucagon
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Insulin and glucagon are the
main hormones which control the
glucose homeostasis in the liver
Insulin and glucagon depend on
plasma glucose concentrations
Insulin and glucagon are counterregulatory hormones
Insulin and glucagon
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Glucagon inhibits HGU (glycogen synthesis and glycolysis) and
activates HGP (gluconeogenesis and glycogenolysis)
Insulin reverse effects (activates HGU and inactivates HGP)
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During fasting the glucagon level is increased and insulin level
decreased
–
Postprandial the insulin level is increased and the glucagon level
decreased
Interconvertible enzymes I
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Insulin and glucagon activate signal
cascades which change the
phosphorylation state of
interconvertible enzymes of HGP and
HGU
–
short term changes in [cAMP]
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PKA phosphorylates most of the
interconvertible enzymes of the
HGP and HGU
–
changed phosphorylation state of
key enzymes
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phosphorylated [p] and
dephosphorylated [dp] form of
the enzymes have different
kinetics
–
changed fluxes
Interconvertible enzymes II
Diabetes
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Diabetes most frequent disease of
glucose homeostasis
In diabetes type II the insulin and
glucagon response is defective
–
blood glucose level increased
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insulin level decreased
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glucagon level increased
Diabetes
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Diabetes most frequent disease of
glucose homeostasis
In diabetes type II the insulin and
glucagon response is defective
–
blood glucose level increased
–
insulin level decreased
–
glucagon level increased
Postprandial hyperglycaemia is due to
reduced glucose clearance by muscle
or adipose tissue and increased HGP.
–
Increased gluconeogenesis
instead of increased glycogenolysis
is the dominant process
Hypotheses
I. Hepatocyte switches between HGP and HGU depending on
blood glucose (insulin and glucagon).
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only the fast regulations via interconvertible enzymes and allosteric
effectors necessary
no changes in gene expression necessary
II. The glycogen store is emptied during HGP and filled during
HGU.
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By the use of the glycogen store the task of glucose homeostasis
can be better fulfilled
III. Defective insulin and glucagon signals lead to an disturbed
switch between HGU and HGP. The hormonal signals in
diabetes should lead to diabetic effects (increased HGP).
Outline
●
●
●
Introduction
–
Glucose homeostasis & role of liver
–
Regulation mechanisms
–
Diabetes
–
Model hypotheses
Kinetic Model
–
Overview model
–
Hormone dose-response curves
–
Signal function 
–
Interconvertible enzymes
Simulations & Results
Overview model
●
●
Glycolysis, gluconeogenesis and glycogen metabolism
–
allosteric regulation
–
hormonal regulation (insulin and glucagon)
Detailed kinetic model
–
36 fluxes, 49 compounds
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3 compartments (blood, cytosol, mitochondrium)
–
detailed kinetics for all reactions
[model view]
Dose-response curves
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Sigmoid insulin and glucagon dose-response curves
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Altered response curves in diabetes type II
Signal function 
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Signal function defines the phosporylation state of the
interconvertible enzymes depending on insulin and glucagon
( [ins], [glu] )
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Simplifies the signal cascade between the hormones and the
change in the phosphorylation state
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completely dephosphorylated
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completely phosphorylated
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 monoton decreasing in insulin | monoton increasing in glucagon
Signal function 
●
phosporylation state depending on insulin and glucagon
Signal function
Interconvertible enzymes
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Different kinetics for phosphorylated [p] and dephophorylated [dp] form.
Enzyme kinetics is combination of [p] and [dp] form depending on the
phosphorylation state 
GS
Outline
●
Introduction
●
Kinetic Model
●
–
Overview model
–
Insulin and glucagon dose-response curves
–
Signal function 
–
Interconvertible enzymes
Simulations & Results
I .Switch between HGU and HGP based on interconvertible enzymes and
allosteric regulation
II. Role of the glycogen storage for glucose homeostasis
III. Diabetes
HGP concentrations [3mM] glucose
HGP fluxes [3mM] glucose
Summary HGP
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Biphasic
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First phase of glycogen depletion
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Second phase with empty glycogen stores
Nearly all fluxes and concentrations change between the two
phases
Glycogen can be used for HGP
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During glycogen depletion an increased HGP can be found
–
The system is able to perform HGP with empty glycogen stores, the
glucose is fully produced by gluconeogenesis
Gluconeogenesis is increased if the glycogen store is depleted
HGU concentrations [8mM] glucose
HGU fluxes [8mM] glucose
Summary HGU
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Biphasic
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First phase of glycogen storage
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Second phase with full glycogen stores
Nearly all fluxes and concentrations change between the two
phases
Glycogen storage can improve the HGU
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During glycogen storage an increased HGU is found
–
The system is able to perform HGU with full glycogen stores, the
glucose is completely used in the glycolysis
Glycolysis is increased if the glycogen store is filled
Switch between HGU and HGP
Important fluxes and concentrations
Summary switch depending on glucose
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The system switches between HGP and HGU depending on the
blood glucose concentration
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Switch at ~5-6.5 mM depending on the glycogen content
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Glycogen stored above ~5mM blood glucose, the glycogen store is
depleted below 5 mM
Glucose-lactate dependency
Summary glucose - lactate dependency
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Lactate nearly no effect on the glycogen storage
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Switch point changed a little
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velocity of glycogen depletion depends on lactate concentration
(HGP)
Lactate has only effect on HGP under hypoglycaemic condition
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low concentrations of lactate limiting factor for the gluconeogenesis
(no HGP possible without lactate)
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increase in lactate (>1mM) does not increase the HGU
Diabetes type II
Diabetes type II
Summary diabetes type II
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Glycogen storage is nearly unaffected by changed insulin and
glucagon dose-response curves
Diabetic effects can be found with the diabetes type II response
curves
–
switch between HGP and HGU is changed to increased glucose
concentrations (~9-10 mM)
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rncreased HGP (mainly due to increased gluconeogenesis)
–
reduced HGU
Insulin and glucagon restoration reduce the diabetic effects
–
glucagon is more effective than insulin
Summary
The hepatocyte switches between HGP and HGU depending on blood
glucose, insulin and glucagon.
–
only the fast regulations via interconvertible enzymes and
allosteric effectors are necessary
–
no changes in gene expression are necessary
The glycogen store is emptied during HGP and filled during HGU.
–
By the use of the glycogen store the task of glucose homeostasis
can be better fulfilled
If the insulin and glucagon signals are defective also the switch between HGU
and HGP is disturbed. In the case of diabetes type II results the insulin and
glucagon defect in an increased HGP.
–
glucagon more effective than insulin
Summary II
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First detailed kinetic model of hepatic glycolysis,
gluconeogenesis and glycogen metabolism with the
incorporation of the interconvertible enzymes and hormones
Many experimental effects can be shown
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Nearly no glycolysis during glycogen storage in HGU
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Effect of gluconeogenic precursors (lactate) – rate limiting in small
concentrations, no further increase in gluconeogenesis in high
concentrations
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Diabetic results
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Higher HGP and higher plasma glucose concentration
Increased gluconeogenesis not increased glycogenolysis main
contributor to HGP
Optimization of states by changes in gene expression (not shown)
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Hermann-Georg Holzhütter
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Sascha Bulik
Gene expression
Regulation by insulin and glucagon
●
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Insulin and glucagon are the
main hormones which control the
glucose homeostasis in the liver
Insulin and glucagon
concentrations depend on the
plasma glucose concentrations