Tutorial: Glucose Metabolism in the b -Cell. Richard Bertram Department of Mathematics And Programs in Neuroscience and Molecular Biophysics. Metabolites as Signaling Molecules. All cells in the body convert glucose and other fuels to adenosine
Department of Mathematics
Programs in Neuroscience and Molecular Biophysics
All cells in the body convert glucose and other fuels to adenosine
triphosphate (ATP), the primary energy molecule. The ATP powers
many of the energy-requiring chemical reactions that occur in the cell.
However, in b-cells the ATP molecule and several intermediates of
metabolism act also as signaling molecules. They tell the b-cell the
level of blood glucose, so that the cell can adjust its electrical and
Ca2+ activity to secrete the appropriate amount of insulin.
A primary target of the signaling molecule ATP is the ATP-dependent
K+ channel (the K(ATP) channel). This is inactivated by ATP, so:
Kahn et al., Nature, 444:840, 2006
Anaerobic production of ATP. Occurs in the cytosol.
However, not much ATP is produced by glycolysis, only
two ATP molecules for each glucose molecule metabolized.
Found in all aerobic organisms, takes place in mitochondria
of eucaryotes. Most of the coenzyme NADH is made here
Through a series of redox reactions (NAD+ is reduced).
Found in eucaryotes, takes place in mitochondria. O2
is consumed by the electron transport chain. Most of the
ATP is produced here, 28 ATP molecules for each
glucose molecule metabolized.
Two ATP molecules
are used to make one
molecule of FBP
Two ATP molecules
produced for each of
total of 4 ATP generated.
Sustained NADH oscillations in yeast,
very simple (single cell) eucaryotes.
Oscillations are in the presence of glucose
and cyanide (which blocks electron
transport, inhibiting oxidative phosphorylation).
Dano et al., Nature, 402:320, 1999
Oscillations in three glycolytic
intermediates in muscle extracts.
Tornheim, JBC, 263:2619, 1988
In muscle extracts the mechanism is
known to be the allosteric enzyme
Phosphofructokinase (PFK). The
key feature of this enzyme is that
its product FBP feeds back and
stimulates the enzyme.
The muscle form of this enzyme,
PFK-M, dominates the PFK
activity in b-cells.
With moderate glucokinase
With high glucokinase
Bertram et al., Biophys. J., 87:3074, 2004
JGK is the glucokinase reaction rate
JPFK is the PFK reaction rate
JGPDH is the GPDH reaction rate
5 minGlycolytic Oscillations Occur Only for Moderate GK Rates
A model prediction is that it should be possible to turn on the
GOs by simply increasing the glucose concentration. We have
evidence for this from Ca2+ measurements in islets:
Nunemaker et al., Biophys. J., 91:2082, 2006
Acetyl group has 2 carbons
Oxaloacetate has 4 carbons
Citrate has 6 carbons
As the cycle progresses, first one
carbon is lost and then another
Cycle ends where it began, except
that 4 NADH, one FADH2, and
one GTP molecule have been made
The coenzymes NADH and FADH2
are electron carriers that are used to
transfer electrons between molecules.
This transfer is key for powering
Anaplerosis is a series of enzymatic reactions in which metabolic
intermediates enter the citric acid cycle from the cytosol.
Cataplerosis is the opposite, a process where intermediates leave the
citric acid cycle and enter the cytosol.
In muscle, anaplerosis is important for increasing citric acid throughput
during periods of exercise.
There is some evidence that anaplerosis is required for a glucose-induced
rise in mitochondrial ATP production.
Some amino acids (the building blocks of proteins) enter and leave the
citric acid cycle through anaplerosis and cataplerosis.
Some of the coenzyme NADH is made during glycolysis. How does
this get into the mitochondria where it can power oxidative
Keeping score of ATP production:
Glycolysis – 2 ATP for each glucose molecule
Citric Acid cycle – No ATP produced
Oxidative Phosphorylation – up to 34 ATP molecules
Without mitochondria (and thus OP), complex life forms could not
Published as a series of papers in the late 1990s. Describes oxidative
Phosphorylation in b-cells.
We have recently published a simpler model that uses curve fitting to
reduce the complexity of the flux and reaction functions (Bertram et al.,
J. Theoret. Biol., 243:575, 2006).
Mitochondrial Variables: NADH concentration
ADP or ATP concentration (ADP+ATP=constant)
Inner membrane potential
O2 consumption is also calculated
NADH flux from citric acid cycle increases NADH concentration.
NADH is oxidized when it supplies electrons to the electron
transport chain, decreasing NADH concentration.
NADH autofluorescence is measured
Bertram et al., Biophys. J., 92:1544, 2007
ADP is phosphorylated to ATP by the F1-F0 ATP-synthase. This is due
to the flux of protons down the concentration gradient from outside
to inside of the mitochondrial inner membrane.
The ATP made in this way is transported out, and ADP transported in,
by the adenine nucleotide transporter.
ATP measured using adenovirally driven expression of
recombinant firefly luciferase.
Ainscow and Rutter, Diabetes, 51:S162, 2002
Calcium enters the mitochondria from the cytosol through calcium
Calcium is pumped out of the mitochondria into the cytosol via
This membrane potential is the driving force for ATP production
by the F1F0 ATP synthase. If membrane potential is 0, then no
ATP will be made.
(Negative terms represent positive charge entering across the inner
Measured using the fluorescent dye rhodamine 123 (Rh 123)
Kindmark et al., J. Biol. Chem., 276:34530, 2001
Measured using an oxygen electrode
Kennedy et al., Diabetes, 51:S152, 2002