Download Integration of Metabolism: Glucose Synthesis

Biochemistry I
Integration of Metabolism:
Glucose Synthesis, Transport and
Regulation
PPT 16.4
Chapters 16, 21, 17, 14
Gluconeogenesis
OUT
Chapter 14, 16, 10
IN
Dr. Ray
Integration of Carbohydrate and Lipid Metabolism
Pentose Phosphate
Pathway (PPP)
• produce
• produce
Liver exports glucose if low blood sugar
Glycogen (polymer of
glucose, energy storage)
Glucose-6-P
Gluconeogenesis:
Glycolysis:
• convert
• utilizes
• convert
• produce
Pyruvate
Triacylglycerol (energy storage)
Pyruvate Dehydrogenase Complex:
• convert
• produce
Ketone Bodies
Cholesterol
Citric Acid Cycle
• convert
• produce
b-oxidation:
• convert
Acetyl CoA
• produce
Oxidative Phosphorylation:
•
•
•
•
oxidize
reduce
create
phosphorylate
Control of Glycolysis
• Flux through the glycolytic pathway must be
adjusted in response to conditions both inside
and outside cell. Rate of conversion of glucose
into pyruvate is regulated for cellular needs:
1) production of ATP, generated by the
degradation of glucose
2) form building blocks for synthetic reactions,
such as the formation of fatty acids.
In metabolic pathways, enzymes
catalyzing essentially irreversible
reactions (decrease in free energy) are
potential sites of control.
Hexokinase
2
Phosphofructo
kinase
3
4
5
6
7
Which glycolytic enzymes are sites of control?
The ________________ reactions at
steps
__________ , which is near
equilibrium in cellular concentrations)
• In gluconeogenesis, these three
irreversible steps are by-passed by
other reactions, in order to overcome
the energetic barrier.
1
8
9
Pyruvate
kinase
10
Gluconeogenesis
 Focus on comparing Gluconeogenesis to Glycolysis
The synthesis of glucose from non-carbohydrate precursors
(such as pyruvate & lactate) is called gluconeogenesis.
1. Why is this metabolic pathway important?
because the _______ depends on glucose as its primary fuel
and _______________ use only glucose as a fuel
RBC: do not have mitochondria, NO aerobic metabolism,ATP only from glycolysis
• The daily glucose requirement of the brain in a typical adult human being is about
120 g out of the160 g of glucose needed daily by the whole body.
• The amount of glucose present in body fluids is about 20 g, and that readily
available from glycogen, a storage form of glucose, is approximately 190 g.
Direct glucose reserves are sufficient to
meet glucose needs for about a day. During
a longer period of not eating, glucose must
be formed from non-carbohydrate sources,
through gluconeogenesis mostly in the
_________ and some in ______________
http://higheredbcs.wiley.com/legacy/college/boyer/0471661791/animations/animations.htm
 Gluconeogenesis  Energetics
Gluconeogenesis
- NOT a reversal of glycolysis
The three exergonic (irreversible)
steps of glycolysis in cell:
phosphorylate
(1) Hexokinase
(2) Phosphofructokinase
(3) Pyruvate kinase
make ATP
by Substrate Level Phosphorylation
ATP
are replaced by other favorable
reactions:
dephosphorylate
(11) Glucose-6-phosphatase
(9) Fructose-1,6-bisphosphatase
happens
twice
GTP
ATP
, HCO3-
PyruvateOxaloacetate
Oxaloacetate 
 PEP
PEP
Pyruvate
(1) Pyruvate carboxylase
(2) Phosphoenolpyruvate
carboxykinase
Make high
Steps 1 &2, NO details
energy PEP,
use ATP, GTP
Energetics of Gluconeogenesis
in glycolysis
(step 3)
Exergonic phosphorylation catalyzed by PFK
DGo’ = -14.2 kJ/mol
DGo’ = -16.3 kJ/mol
in gluconeogenesis
(step 8)
Exergonic hydrolysis catalyzed by Fructose 1,6-bisphosphatase
Both kinase & phosphatase
reactions are exergonic.
Kinase uses ATP for
phosphate source and
for energy source .
What is the phosphoryl transfer
potential of Pi (inorganic phosphate?
Phosphatases hydrolyze
phosphorylated alcohols
(ROP), which have some
small amount of free
energy of hydrolysis.
Non-Carbohydrate Precursors
of Gluconeogenesis
• Lactate – formed by lactate dehydrogenase in
active skeletal muscle when rate of glycolysis
exceed rate of oxidative metabolism (because
of insufficient levels of oxygen).
• Amino acids – from proteins in the diet and
during starvation from breakdown of proteins
in skeletal muscle.
• Glycerol – from hydrolysis of triacylglycerols
in fat cells. Glycerol can enter the glycolytic
or gluconeogenic pathways through DHAP:
Phosphorylate at C3
Oxidize at C2
The Cori Cycle (Fig 16.33)
CORI CYCLE - Lactate formed by active muscle travels through the
blood and is converted into glucose by the liver. This cycle shifts part
of the metabolic burden of active muscle to the liver.
Anaerobic
glycolysis in
muscle
• The main site of gluconegenesis is in the
liver, with a small amount occurring in the
kidneys.
• Gluconeogenesis in these two organs helps
maintain glucose levels in the blood so that
the brain, red blood cells and muscles can
extract sufficient glucose from blood to
satisfy their energy needs.
Gluconeogenesis costs 6 ATP
equivalents per glucose made
Gluconeogenesis
in liver
In the liver, lactate is oxidized to pyruvate by lactate dehydrogenase. This
pyruvate undergoes gluconeogenesis to produce free glucose, which is released
into the blood. The liver restores the level of glucose necessary for active
muscle cells to continue anaerobic glycolysis for immediate energy needs.
Reciprocal Regulation of Related Pathways
Glycolysis and Gluconeogenesis are coordinated, in a tissuespecific fashion, to ensure that the glucose-dependent energy
needs of ALL cells are met. In a particular cell, when one
pathway is upregulated, the other is downregulated.
• Glycolysis produces 2 ATP
• Gluconeogenesis costs 6 ATP
Overall, both pathways
are highly exergonic
(spontaneous)!
Workbook, Chapter 16 Self-Test, Q27:
1. Which of the following statements about the Cori cycle and its
physiologic consequences are true?
A)
B)
C)
D)
E)
It involves the synthesis of glucose in muscle.
It involves release of lactate by muscle.
It involves lactate synthesis in the liver.
It involves ATP synthesis in muscle.
It involves release of glucose by the liver.
Hormone Triggered Regulation of Glycogen Metabolism
Epinephrine
Hormones secreted by pancreas and adrenal glands:
1) Insulin (peptide) = fed state (High blood sugar)
2) Glucagon (peptide) = fasting state (Low blood sugar)
3) Epinephrine (catecholamine) = rapid energy needed
Glycogen
Glucose
Insulin binds to Tyrosine Kinase Receptors on both Liver & Muscle
cells, causing an increase in glucose uptake & glycogen degradation.
Liver Cell
Epinephrine binds to
G-protein Coupled
Receptors (GPCR) on
muscle cells, causing
glycogen degradation.
Glucagon only binds to
liver cells, causing an
increase in glycogen
degradation via a cAMP
cascade, so that glucose
is secreted into blood.
Muscle Cell
10
Regulation of Glucose Uptake by GLUT Transporters
• Glucose enters a cell through
a homologous set of transporters
called GLUT1 to GLUT5.
Transporter expression is a
primary means of controlling
metabolism, since the plasma
membrane of each type of
cell expresses a different set
of transporters.
• Nerve cells express high
concentrations of GLUT 3
(high glucose affinity).
• Muscle and adipose tissue
express GLUT 4 (medium
glucose affinity)
• Pancreatic b-cells and liver express
GLUT2 (low glucose affinity)
A cell can only perform biochemical
reactions on compounds which it
has taken up from its environment.
Secretion of Insulin by Pancreatic b-cells
How does the pancreas sense
blood glucose levels?
GLUT2
12
• Pancreatic b-cells express
GLUT2 on their plasma
membrane, which has a low
affinity for glucose
(KM = 15-20 mM), so glucose
will enter these cells only when
blood glucose is plentiful
(after a meal).
• Metabolism of glucose to CO2
increases ATP/AMP ratio
(cell’s energy charge).
• ATP-sensitive K+ channel
closes, which alters membrane
potential.
• This causes a Ca2+ ion channel
to open (secondary messenger),
which in turn causes cell to
secrete insulin into blood.
Insulin Receptor – A Regulatory Protein
Signal transduction (information transfer) from
exterior to interior of cells, although signal molecule
is NOT permeable across plasma membrane
(1) How does the insulin receptor function in the plasma membrane?
Inactive State:
Monomer
Active State:
Dimer
Insulin
• The insulin receptor is a
dimer of two identical subunits.
• The separate monomers are
inactive (do not bind insulin),
but they come together
(dimerize) around a single
insulin molecule.
• The dimer is active (binds
insulin and becomes
functional).
Regulation of Glucose Uptake by GLUT Transporters
Glucose transport
into muscle cells is
regulated by INSULIN:
• GLUT 4 is stored in
intracellular vesicles
• When stimulated by
insulin, GLUT4
transporters move to
plasma membrane
• Passive glucose uptake
increases because more
GLUT4 transporters are
present in plasma
membrane
• When insulin
decreases, GLUT4
transporters reenter
cell
Regulation of Glucose Uptake:
http://www.wiley.com/college/f
ob/quiz/quiz15/15-21.html
1) Does GLUT4 have high
or low KM for glucose since
muscle cells need to uptake
glucose rapidly?
http://www.wiley.com/college/
fob/quiz/quiz21/21-7.html
OUT
IN
14
Metabolic Enzymes
• What types of reactions do the
following enzymes catalyze?
(A) Kinase –
(B) Phosphatase –
(C) Mutase –
(D) Glycosylase –
(E) Phosphorylase – breaks a glycosidic
bond via phosphorolysis, thus breaks an
acetal into a hemiacetal and an RO P
using Pi as the nucleophile
Glycogen Breakdown
Removal of glucose molecules from glycogen involves 3 enzymes:
Glycogen + Pi  Glucose-1-P + Glycogen
(n-1 residues)
- Cleaves a-1,4-glycosidic bond in (n residues)
glycogen, but not by hydrolysis
(1) Glycogen phosphorylase:
- catalyzes phosphorolysis
(cleavage of a glycosidic bond
[acetal] by the nucleophile Pi
instead of H2O)
(2) Debranching enzyme:
a-1,6-glucosidase
Hydrolyzes a(16) glycosidic bonds
at branch points in glycogen
(3) Phosphoglucomutase:
Glucose-1-P  Glucose-6-P
(G6P feeds into glycolysis)
Glycogen Metabolism in Muscle and Liver
Glycogen
Breakdown
Glycogen
Synthesis
(1) Muscle cells
- Produce ATP for motion
(exercise)
- Store glucose as glycogen
when energy is not needed
(resting)
(2) Liver
Glucose
(only in liver) catalyzed by
glucose-6-phosphatase
- Control blood sugar
- Store glucose when blood
sugar is high (feeding)
- Release glucose when blood
sugar is low (exercise)
• Several synthesis and degradation enzymes are allosterically
regulated by metabolites, so that enzyme activity is adjusted to
meet current needs of the cell.
• Metabolism of glycogen is tightly controlled by hormones to
meet metabolic needs of entire organism.
General Metabolism of Glucose in Tissues
Glycolysis, Gluconeogenesis, and Glycogen metabolism are
coordinated, in a tissue-specific fashion, to ensure that the
glucose-dependent energy needs of all cells are met.
Metabolism Regulation
In multistep biochemical pathways such as glycolysis, enzymes that
catalyze irreversible steps are regulated, rather than enzymes that
catalyze steps at equilibrium. Often feedback inhibition or
feedforward activation is used for regulation of pathways.
Regulation of the Citric Acid Cycle
Control of the Citric Acid Cycle:
• Regulated primarily by the
concentrations of ATP and NADH
(final products of pathway)
• Key control points are two of the
reactions with the largest driving
force (DG’):
isocitrate dehydrogenase
a-ketoglutarate dehydrogenase
- If cell’s energy charge
[ATP]/[AMP] is high (because
excess ATP is present) then these
two enzymes and the entire TCA
pathway is downregulated via
_______________________.
Amphibolic Functions of the Citric Acid Cycle
TCA is both catabolic and anabolic
1. Do anabolic reactions
require free energy to occur?
2. For catabolism of Acetyl CoA, are
the cellular concentrations of OAA
and Acetyl CoA the same?
• ______________ amounts of TCA
intermediates are needed to maintain
the degradative (catabolic) function
of the cycle.
3. Is there NET production of OAA
during the Citric Acid cycle?
4. Can TCA intermediates by used to
make other macromolecules?
5. Can TCA intermediates be made
from sources other than acetyl CoA?
http://www.wiley.com/college/fob/quiz/quiz16/16-15.html
Receptor Tyrosine Kinases: Insulin Receptor
From chapter 14 (FYI)
Insulin:
• Activation of Tyr
Kinase receptors
initiates a kinase
cascade.
• Soluble proteins
are phosphorylated
which converts
them from inactive
to activate form.
• Through the sequential interaction of many
different proteins on the interior side of the
plasma membrane, the cell increases its
uptake of glucose.
• Excess glucose not needed for immediate
energy is stored as glycogen.
Receptor activation by covalent modification (phosphorylation)
Signal Transduction Pathways (Chapter 14)
FYI
• Signal transduction – the multistep process by which an
extracellular signal (chemical, mechanical or electrical) is
amplified and converted to a intra-cellular response.
• Signal transduction leads to production of intracellular second
messenger (cAMP), which leads to activation of many target
proteins.
Adenylate Cyclase Signaling System
= epinephrine (adrenaline)
= cell surface receptor
= cAMP cascade
activates kinases
= glycogen breakdown  glycolysis
 ATP synthesis  energy to run fast
• Cascade quickly amplifies effect of binding a small number of hormones into
release of large number of sugar (glucose) units
Activating and Deactivating G-proteins
in Signal Transduction Pathways
FYI
ON SWITCH: activate G protein (which binds to either GTP or GDP):
Hormone binding to receptor causes conformational change in
receptor so that G-protein releases GDP, binds GTP, and dissociates
into two active components
• Dephosphorylation
deactivates G-protein
Function of Cyclic AMP in Metabolic Regulation
Activates Protein Kinase A (Chapter 10) FYI
Protein Kinase A is composed
of 4 subunits: 2 R + 2 C
• R is regulatory subunit
• C is catalytic subunit
Inactive R2C2 tetramer
Pseudosubstrate
sequence
Two active C monomers
Active
C
cAMP binding
domains
Active
C
cAMP
• No cAMP present: inactive R2C2 tetramer, because the active sites on
the C subunits are occupied by a pseudosubstrate portion of R (shown
in red), which prevents access to the active site by substrates.
• When cAMP binds (green squares): causes allosteric conformational
change of R, removes the pseudosubstrate sequence from the active
sites, and tetramer dissociates into a regulatory subunit (R2) and two
catalytically active subunits (2 C).
• Inhibition of catalytic subunits is relieved, so activated C is now able to
bind a protein substrate (phosphorylase kinase) and phosphorylate it.
Slide
FYI
Adenylate Cyclase Signaling System
Epinephrine
activate Glycogen Phosphorylase
Glycogen
Glucose
Result: Glycogen releases Glucose-1-phosphate which enters
glycolysis to generate ATP for muscle movement.
Integration of Carbohydrate and Lipid Metabolism
Pentose Phosphate
Pathway (PPP)
Liver exports glucose if low blood sugar
• produce NADPH
• produce ribose-5-P
Glycogen (polymer of
glucose, energy storage)
Glucose-6-P
Gluconeogenesis:
Glycolysis:
• convert pyruvate into glucose
• utilizes 4 ATP, 2 GTP & 2 NADH
• convert glucose into pyruvate
• produce 2 ATP & 2 NADH
Pyruvate
Triacylglycerol (energy storage)
Pyruvate Dehydrogenase Complex:
• convert pyruvate into acetyl CoA
• produce NADH
Ketone Bodies
b-oxidation:
• convert fatty acids into
acetyl CoA
• produce NADH & FADH2
Acetyl CoA
Cholesterol
Citric Acid Cycle
• convert acetyl CoA to CO2
• produce NADH, FADH2 & GTP
Oxidative Phosphorylation:
•
•
•
•
oxidize NADH & FADH2
reduce O2 to H2O
create H+ gradient across IMM
phosphorylate ADP to ATP
Metabolic Enzymes
• What types of reactions do the
following enzymes catalyze?
(A) Kinase – transfers phosphate from
ATP to alcohol to make RO P
(B) Phosphatase – hydrolyzes phosphate
from phosphorylated alcohol, thus
transfers phosphate from RO P to H2O
(C) Mutase – an isomerase that moves a
phosphate from one alcohol to another
alcohol in the same molecule
(D) Glycosylase – hydrolyzes a
glycosidic bond (with specific
stereochemistry), thus breaks an acetal
into a hemiacetal and an alcohol
(E) Phosphorylase – breaks a glycosidic
bond via phosphorolysis, thus breaks an
acetal into a hemiacetal and an RO P
using Pi as the nucleophile