Download pentose phosphate pathway

Chapter 6
Gluconeogenesis, Glycogen Metabolism,
and the Pentose Phosphate Pathway
A basket of fresh bread.
Carbohydrates such as these
provide a significant portion of
human caloric intake
Outline
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What is gluconeogenesis, and how does it operate?
How is gluconeogenesis regulated?
How are glycogen and starch catabolized in animals?
How is glycogen synthesized?
How is glycogen metabolism controlled?
Can glucose provide electrons for biosynthesis?
What is PPP?
What Is Gluconeogenesis, and How Does
It Operate?
Definition: the biosynthesis of glucose from simpler
molecules, primarily pyruvate and its precursors.
• The liver is the major location for gluconeogenesis
• The gluconeogenesis pathway is similar to the reverse of
glycolysis but differs at critical sites.
• Glucose is needed to maintain glucose levels range in
blood.
• Some tissue for example brain, erythrocytes,and muscles
use glucose at a rapid rate and sometimes require glucose
in addition to dietary glucose.
• The brain uses mostly glucose and erythrocytes can use
only glucose as a source of energy.
The Gluconeogenesis Pathway
1. pyruvate carboxylase
BYPASS 3
11
BYPASS 2
10
2. Phosphoenolpyruvate carboxykinase
3. enolase
4. phosphoglyceromutase
9
5. phosphoglycerate kinase
8
6. Glyceraldehyde-3- phosphatedehydrogenase
7
7. triose phosphate isomerase
6
8. aldolase
5
9. Fructose-1,6-bisphosphatase
4
10. phosphogluco-isomerase
3
11. Glucose-6-phosphatase
2
1
(3C)
BYPASS 1
lactate, alanine, other amino acids
Gluconeogenesis
Cost: The production of glucose is energy expensive.
Input: 2 pyruvate + 4 ATP + 2 GTP + 2 NADH
Output: glucose + 4 ADP + 2 GDP + 2 NAD++ 6 Pi
6 high energy bonds used per glucose synthesized
Pyruate is the major precursor for gluconeogenesis.
Lactate is the primary source for pyruvate.
In muscle, lactate is produced in great quantities during
exercise and this extra lactate cannot be further oxidized in
muscle.
So, Lactate is released from the muscles to the blood and
travels to the liver for conversion to pyruvate and, ultimately to
glucose.
Gluconeogenesis
In glycolysis, there are three irreversible kinase reactions at
control points involving: hexokinase, phosphofructokinase, and
pyruvate kinase.
In gluconeogenesis, these reactions must be forced the
opposite way.
The control points are the same for gluconeogenesis and for
glycolysis.
Four unique enzymes are used to replace or bypass these
irreversible steps.
The rest of the steps use the same enzymes as glycolysis.
Gluconeogenesis
• Gluconeogenesis retains seven steps of glycolysis:
• Steps 2 and 4-9
• Three steps are replaced or bypassed.
1. Pyruvate carboxylase and PEP carboxykinase
replace the pyruvate kinase reaction of glycolysis.
2. Fructose-1,6-bisphosphatase replaces the
phosphofructokinase reaction of glycolysis.
3. Glucose-6-phosphatase replaces the hexokinase
reaction of glycolysis.
• The new reactions provide for a spontaneous
pathway (DG negative in the direction of sugar
synthesis), and they provide new mechanisms of
regulation.
Bypass No. 1) Pyruvate to oxaloacetate & then PEP
 The enzyme Pyruvate Carboxylase is located inside
mitochondria. Only this enzyme of the gluconeogenesis
pathway is mitochondrial.
pyruvate + CO2 + ATP + H2O  oxaloacetate + ADP + Pi
Carboxylations reaction involve CO2 and almost
always use the vitamin biotin (Vit B) as a cofactor.
For gluconeogenesis, oxaloacetate must leave the
mitochondria because all the rest of the
gluconeogenesis enzymes are in the cytosol.
mitochondrial membranes are nearly impermeable to
oxaloacetate.
How does oxaloacetate transported from
Mitochondria to cytosol??
1. At first Mitochindrial enzyme malate dehydrogenase
convert Oxaloacetate to malate. This is reversible
reaction.
2. Then malate is permeable to mitochnodrial
membrane and goes out.
3. Now again the cytosolic malate dehydrogenase
convert Malate to oxaloacetate.
Now this Oxaloacetate is converted to the
phosphoenolpyruvate by enzyme enzyme PEP
carboxykinase.
(Other steps are simple reaction)
REGULATION OF GLUCONEOGENESIS
Gluconeogenesis serves as an alternative source of glucose
when supplies are low and is largely controlled by diet.
High carbohydrate in meal reduce gluconeogenesis and
fasting increases.
key enzymes targeted.
Gluconeogenesis and glycolysis are controlled in reciprocal
fashion.
 Glucose-6-phosphatase is under substrate-level control,
not allosteric control.
 The fate of pyruvate depends on acetyl-CoA.
 Fructose-1,6-bisphosphatase is inhibited by AMP, activated
by citrate - the reverse of glycolysis.
 Fructose-2,6-bisphosphate is an allosteric inhibitor of
fructose-1,6-bisphosphatase.
REGULATION OF
GLUCONEOGENESIS
The principal
regulatory
mechanisms in
glycolysis and
gluconeogenesis.
Activators are
indicated by plus
signs and
inhibitors by
minus signs.
REGULATION OF GLUCONEOGENESIS
Glucagon: hormone released when glucose levels are low.
This is a signal to elevate blood glucose levels which increases
intracellular levels of cAMP in liver.
cAMP activates protein kinase A that stimulate
gluconeogenesis and glycogenolysis.
Summary of Gluconeogenesis
purpose- alternative source of glucose rather than dietary
carbohydrates or glycogen breakdown.
primary precursors are pyruvate, lactate, glycerol, part of
fatty acids and certain amino acids.
 Three essentially irreversible steps of glycolysis are
bypassed.
Regulated via pyruvate carboxylase, fructose 1,6
bisphosphatase, and 2-phosphofructokinase.
Lactate Formed in Muscles is Recycled to
Glucose in the Liver
How your liver helps you during exercise:
 Recall that vigorous exercise can lead to a
buildup of lactate and NADH, due to oxygen
shortage and the need for more glycolysis.
 NADH can be reoxidized during the reduction of
pyruvate to lactate.
 Lactate is then returned to the liver, where it
can be reoxidized to pyruvate by liver LDH.
 Liver provides glucose to muscle for exercise and
then reprocesses lactate into new glucose.
• This is referred to as the Cori cycle.
Lactate Formed in Muscles is Recycled to
Glucose in the Liver
The Cori cycle.
GLYCOGEN METABOLISM
Glycogen: These are highly branched polymer of glucose.
Chains have glycosidic links α 14.
Branches are linked α 16.
Glucose stored in polymeric form as glycogen mostly in the liver and
skeletal muscle.
Glucose can be rapidly delivered to the blood stream when needed upon
degradation of glycogen. (This process is called as glycogenolysis).
Enough glucose and energy triggers synthesis of glycogen. This process
is called as glycogenesis.
Glycogenesis: GLYCOGEN BIOSYNTHESIS
Glucose is transported into the liver cell by a
specific glucose transporter and immediately
phosphorylated.
Most of the glucose in a cell is in the form of
glucose-6-phosphate.
There are four steps in Glycogen Biosynthesis
1. Conversion of glucose-6-phosphate to glucose-1-phosphate
2. Synthesis of Uridine Diphosphoglucose
3. Glycogen synthesis
4. Branching
Step1- Conversion of glucose-6-phosphate to glucose-1phosphate by Enzyme phosphoglucomutase
α-D-glucose-6phosphate
phosphoglucomutase
α-D- glucose-1phosphate
Reversible reaction allows G1P conversion to G6P
in glycogenolysis.
 Mechanism involves phosphorylated enzyme
intermediate and glucose-1,6 bisphosphate bound
intermediate similar to phosphoglycerate mutase
Step 2- Synthesis of Uridine Diphosphoglucose
With help of enzyme UDP-glucose pyrophosphorylase.
UDP-glucose pyrophosphorylase
α-D- glucose-1- phosphate
Uridine diphosphate glucose
UDP-glucose is one of the sugar nucleotides. Discovered by
Luis Leloir in the 1950s, they are “activated forms of sugar”.
Step 3- Glycogen synthesis by the enzyme glycogen synthase.
UDP-glucose + (glucose)n
UDP+(glucose)n+1
Glycogen Synthase Catalyzes
Formation of α(1→4) Glycosidic
Bonds in Glycogen.
Very large glycogen particle is
built around a single protein,
glycogenin, at the core.
The first glucose is linked to a
tyrosine -OH on the protein.
Sugar units are then added by
the action of glycogen synthase.
Glycogen synthase transfers
glucosyl units from UDP-glucose
to C-4 hydroxyl at a nonreducing
end of a glycogen strand.
Step 4- Branching by the enzyme called branching enzyme.
This branching enzyme introduces branching by transferring
a teminal fragment of 6-7 residues to a growing chain at 6position.
Makes a branch with an α (16) link creating two ends to
add glucose.
Branching accelerates the rate of glucose release during
degradation.
GLYCOGENOLYSIS (DEGRADATION OF GLYCOGEN)
Release of glucose-1-phosphate by the
enzyme called glycogen phosphorylase. This
enzyme always acts at nonreducing end. The 1,4
glycosidic link is cleaved by phosphorylysis.
This process stops at fourth glucose from a
1,6 branch point.
The de-branching enzyme such glucosidase or
transferase breaks 1,6 branch and release
glucose and glucose-1-phosphate molecules.
How Is Glycogen Metabolism Controlled?
Glycogen reserves are the most immediately available large
source of metabolic energy for mammals.
Storage and utilization are under dietary and hormonal control.
1. Primary hormones
“fight-or-flight”)
a).epinephrine (adrenaline,
b).glucagon
c).insulin
2. Primary enzyme targets in glycogen metabolism
are glycogen phosphorylase and glycogen
synthase. The actions of the hormones are indirect.
1. Control by hormones
a). Glucagon - released at low glucose levels
This is a polypeptide hormone produced in α-cells of the
islets of Langerhan of the pancreas.
Acts primarily on liver cells because the
receptors of this hormones are found on surface of liver cells.
Stimulates glycogen breakdown & inhibits glycogenesis.
Glucagon also blocks glycolysis & stimulates
gluconeogenesis.
b). Epinephrine – Also released at low glucose levels
 Acts primarily on skeletal muscle.
Stimulates glycogen breakdown & inhibits glycogenesis.
Glucagon and epinephrine both stimulate intracellular
pathway via increasing levels of cAMP.
1. Control by hormones
c). Insulin: -High levels of glucose induce release of insulin from β-cells of
islets of Langerhan in the pancreas.
Insulin is polypeptide hormone and detected by receptors at surface of
muscle cells.
Increases glycogenesis in muscle.
Intracellular signal pathway involves complex sequential Phosphorylations
and dephosphorylations.
Insulin triggers
protein kinases
that stimulate
insulin synthesis.
Insulin Modulates the Action of Glycogen Synthase in Several Ways
The Actions of Insulin on Metabolism
The metabolic effects of insulin are mediated through protein
phosphorylation and second messenger modulation.
2. Control by Enzymes
Glycogen metabolism is a highly regulated process which involve the
mutual control of glycogen phosphorylase and glycogen synthase.
GP allosterically activated by AMP and inhibited by ATP, glucose-6-P and
caffeine.
GS is stimulated by glucose-6-P. Both enzymes are regulated by
covalent modification called phosphorylation.
SIMPLISTIC SUMMARY
Epinephrine and glucagon stimulate glycogenolysis and inhibit
glycogenesis via a cAMP and a phosphorylation cascade.  releases
of glucose
Glycogenesis is stimulated by insulin in a pathway ending in the
dephosphorylation of glycogen
synthase.
Glycogenolysis is also inhibited via dephosphorylation.  takes up
glucose
Pentose Phosphate Pathway
 Cells are provided with a constant supply of NADPH
for biosynthesis by the pentose phosphate
pathway.
 Also called the hexose monophosphate shunt.
 This pathway also produces ribose-5-P .
 This pathway consists of two oxidative processes
followed by five non-oxidative steps.
 It operates mostly in the cytosol of liver and
adipose cells.
 NADPH is used in cytosol for fatty acid and other
biosynthesis.
pentose phosphate pathway
The pentose phosphate
pathway. The numerals
in the blue circles
indicate the steps
discussed in the text.
pentose phosphate pathway
The pentose phosphate pathway. The numerals in the blue
circles indicate the steps discussed in the text.
The pentose
phosphate pathway.
1. The Oxidative Steps of the Pentose
Phosphate Pathway
1. Glucose-6-P Dehydrogenase: Irreversible 1st step - highly regulated!
2. Gluconolactonase
3. 6-Phosphogluconate Dehydrogenase: An oxidative decarboxylation
The glucose-6-phosphate dehydrogenase reaction is the
committed step in the pentose phosphate pathway.
1. The Oxidative Steps of the Pentose
Phosphate Pathway
2. The Gluconolactonase Reaction
3. The 6-phosphogluconate dehydrogenase reaction
2. The Nonoxidative Steps of the Pentose
Phosphate Pathway
There are Five steps, but only 4 types of reactions...
• Phosphopentose isomerase
• converts ketose to aldose
• Phosphopentose epimerase
• epimerizes at C-3
• Transketolase ( a TPP-dependent reaction)
• transfer of two-carbon units
• Transaldolase (uses a Schiff base mechanism)
• transfers a three-carbon unit
2. The Nonoxidative Steps of the Pentose
Phosphate Pathway
The phosphopentose isomerase reaction converts a ketose to an aldose. The reaction
involves an enediol intermediate.
The phosphopentose epimerase reaction interconverts ribulose-5-P and
xylulose-5-phosphate
2. The Nonoxidative Steps of the Pentose
Phosphate Pathway
The transketolase reaction of step 6 in the pentose phosphate
pathway.
The transaldolase reaction
2. The Nonoxidative Steps of the Pentose
Phosphate Pathway
The transketolase reaction of step 8 in the pentose
phosphate pathway. This is another two-carbon transfer, and
it also requires TPP as a coenzyme.
Utilization of Glucose-6-P Depends on the
Cell’s Need for ATP, NADPH, and Rib-5-P
• Glucose can be a substrate either for glycolysis or
for the pentose phosphate pathway
• The choice depends on the relative needs of the cell
for biosynthesis and for energy from metabolism
• ATP can be made if G-6-P is sent to glycolysis
• Or, if NADPH or ribose-5-P are needed for
biosynthesis, G-6-P can be directed to the pentose
phosphate pathway
• Depending on these relative needs, the reactions of
glycolysis and the pentose phosphate pathway can
be combined in four principal ways
Four Ways to Combine the Reactions of
Glycolysis and Pentose Phosphate
1) Both Ribose-5-P and NADPH are needed by the
cell
• In this case, the first four reactions of the pentose
phosphate pathway predominate
• NADPH is produced and ribose-5-P is the principal
product of carbon metabolism
2) More Ribose-5-P than NADPH is needed by the cell
• Synthesis of ribose-5-P can be accomplished
without making NADPH, by bypassing the oxidative
reactions of the pentose phosphate pathway
Four Ways to Combine the Reactions of
Glycolysis and Pentose Phosphate
3) More NADPH than ribose-5-P is needed by the cell
• This can be accomplished if ribose-5-P produced in
the pentose phosphate pathway is recycled to
produce glycolytic intermediates
4) Both NADPH and ATP are needed by the cell, but
ribose-5-P is not
• This can be done by recycling ribose-5-P, as in case
3 above, if fructose-6-P and glyceraldehyde-3-P
made in this way proceed through glycolysis to
produce ATP and pyruvate, and pyruvate continues
through the TCA cycle to make more ATP
Xylulose-5-Phosphate is a Metabolic
Regulator
• In addition to its role in the pentose phosphate
pathway, xylulose-5-P is also a signaling molecule
• When blood glucose rises, glycolysis and the
pentose phosphate pathways are activated in the
liver
• The latter pathway makes xylulose-5-P, which
stimulates protein phosphatase 2A (PP2A)
• PP2A dephosphorylates PFK-2/F-2,6-Bpase and
also carbohydrate-responsive element-binding
protein (ChREBP)
• Glycolysis and lipid biosynthesis are both activated
as a result