Transcript No Slide Title

Gluconeogenesis
(formation of new sugar)
1. Why gluconeogenesis?
2. How different precursors enter
gluconeogenesis.
3. The synthesis of glycogen, starch and
sucrose; also lactose.
4. The role of sugar-nucleotide.
Gluconeogenesis
• Gluconeogenesis happens in all animals, plants, and
fungi. All the reactions are the same except the
regulation.
• In higher animals, gluconeogenesis happens in liver
and renal cortex. Gluconeogenesis can also happen
in brain, skeletal and heart muscle. However, liver
and kidney remain the main site for this pathway. In
liver the major function of gluconeogenesis is to
maintain blood glucose.
• The gluconeogenesis described here is the
mammalian pathway.
Why gluconeogenesis?
• Brain, nervous system, erythrocytes, testes,
renal medulla, and embryonic tissues can
only utilize glucose from blood as their
major or only energy source.
• Between meals and during longer fasts, or
after vigorous exercise, glycogen is
depleted. In order to keep the above systems
functional, organisms need a method for
synthesizing glucose from noncarbohydrate
precursors.
Noncarbohydrate precursors for
gluconeogenesis
Animals
Lactate
Glycerol
Pyruvate
Glucogenic amino acids
Stored fats Plants
Stored proteins
Microorganisms
Acetate, lactate, propionate
Three reactions are irreversible in
glycolysis and must be bypassed during
gluconeogenesis
1. Glucose  glucose 6-phosphate (hexokinase)
2.
Glucose 6-phosphate  fructose 6-phosphate (phosphohexose isomerase)
3. Fructose 6-phosphate  fructose 1,6-bisphosphate (PFK-1)
4.
5.
6.
7.
8.
9.
Fructose 1,6-bisphosphate  dihydroxyacetone phosphate + glyceraldehyde
3-phosphate (aldolase)
Dihydroxyacetone phosphate  glyceraldehyde 3-phosphate (triose
phosphate isomerase)
Glyceraldehyde 3-phosphate  1,3-bisphosphoglycerate (glyceraldehyde
3-phosphate dehydrogenase)
1,3-bisphosphoglycerate  3-phosphoglycerate (phosphoglycerate kinase)
3-phosphoglycerate  2-phosphoglycerate (phosphoglycerate mutase)
2-phosphoglycerate  phosphoenolpyruvate (enolase)
10. Phosphoenolpyruvate  pyruvate (pyruvate kinase)
These irreversible reactions will be bypassed in
gluconeogenesis.
Reactions in gluconeogenesis
1. Pyruvate  phosphoenolpyruvate
2.
3.
4.
5.
6.
7.
Phosphoenolpyruvate  2-phosphoglycerate (enolase)
2-phosphoglycerate  3-phosphoglycerate (phosphoglycerate mutase)
3-phosphoglycerate  1,3-bisphosphoglycerate (phosphoglycerate kinase)
1,3-bisphosphoglycerate  glyceraldehyde 3-phosphate (glyceraldehyde 3phosphate dehydrogenase)
Glyceraldehyde 3-phosphate  dihydroxyacetone phosphate (triose
phosphate isomerase)
Dihydroxyacetone phosphate + glyceraldehyde 3-phosphate  fructose 1,6bisphosphate (aldolase)
8. Fructose 1,6-bisphosphate  fructose 6-phosphate
(fructose 1,6-bisphosphatase)
9.
Fructose 6-phosphate  glucose 6-phosphate (phosphohexose isomerase)
10. Glucose 6-phosphate  glucose (glucose 6-phosphatase)
Three bypasses in
gluconeogenesis
• Because both glycolysis
and gluconeogenesis
happen in cytosol,
reciprocal and
coordinated regulation
is necessary.
First bypass: from pyruvate to
phosphoenolpyruvate (PEP)
• There are two pathways from pyruvate to
PEP.
• The major pathway uses pyruvate/alanine as
glucogenic precursor; however the second
pathway will dominate when lactate is the
glucogenic precursor.
• This step involved both cytosolic and
mitochondiral enzymes.
Main pathway of the first bypass (1)
(mito)
• The carboxylation of
pyruvate by pyruvate
carboxylase activates
it, initiating the
process of
gluconeogenesis.
• Pyruvate carboxylase
uses biotin as a carrier
of activated HCO3-.
Reaction mechanism of pyruvate carboxylase
Structure of pyruvate carboxylase
Reaction mechanism of pyruvate carboxylase
• Reaction of pyruvate
carboxylase happens
in two phases, which
occur at two different
sites in the enzyme.
• Biotin is covalently
linked to the e-amino
group of a Lys residue
and acts as a flexible
arm between two
active sites.
Main pathway of the first bypass (2)
• Oxaloacetate (OAA) produced by pyruvate
carboxylase is then transported out of
mitochondria in the form of malate
(mitochondrial membrane has no OAA
transporter).
(mito)
OAA+NADH+H+  L-malate + NAD+
(mito)
malate dehydrogenase
Main pathway of the first bypass (3)
• After OAA left mitochondria in the form of
malate, it will be converted back to OAA by
the cytosolic malate dehydrogenase (p.546,
eq. 14-6).
• This reaction also brings NADH from
mitochondria to cytosol, which will help
gluconeogenesis to proceed in the latter
stage.
Main pathway of the first bypass (4)
• OAA is then converted
to PEP by PEP
carboxykinase with
GTP as the phosphoryl
group donor.
• The same CO2 that
activates pyruvate at
the first step is lost
during this reaction.
Alternative pathway of first
bypass: when lactate is precursor
• Lactate produced from
erythrocytes or
anaerobic muscle will
be converted to
pyruvate first by
lactate dehydrogenase
(LDH) in hepatocytes
(the reverse of lactate
fermentation).
Alternative pathway of first
bypass (2)
• Pyruvate is then
transported into
mitochondria, where it
is converted to OAA
by pyruvate
carboxylase.
Alternative pathway of first
bypass (3)
• OAA is then converted
to PEP by
mitochondrial PEP
carboxykinase
(encoded by separate
nuclear gene).
Second bypass: conversion of fructose
1,6-bisphosphate to fructose 6-phosphate
• Because the
conversion of fructose
6-phosphate to
fructose 1,6bisphosphate is highly
exergonic, the reverse
reaction in
gluconeogenesis is
catalyzed by a
different enzyme,
FBPase-1.
Third bypass: conversion of
glucose 6-phosphate to glucose
• Similar condition also
happened in third
bypass.
Dephosphorylation of
glucose 6-phosphate
yielding glucose is
catalyzed by glucose
6-phosphatase.
However, this reaction
does not happen in
every tissue.
Third bypass
• Glucose 6-phosphatase is found on the lumenal
side of the ER of hepatocytes and renal cells. It is
activated by Mg2+. Muscle and brain tissue do not
contain this enzyme and so cannot carry out
gluconeogenesis.
Gluconeogenesis is energetically
expensive, but essential
• For glycolysis, every glucose generate
2ATP and 2NADH (p.548).
• However, 6ATP (4ATP+2GTP) and
2NADH were spent to generate 1 glucose
from 2 pyruvate (p.548, eq. 14-9).
• The extra energy spent is to ensure the
irreversibility of gluconeogenesis.
Many amino acids are glucogenic
Alanine
Cysteine
Glycine
Serine
Tryptophan
Pyruvate
Asparagine
aspartate
Phenylalanine
tyrosine
Isoleucine
Methionine
Threonine
valine
Glutamine
Arginine
Glutamate
Histidine
proline
Glycolysis and Gluconeogenesis
must be reciprocally regulated
• ATP + Fructose 6-phosphate  ADP +
Fructose 1,6-bisphosphate
• Fructose 1,6-bisphosphate + H2O 
fructose 6-phosphate + Pi
• ATP + H2O  ADP + Pi + Heat