CHAPTER 16
Glycolysis
Net reaction of Glycolysis
Converts:
1 Glucose
Hexose
stage
2 pyruvate
Triose
stage
-
Two molecules of ATP are produced
-
Two molecules of NAD+ are reduced to NADH
Glucose + 2 ADP + 2 NAD+ + 2 Pi
2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O
Glycolysis can be divided into two stages
Hexose
stage
Triose
stage
2 ATP are consumed
per glucose
4 ATP are produced
per glucose
NET: 2 ATP produced
per glucose
Enzymes are needed to speed up
the reactions.
Hexose
stage
Note the near equilibrium rxns
 concentration affects flux
Vs.
Those that are irreversible
 points of regulation
Triose
stage
Enzymes are needed to speed up
The reactions.
Hexose
stage
Note the near equilibrium rxns
 concentration affects flux
Vs.
Those that are irreversible
 points of regulation
Two molecule of NADH are produced in triose
stage and are equivalent to several ATP in
terms of redox potential.
Triose
stage
Enzyme 1 Hexokinase
- Transfers the g-phosphoryl of ATP to glucose C-6 oxygen to
generate glucose 6-phosphate
- Mechanism: Mg2+ is an important cofactor, attack of C-6 hydroxyl
oxygen of glucose on the g-phosphorous of MgATP2- displacing
MgADP-
This is an irreversible reaction
Figure 16.2: Enzyme 1 Hexokinase
2 lobes show induced fit model
Enzyme 2 Phosphoglucose Isomerase
- Converts glucose 6-phosphate (an aldose) to
fructose 6-phosphate (a ketose)
- Enzyme converts glucose 6-phosphate to open chain
form of fructose 6-phosphate in the active site.
A near equilibrium reaction
Enzyme 3 Phosphofructokinase-1 (PFK-1)
- Catalyzes transfer of a phosphoryl group from ATP to the
C-1 hydroxyl group of fructose 6-phosphate to form
fructose 1,6-bisphosphate
- PFK-1 is metabolically irreversible and a critical regulatory
point for glycolysis in most cells (PFK-1 is the first committed
step of glycolysis.
Enzyme 4 Aldolase
- Aldolase cleaves the hexose, fructose 1,6-bisphosphate
into two triose phosphates: glyceraldehyde 3-phosphate (GAP)
and dihydroxyacetone phosphate (DHAP)
This reaction is also near equilibrium
but products are favored
Enzyme 5 Triose Phosphate Isomerase
- Conversion of dihydroxyacetone phosphate (DHAP)
into glyceraldehyde 3-phosphate (GAP)
This reaction is also near equilibrium
but products are favored due to GAP metabolism
Fate of carbon atoms from hexose stage to triose stage
Consumed 2 ATP in the
process - per one glucose
Figure 11.7
Enzyme 6 Glyceraldehyde 3-phosphate
Dehydrogenase
- Conversion of glyceraldehyde 3-phosphate (GAP) to
1,3-bisphosphoglycerate (1,3-BPG)
- One molecule of NAD+ is reduced to NADH
Oxidation and phosphorylation,
yielding a high energy mixed
acid hydride (NADH)
NADH can be reoxidized elsewhere, such as
the membrane associated electron transport chain
Enzyme 6 Glyceraldehyde 3-phosphate
Dehydrogenase
The coupling of these two reactions make the overall conversion
near equilibrium
Very unfavorable
reaction
Enzyme 7 Phosphoglycerate Kinase
- Uses the high-energy compound 1,3-bisphosphoglycerate
to transfer a phosphoryl group to ADP to form ATP
- 3-phosphoglycerate is formed in the process
1,3-BPG has more stored chemical energy than ATP
Enzyme 8 Phosphoglycerate Mutase
- Catalyzes transfer of a phosphoryl group from one part of
of the substrate molecule to another
- Reaction occurs without input of ATP
Enzyme 8
Phosphoglycerate Mutase
A different phosphate is placed
on carbon-2.
Enzyme 9 Enolase: 2-phosphoglycerate
to phosphoenolpyruvate (PEP)
- Elimination of water (dehydration) yields PEP
- PEP has a very high phosphoryl group transfer potential
because it exists in its unstable enol form
Enzyme 10 Pyruvate Kinase:
- Metabolically irreversible reaction
- This reaction is another site of regulation
- Pyruvate kinase is regulated both allosterically and
by covalent modification with a phosphate group
unstable
Triose Stage
2 glyceraldehyde 3phosphate molecules
generated
per one glucose
2 NADH generated
per one glucose
4 ATP generated
per one glucose
Table 16.1: Irreversible reactions have
large negative free energy (DG)
When DG = 0, equilibrium is established
Fates of pyruvate
- For centuries, bakeries
and breweries have
exploited the conversion
of glucose to ethanol and
CO2 by glycolysis in yeast
Metabolism of Pyruvate
1. Aerobic conditions: pyruvate is oxidized to acetyl CoA
which enters the citric acid cycle for further oxidation to CO2
2. Anaerobic conditions: (microorganisms): pyruvate is converted
to ethanol
3. Anaerobic conditions: (muscles, red blood cells): pyruvate
is converted to lactate
4. Amino acid synthesis: pyruvate is a precursor to alanine
Anaerobic Conversion: Pyruvate to Ethanol
-
Yeast cells convert pyruvate to ethanol and CO2 and oxidize NADH to NAD+.
-
Two steps (and enzymes) are required:
- Decarboxylation to form acetaldehyde by pyruvate decarboxylase.
- Coenzyme thiamine pyrophosphate(TPP) is also needed
- Reduction of acetaldehyde to ethanol by alcohol dehydrogenase.
This is coupled with the oxidation of NADH
Redox is
balance
Reduction of Pyruvate to Lactate
muscles - anaerobic
- Muscles lack pyruvate dehydrogenase and cannot produce
ethanol from pyruvate
- Muscle have lactate dehydrogenase to convert pyruvate
to lactate
Reduction of Pyruvate to Lactate
muscles - anaerobic
This reaction regenerates NAD+ for
use by glyceraldehyde 3-phosphate
dehydrogenase in glycolysis.
This will maintain glycolytic flux.
-
Lactate formed in skeletal muscles during
exercise is transported to the liver via the
bloodstream.
-
Liver lactate dehydrogenase can convert lactate
to pyruvate
-
Lactic acidosis can result from insufficient
oxygen (an increase in lactic acid and decrease in
blood pH)
Figure 16.7 Entry of other sugars into glycolysis
From ingested
fructose
From ingested
fructose
Figure 16.8:Fructose is converted to
Glyceraldehyde 3-phosphate
Conversion requires
2 ATP molecules same
as the hexose stage
for glucose.
Figure 16.9: Gal-1-P is activated by attachment of a UMP.
Activated
form
Glu-6P
phosphoglucomutase
Regulation of Glycolysis
-
Enzymes that are not reversible:
Reaction 1 – hexokinase
Reaction 3 – phosphofructokinase
Reaction 10 – pyruvate kinase
-
These steps are metabolically irreversible, and these enzymes are regulated.
All other steps of glycolysis are near equilibrium in cells and not regulated
1.
When ATP is needed, glycolysis is activated
- Inhibition of phosphofructokinase-1 is relieved
by accumulation of AMP or fructose 2,6-bisphosphate
- Pyruvate kinase is activated by fructose 1,6-bisphosphate
2.
When ATP levels are sufficient, glycolysis activity decreases
- Phosphofructokinase-1 is inhibited by ATP and Citrate
- Hexokinase is inhibited by glucose 6-phosphate
Glucose transport into
the cell- the first step
in regulation
Transporters are proteins consisting
of ~500 AA single chain polypeptide
(Integral membrane proteins)
Insulin binding activates GLUT 4 embedded
in plasma membrane.
Glucose uptake into skeletal and heart muscles
Regulation of Hexokinase
- Hexokinase reaction is metabolically irreversible
- Glucose 6-phosphate (product) levels increase when
glycolysis is inhibited at sites further along in the pathway
- Glucose 6-phophate inhibits hexokinase
Regulation of Phosphofructokinase-1 (PFK-1)
- ATP is a substrate and an allosteric inhibitor of PFK-1
- High concentrations of AMP (and ADP) allosterically activate
PFK-1 by relieving the ATP inhibition. ([ATP] does
not vary much in cells. [ADP] and [AMP]<< [ATP])
- Elevated levels of citrate in the liver (indicate ample substrates for
citric acid cycle) also inhibit phosphofructokinase-1
Figure 16.12
Figure 16.13: Allosteric regulation of hexokinase and PFK
Glycolysis regulation in the Liver
PFK-1 by Fructose 2,6-bisphosphate (F-2,6-BP)
Fructose 2,6-bisphosphate is formed form fructose 6-phosphate
and allosterically activates PFK-1 to increase its affinity for fructose
6-phosphate.
PFK-2
PFK-1
Figure 16.14
Regulation of Liver Pyruvate Kinase
allosteric effectors and covalent modification
Protein
Kinase A
The hormone glucagon stimulates a protein kinase A, which
phosphorylates pyruvate kinase, converting it to a less active form
Assignment
Read Chapter 15
Read Chapter 16