Download File

Glycolysis
Chapter 17
Dr. M. khalifeh
Coupling of Production and Use of Energy
• ATP is an essential high energy bond-containing
compound
• It does not store energy, it transfer it
• (electric current does not store energy)
• A lot of negative charges make it unsuitable for storage
(repulsion)
• Fat and carbohydrate store energy
• Hydrolysis of ATP to ADP produce energy
• Phosphorylation of ADP to ATP require energy
• Supplied from oxidation of nutrients
Dr. M. khalifeh
ATP
• Why is ATP less stable,
charge-wise, than ADP?
• Four –ve charges on ATP
and 3 on ADP, therefore
• ATP is less stable due to
electrostatic repulsion.
• Energy must be expended
to put an additional
negatively charged
phosphate on ADP
Dr. M. khalifeh
Organophosphate as an energy source
• Organophosphates release higher energy when
hydrolyzed than ATP (Table 15.1)
• We can use the energy release from hydrolysis of
these compound to phosphorylate ADP to ATP
Dr. M. khalifeh
The Overall Pathway of Glycolysis
• Glycolysis is the first stage of glucose metabolism
• One molecule of glucose is converted to pyruvate
which gives rise to 2 molecules of ATP via substrate
level phosphorylation.
• Note that all of the steps of glycolysis are
enzymatically catalyzed.
• It plays a key role in the way organisms extract
energy from nutrients
• Once Pyruvate is formed, it has one of several fates
Dr. M. khalifeh
Fates of Pyruvate From Glycolysis
Dr. M. khalifeh
The Reactions of Glycolysis
• Phosphorylation of glucose to give glucose-6phosphate
• Isomerization of glucose-6-phosphate to give
fructose-6-phosphate
• Phosphorylation of fructose-6-phosphate to yield
fructose-1,6-bisphosphate
• Cleavage of fructose-1,6,-bisphosphate to give
glyceraldehyde-3-phosphate and
dihydrooxyacetone phosphate
• Isomerization of dihyroxyacetone phosphate to give
glyceraldehyde-3-phosphate
Dr. M. khalifeh
The Reactions of Glycolysis (Cont’d)
• Oxidation of glyceraldehyde-3-phosphate to give
I,3-bisphosphoglycerate
• Transfer of a phosphate group from
1,3-bisphosphoglycerate to ADP to give ATP
• Isomerization of 3-phosphoglycerate to give 2phosphoglycerate
• Dehydration of 2-phosphoglycerate to give
phosphoenolpyruvate
• Transfer of a phosphate group from
phosphoenolpyruvate to ADP to give pyruvate
Dr. M. khalifeh
Glycolysis phases
• The pathway of glycolysis can
be seen as consisting of 2
separate phases.
• the first is the chemical priming
phase requiring energy in the
form of ATP:
• 2 equivalents of ATP are used
to convert glucose to fructose
1,6-bisphosphate (F1,6BP).
• the second is considered the
energy-yielding phase:
• F1,6BP is degraded to
pyruvate, with the production
of
• 4 equivalents of ATP and 2
equivalents of NADH.
Dr. M. khalifeh
Conversion of Glucose to Glyceraldehyde-3-Phosphate
• In step 1 of glycolysis, glucose is phosphorylated to
give glucose-6-phosphate
• The reaction is endergonic, as it is driven by the free
energy of ATP hydrolysis
Dr. M. khalifeh
The Hexokinase Reaction
• The phosphorylation by hexokinase converts
nonionic glucose into an anion that is trapped in the
cell, since cells lack transport systems for
phosphorylated sugars.
• Glucokinase is the form of the enzyme found in
hepatocytes
Dr. M. khalifeh
Conversion of Glucose to Glyceraldehyde-3Phosphate
• The second step is the Isomerization of glucose-6phosphate to fructose-6-phosphate
• The C-1 aldehyde of glucose-6-phosphate is
reduced to a hydroxyl group
• The C-2 hydroxyl group is oxidized to give the
ketone group of fructose-6-phosphate
Dr. M. khalifeh
Conversion of Glucose to Glyceraldehyde-3-Phosphate
• Fructose-6-phosphate is then phosphorylated again
to generate fructose-1,6-bisphosphate
• This is the second reaction to be coupled with ATP
hydrolysis
Dr. M. khalifeh
Phosphofructokinase is a key regulatory enzyme
• Phosphofructokinase (PFK):
• Exists as a tetramer and subject to allosteric feedback
regulation
• The tetramer is composed of L and M subunits
• M4, M3L, M2L2, ML3, and L4 all exist. Combinations of these
subunits are called Isozyme
• Muscles are rich in M4; the liver is rich in L4
• ATP is an allosteric effector;
• high levels inhibit the enzyme
• low levels activate it
• Fructose-1,6-bisphosphate is also an allosteric effector
Dr. M. khalifeh
Conversion of Glucose to Glyceraldehyde-3-Phosphate
• Fructose-1,6-bisphosphate is split into
• two three carbon fragments
• Reaction catalyzed by Aldose
• Side chains of an essential Lys and Cys play key roles in
catalysis
Dr. M. khalifeh
Conversion of Glucose to Glyceraldehyde-3-Phosphate
• In step 5, dihydroxyacetone phosphate (DHAP) is converted
to glyceraldehyde-3-phosphate
• These compounds are trioses
• This reaction has small +G (0.58kcal/mol-1)
• Remember that glycolysis has several reactions that have very
negative G values, and drive other reactions to completion
Dr. M. khalifeh
Summary
• In the first stages of glycolysis, glucose is converted
to two molecules of glyceraldehyde-3-phosphate
• The key intermediate in this series of reactions is
fructose-1,6-bisphosphate.
• The enzyme that catalyzes this reaction,
phosphofructokinase, is subject to allosteric control
Dr. M. khalifeh
Phase II starts (payoff phase)
Glyceraldehyde-3-Phosphate is Converted to Pyruvate
• The first reaction that begins the conversion to pyruvate
involves the oxidation of glyceraldehyde-3-phosphate to
1,3-bisphosphoglycerate
• This reaction involves addition of a phosphate group, as well
as an electron transfer reaction from glyceraldehyde-3phosphate to NAD+
• The oxidizing agent, NAD+, is reduced to NADH
• Mixed anhydride of two acid by loss of water
Dr. M. khalifeh
Oxidation and Phosphorylation Reaction
Dr. M. khalifeh
Cysteine’s Role in Glyceraldehyde-3-phosphate Dehydrogenase
Dr. M. khalifeh
Glyceraldehyde-3-Phosphate to Pyruvate
• 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate
• This step involves reaction in which ATP is produced by
phosphorylation of ADP
• 1,3-bisphosphoglycerate transfers a phosphate group to ADP.
This is known as substrate level phosphorylation.
• Reaction is catalyzed by phosphoglycerate Kinase
• This reaction is the sum of the endergonic phosphorylation of
ADP and the exergonic hydrolysis of the mixed phosphate
anhydride
• The standard free energy of the hydrolysis rxn is more negative
than the standard free energy of the hydrolysis of the new
phosphate group being formed
• Hydrolysis of 1,3-bisphosphoglycerate yield more negative free
energy than ATP hydrolysis
Dr. M. khalifeh
Glyceraldehyde-3-Phosphate to Pyruvate
Dr. M. khalifeh
Glyceraldehyde-3-Phosphate to Pyruvate
• The next step involves the transfer of
3-phosphoglycerate to 2-phosphoglycerate
• This reaction is catalyzed by phosphoglyceromutase
Dr. M. khalifeh
Glyceraldehyde-3-Phosphate to Pyruvate
• Next, 2-phosphoglycerate loses one molecule of
water, producing Phosphoenolpyruvate
• It is just dehydration rxn
• Enolase catalyzes the reaction and requires a Mg as
cofactor
• Phosphoenolpyruvate contains a high energy bond
Dr. M. khalifeh
Glyceraldehyde-3-Phosphate to Pyruvate
• Phosphenolpyruvate (PEP) transfers its phosphate group
to ADP, producing ATP and pyruvate
• G of hydrolysis of PEP is more than that of ATP
(-61.9kJ mol-1 vs. -30.5kJ mol-1)
• Reaction is catalyzed by pyruvate kinase
• Allosteric enzyme
Dr. M. khalifeh
Control Points in Glycolysis
Allosteric enzymes
Hexokinase
Allosteric enzymes
Phosphofrectokinase
Lack of NAD+ mean
storage is fully loaded
Allosteric enzymes
Pyruvate Kinase
Dr. M. khalifeh
Summary
• In the final stages of glycolysis, two molecules of
pyruvate are produced for each molecule of
glucose that entered the pathway
• These reactions involve electron transfer, and the
net production of two ATP for each glucose
• There are three control points in the glycolytic
pathway
Dr. M. khalifeh
Anaerobic Metabolism of Pyruvate
• Under anaerobic conditions, the most important pathway for
the regeneration of NAD+ is reduction of pyruvate to lactic
acid
• Lactate dehydrogenase (LDH) is a tetrameric isoenzyme
consisting of H and M subunits; H4 predominates in heart
muscle, and M4 in skeletal muscle
Dr. M. khalifeh
NAD+ Needs to be Recycled to Prevent
Decrease in Oxidation Reactions
Dr. M. khalifeh
Alcoholic Fermentation
• Two reactions lead to the production of ethanol:
• Decarboxylation of pyruvate
to acetaldehyde
• Reduction of acetaldehyde
to ethanol
• Pyruvate decarboxylase catalyzes the first reaction
• This enzyme require Mg2+ and the cofactor Thiamine
pyrophosphate (TPP)
• Alcohol dehydrogenase catalyzes the conversion of
acetaldehyde to ethanol
Dr. M. khalifeh
Structures of Thiamine and TPP
Dr. M. khalifeh
Summary
• Pyruvate is converted to lactate in anaerobic
tissues, such as actively metabolizing muscle.
NAD+ is recycled in the process
• In some organisms, pyruvate is converted to
ethanol in a process requiring thiamine
pyrophosphate as a coenzyme
Dr. M. khalifeh
The Energy Derived from Glucose Oxidation
• Aerobic glycolysis of glucose to pyruvate, requires
two ATP to activate the process, with production of
four ATP and two NADH.
Glucose + 2 ADP + 2 NAD+ + 2 Pi
----->
2 Pyruvate + 2 ATP + 2 NADH + 2 H+
Dr. M. khalifeh
Glycolysis NADH
• The NADH generated during glycolysis is used to
fuel mitochondrial ATP synthesis via oxidative
phosphorylation, producing either two or three
equivalents of ATP
• Depending upon whether the glycerol phosphate
shuttle or the malate-aspartate shuttle is used to
transport the electrons from cytoplasmic NADH into
the mitochondria.
Dr. M. khalifeh
Complete energy yield from Glycolysis
• The net yield from the oxidation of 1 mole of glucose
to 2 moles of pyruvate is, therefore, either 6 or 8
moles of ATP.
• Released or stored
Dr. M. khalifeh