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Glycolysis
Biochemistry
All organisms produce ATP by
breaking down molecules and by
releasing energy stored in glucose and
other sugars.
Aerobic respiration - the process by
which a cell uses O2 to "burn" molecules and
release energy
The reaction:
C6H12O6 + 6O2 >> 6CO2 + 6H2O
Aerobic respiration - the process by
which a cell uses O2 to "burn" molecules and
release energy
The reaction:
C6H12O6 + 6O2 >> 6CO2 + 6H2O
Note: this reaction is the opposite of photosynthesis
three major reaction pathways
•Glycolysis
•The Kreb's Cycle
•Electron Transport
Phosphorylation (chemiosmosis)
Glucose is metabolized to pyruvate by the
pathway of glycolysis
Aerobic tissues metabolize pyruvate to acetyl-CoA,
which can enter the citric acid cycle for complete
oxidation to CO2 and H2O, linked to the formation of
ATP in the process of oxidative phosphorylation
Glycolysis (glyco = sugar; lysis = breaking)
•Goal: break glucose down to form two
pyruvates
•Who: all life on earth performs
glycolysis
•Where: the cytoplasm
Glycolysis literally means
"splitting sugars."
glucose (a six carbon sugar) is split
into two molecules of a threecarbon sugar.
Glycolysis Is Regulated at Three Steps
Involving Nonequilibrium Reactions
Although most of the reactions of glycolysis are reversible,
three are markedly exergonic and must therefore
be considered physiologically irreversible.
These reactions, catalyzed by
1. hexokinase (and glucokinase)
2. Phosphofructokinase
3. pyruvate kinase,
are the major sites of regulation of glycolysis.
The First Stage
of Glycolysis
•Glucose (6C) is broken down into 2 (3C)
•This requires two ATP's
The Second Stage
of Glycolysis
•2 (3C) are converted to 2 pyruvates
•This creates 4 ATP's and 2 NADH's
•The net ATP production of Glycolysis is 2
ATP's
10 Steps of Glycolysis
Step 1
The enzyme hexokinase phosphorylates
(adds a phosphate group to) glucose in the
cell's cytoplasm. In the process, a phosphate
group from ATP is transferred to glucose
producing glucose 6-phosphate.
Glucose (C6H12O6) + hexokinase + ATP →
ADP + Glucose 6-phosphate (C6H11O6P1)
Step 2
The enzyme phosphoglucoisomerase converts glucose 6phosphate into its isomer fructose 6-phosphate. Isomers
have the same molecular formula, but the atoms of each
molecule are arranged differently.
Glucose 6-phosphate (C6H11O6P1) +
Phosphoglucoisomerase → Fructose 6-phosphate
(C6H11O6P1)
Step 3
The enzyme phosphofructokinase uses another
ATP molecule to transfer a phosphate group to
fructose 6-phosphate to form fructose 1, 6diphosphate.
Fructose 6-phosphate (C6H11O6P1) +
phosphofructokinase + ATP → ADP + Fructose 1,
6-diphosphate (C6H10O6P2)
Step 4
The enzyme aldolase splits fructose 1, 6bisphosphate into two sugars that are isomers of
each other. These two sugars are
dihydroxyacetone phosphate and glyceraldehyde
phosphate.
Fructose 1, 6-bisphosphate (C6H10O6P2) + aldolase
→ Dihydroxyacetone phosphate (C3H5O3P1) +
Glyceraldehyde phosphate (C3H5O3P1)
Step 5
The enzyme triose phosphate isomerase rapidly interconverts the molecules dihydroxyacetone phosphate and
glyceraldehyde phosphate. Glyceraldehyde phosphate is
removed as soon as it is formed to be used in the next step
of glycolysis.
Dihydroxyacetone phosphate (C3H5O3P1) → Glyceraldehyde
phosphate (C3H5O3P1)
Net result for steps 4 and 5: Fructose 1, 6-diphosphate
(C6H10O6P2) ↔ 2 molecules of Glyceraldehyde phosphate
(C3H5O3P1)
Step 5
The enzyme triose phosphate isomerase rapidly interconverts the molecules dihydroxyacetone phosphate and
glyceraldehyde phosphate. Glyceraldehyde phosphate is
removed as soon as it is formed to be used in the next step
of glycolysis.
Dihydroxyacetone phosphate (C3H5O3P1) → Glyceraldehyde
phosphate (C3H5O3P1)
Net result for steps 4 and 5: Fructose 1, 6-diphosphate
(C6H10O6P2) ↔ 2 molecules of Glyceraldehyde phosphate
(C3H5O3P1)
Step 6
The enzyme triose phosphate dehydrogenase serves two functions in
this step. First the enzyme transfers a hydrogen (H-) from
glyceraldehyde phosphate to the oxidizing agent nicotinamide adenine
dinucleotide (NAD+) to form NADH. Next triose phosphate
dehydrogenase adds a phosphate (P) from the cytosol to the oxidized
glyceraldehyde phosphate to form 1, 3-bisphoshoglyceric acid. This
occurs for both molecules of glyceraldehyde phosphate produced in
step 5.
A. Triose phosphate dehydrogenase + 2 H- + 2 NAD+ → 2 NADH + 2 H+
B. Triose phosphate dehydrogenase + 2 P +
2 glyceraldehyde
phosphate (C3H5O3P1) → 2 molecules of 1,3-bisphoshoglyceric
acid (C3H4O4P2)
Step 7
The enzyme phosphoglycerokinase transfers a P
from 1,3-diphoshoglyceric acid to a molecule of ADP
to form ATP. This happens for each molecule of 1,3diphoshoglyceric acid. The process yields two 3phosphoglyceric acid molecules and two ATP
molecules.
2 molecules of 1,3-diphoshoglyceric acid (C3H4O4P2)
+ phosphoglycerokinase + 2 ADP → 2 molecules of
3-phosphoglyceric acid (C3H5O4P1) + 2 ATP
Step 8
The enzyme phosphoglyceromutase relocates the
P from 3-phosphoglyceric acid from the third
carbon to the second carbon to form 2phosphoglyceric acid.
2 molecules of 3-Phosphoglyceric acid (C3H5O4P1)
+ phosphoglyceromutase → 2 molecules of 2Phosphoglyceric acid (C3H5O4P1)
Step 9
The enzyme enolase removes a molecule of water
from 2-phosphoglyceric acid to form
phosphoenolpyruvic acid (PEP). This happens for
each molecule of 2-phosphoglyceric acid.
2 molecules of 2-Phosphoglyceric acid (C3H5O4P1)
+ enolase → 2 molecules of phosphoenolpyruvic
acid (PEP) (C3H3O3P1)
Step 10
The enzyme pyruvate kinase transfers a P from
PEP to ADP to form pyruvic acid and ATP. This
happens for each molecule of PEP. This reaction
yields 2 molecules of pyruvic acid and 2 ATP
molecules.
2 molecules of PEP (C3H3O3P1) + pyruvate kinase +
2 ADP → 2 molecules of pyruvic acid (C3H4O3) + 2
ATP
1
2
1
3
2
4
Glycolysis
produces 4 ATP's and 2 NADH, but uses 2
ATP's in the process
a net of 2
ATP and 2 NADH
NOTE: this process does not require O2 and does not
yield much energy
2 ATP are used in steps 1-3
2 ATP are generated in step 7
2 ATP are generated in step 10.
4 ATP molecules produced.
- 2 ATP molecules used
---------
2 ATP molecules (Total)
Summary of Glycolysis
a single glucose molecule in
glycolysis produces a total of
2
2
2
2
molecules
molecules
molecules
molecules
of
of
of
of
pyruvic acid
ATP
NADH
water
Kreb’s Cycle
Citric Acid Cycle / Tricarboxylic Acid Cycle
What is Krebs Cycle?
Used to oxidize the pyruvate
formed during the glycolytic breakdown of
glucose into Carbon Dioxide (CO2) and
Water (H2O).
It also oxidizes acetyl CoA which
arises from breakdown of carbohydrate, lipid,
and protein.
Metabolic pathways
fall into three categories:
1) Anabolic pathways are those involved in the synthesis of
compounds. Anabolic pathways are endergonic.
(2) Catabolic pathways are involved in the breakdown of
larger molecules, commonly involving oxidative reactions;
they are exergonic, producing reducing equivalents and,
mainly via the respiratory chain, ATP.
(3) Amphibolic pathways occur at the “crossroads” of
metabolism, acting as links between the anabolic and
catabolic pathways, eg, the citric acid cycle.
GLUCOSE
GLYCOLYSIS
PYRUVATE
PYRUVATE
KREB’S CYCLE
CO2 & H20
CO2 & H20
It begins when acetyl –CoA
enters into a reaction to form citric Acid.
This cycle was discovered by British
biochemist Sir Hans Krebs.
The first product of Krebs cycle is
citric acid (citrate).Therefore, it is also
known as citric
acid cycle.
Pyruvate dehydrogenase
6C
4C
THE OXIDATION OF PYRUVATE TO
ACETYL-CoA IS THE IRREVERSIBLE
ROUTE FROM GLYCOLYSIS TO THE
CITRIC ACID CYCLE
Pyruvate Dehydrogenase Is Regulated
by End-Product Inhibition & Covalent
Modification
Pyruvate dehydrogenase is inhibited by its
products
1. acetyl-CoA
2. NADH
All the products of digestion are
metabolized to
a common product, acetyl- CoA,
which is then oxidized by the
citric acid cycle
Steps of Krebs cycle
cyclic oxidation process
nine steps occur in overall Krebs cycle:
1. Condensation
2. Isomerisation
3. Dehydrogenation
4. Decarboxylation
5. Oxidative Decarboxylation
6. Substrate level ATP/GTP synthesis.
7. Dehydrogenation (oxidation) of Succinate
8. Hydration
9. Dehydrogenation (Oxidation) of Malate
Step 1: Condensation
In first step of Krebs cycle, Acetyl CoA combines with
oxaloacetate in the presence of condensing
enzymes citrate synthetase. CoA is released out.
The product of condensation is citrate which is a
tricarboxylic 6-carbon compound.
CATALYTIC
ROLE
Step 2: Isomerisation
Citrate formed in first step is converted into its isomer
isocitrate in a two – step reaction in the presence of iron
containing enzyme aconitase.
(i) Dehydration : A molecule of water is released and citric
acid is changed into cis-aconitate.
Step 2: Isomerisation
Citrate formed in first step is converted into its isomer
isocitrate in a two – step reaction in the presence of iron
containing enzyme aconitase.
(i) Dehydration : A molecule of water is released and citric
acid is changed into cis-aconitate.
(ii) Rehydration : Cis – aconitate combines with a molecule of
water and form isocitrate.
Step 2: Isomerisation
Citrate formed in first step is converted into its isomer
isocitrate in a two – step reaction in the presence of iron
containing enzyme aconitase.
(i) Dehydration : A molecule of water is released and citric
acid is changed into cis-aconitate.
(ii) Rehydration : Cis – aconitate combines with a molecule of
water and form isocitrate.
Step 3: Dehydrogenation
Now isocitrate undergoes dehydrogenation in the
presence of an enzyme isocitrate dehydrogenase. Mn2+
ion is required for the functioning of enzyme. Hydrogen
given out by isocitrate is picked up by NAD+
(Nicotinamide adenine dinucleotide) to form NADH2.
After losing hydrogen, isocitrate is changed into
oxalosuccinate (6C).
Step 4: Decarboxylation
Oxalosuccinate in Step 4 undergoes
decarboxylation. In the presence of
oxalosuccinate decarboxylase enzyme,
oxalosuccinate is changed into α-ketoglutarate.
Step 5: Oxidative Decarboxylation
In this step 5-carbon compound, α – Ketoglutarate
undergoes simultaneous dehydrogenation and
decarboxylation in the presence of enzyme α –
ketoglutarate dehydrogenase complex. This enzyme
complex contain Thiamine Pyrophosphate (TPP), Lipoic
Acid , Mg2+ and trans – succinylase. NAD+ and CoA are
required. The products formed are 4 – carbon compound
succinyl CoA, NADH2 and CO2.
Step 6: Substrate level ATP/GTP Synthesis
In the presence of enzyme succinyl thiokinase,
succinyl CoA is hydrolyzed. CoA and Succinate are
formed. The energy liberated during the process
is used in synthesis of ATP in Plants and GTP
(Guanosine triphosphate) or ITP (Inosine
triphosphate) in animals. CoA is released out.
Step 7: Dehydrogenation (Oxidation)
4 – Carbon compound Succinate is oxidized to another 4-carbon
compound fumarate with the help of enzyme succinate
dehydrogenase and hydrogen acceptor FAD (Flavin Adenine
Dinucleotide). The enzyme is attached to inner mitochondrial
membrane. It contains non haem iron (Fe–S) protein. This enables
the enzyme to get directly linked to electron transport chain.
Step 8: Hydration
Fumarate reacts with a molecule of water, in the
presence of an enzyme fumarase forming another
4-carbon dicarboxylic acid called Malate.
Step 9: Dehydrogenation (Oxidation)
With the help of enzyme malate dehydrogenase,
Malate formed in step 8 is oxidized to
oxaloacetate. NAD+ reduced to NADH2.
Malate + NAD+
N
oxaloacetate + NADH2
An oxaloacetate formed in this reaction
becomes available to combine with acetyl CoA to
start a new cycle all over again. Back again to
step 1
Note : The overall equation of oxidative
catabolism of pyruvate can be written as
follows:3
3
NADH2 & FADH2 are linked to electron transport system
and formation of ATP by Oxidative Phosphorylation.
The Conversion of Pyruvate to
Acetyl CoA for Entry Into the
Kreb's Cycle
•2 NADH's are generated
•2 CO2 are released
The Kreb's Cycle
•6 NADH's are generated
•2 FADH2 is generated
•2 ATP are generated
•4 CO2's are released
•Therefore, for each glucose molecule
that enters into the Kreb's cycle
(including the prepatory conversion to
Acetyl CoA), the net production of
products are:
8 NADH
2 FADH2
2 ATP
6 CO2
Stages of the TCA cycle
The cycle can be divided into three stages according to the role
of OAA:
– Stage 1:
• The attachment of acetyl CoA to oxaloacetate carrier
(reaction1)
– Stage 2:
• Breaking of oxaloacetate carrier (reaction 2-5)
– Stage 3:
• Regeneration of the carrier (reaction 6-8)
Functions of TCA cycle:
Amphibolic Function:
TCA cycle has both anabolic and catabolic functions
a- Catabolic role:
It is the final common pathway for oxidation of
carbohydrate, lipids and proteins with energy
production.
B-Anabolic role:
Source of the intermediates used in biosynthesis e.g.
1. Oxaloacetic acid is used in gluconeogenesis.
2. α- ketoglutarate is used for synthesis of some non
essential amino a.
3. Succinyl CoA is used in heme synthesis
Besides Glycolysis
and kreb’s,
Is there
anything more?
YES!
E.T.P.
Electron Transport Phosphorylation
(Chemiosmosis)
•Goal: to break down NADH and
FADH2, pumping H+ into the
outer compartment of the
mitochondria
•Where: mitochondria
The figure is found at http://plaza.ufl.edu/tmullins/BCH3023/cell%20respiration.html (December 2006)
The figure is found at http://academic.brooklyn.cuny.edu/biology/bio4fv/page/mito_ox.htm (December 2006)
inner mitochondrial
membrane
ATP synthase
The figure is found at http://plaza.ufl.edu/tmullins/BCH3023/cell%20respiration.html (December 2006)
In this reaction, the ETS creates a gradient
which is used to produce ATP, quite like in the
chloroplast
Electron Transport
Phosphorylation
typically produces
32 ATP's
Net Energy Production from Aerobic Respiration
•Glycolysis:
•Kreb's Cycle:
•Electron Transport
Phosphorylation:
•Net Energy Production:
Net Energy Production from Aerobic Respiration
•Glycolysis:
LET’S COUNT THE ATP’S ONLY
•Kreb's Cycle:
AND EXCLUDE NADH & FADH
•Electron Transport
Phosphorylation:
•Net Energy Production:
Net Energy Production from Aerobic Respiration
•Glycolysis:
•Kreb's Cycle:
•Electron Transport
Phosphorylation:
•Net Energy Production:
Net Energy Production from Aerobic Respiration
•Glycolysis: 2 ATP
•Kreb's Cycle:
•Electron Transport
Phosphorylation:
•Net Energy Production:
Net Energy Production from Aerobic Respiration
•Glycolysis: 2 ATP
•Kreb's Cycle: 2 ATP
•Electron Transport
Phosphorylation:
•Net Energy Production:
Net Energy Production from Aerobic Respiration
•Glycolysis: 2 ATP
•Kreb's Cycle: 2 ATP
•Electron Transport
Phosphorylation: 32 ATP
•Net Energy Production:
Net Energy Production from Aerobic Respiration
•Glycolysis: 2 ATP
•Kreb's Cycle: 2 ATP
•Electron Transport
Phosphorylation: 32 ATP
•Net Energy Production: 36 ATP!