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Transcript of After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic...
After pyruvate is oxidized, the citric acid cycle completes the energy-yielding
oxidation of organic molecules.
Chapter 9, Section 3
Oxidation of Pyruvate to Acetyl CoA In the presence of O2, pyruvate enters the
mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed.
Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycle.
This step is carried out by a multienzyme complex that catalyses three reactions.
Figure 9.10
Pyruvate
Transport protein
CYTOSOL
MITOCHONDRION
CO2 Coenzyme A
NAD + HNADH Acetyl CoA
1
2
3
The Citric Acid Cycle The citric acid cycle, also called the Krebs
cycle, completes the break down of pyruvate to CO2. If acetyl-CoA is NOT stored then it enters the
Citric Acid Cycle. The cycle oxidizes organic fuel derived from
pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn.
Figure 9.12-1
1
Acetyl CoA
Citrate
Citricacidcycle
CoA-SH
Oxaloacetate
Step One: A Condensation Reaction 2-C acetyl-CoA + 4-C
molecule 6-C molecule+ CoA [which can be used over and over!].
Irreversible. Inhibited by large amounts
of ATP already present.
Figure 9.12-2
1
Acetyl CoA
CitrateIsocitrate
Citricacidcycle
H2O
2
CoA-SH
Oxaloacetate
Step Two: Isomerization Hydroxyl group repositioned. Water removed from one
carbon and then added to a different carbon.
RESULT: a change in position of an −H and −OH.
Molecule is still (6-C), but the –OH has moved!
Figure 9.12-3
1
Acetyl CoA
CitrateIsocitrate
-Ketoglutarate
Citricacidcycle
NADH+ H
NAD
H2O
3
2
CoA-SH
CO2
Oxaloacetate
Step Three: First Oxidation Molecule undergoes oxidative
decarboxylation--fancy talk for chopping off a carbon and losing a pair of e-’s in the process.
The pair of e- reduce NAD+ to NADH.
The chopped off C becomes CO2.
Now we have a 5-C molecule.
Figure 9.12-4
1
Acetyl CoA
CitrateIsocitrate
-Ketoglutarate
SuccinylCoA
Citricacidcycle
NADH
NADH
+ H
+ H
NAD
NAD
H2O
3
2
4
CoA-SH
CO2
CoA-SH
CO2
Oxaloacetate
Step Four: Second Oxidation The 5-C molecule
undergoes oxidative decarboxylation. Releases CO2. 2 more e- reduce
another NAD+ to NADH The 4-C fragment that
is left receives a CoA group.
Figure 9.12-5
1
Acetyl CoA
CitrateIsocitrate
-Ketoglutarate
SuccinylCoA
Succinate
Citricacidcycle
NADH
NADH
ATP
+ H
+ H
NAD
NAD
H2O
ADP
GTP GDP
P i
3
2
4
5
CoA-SH
CO2
CoA-SH
CoA-SH
CO2
Oxaloacetate
Step Five: Substrate Level Phosphorylation CoA leaves the 4-C
molecule. Breaking of bond
releases energy. GDP + Pi GTP [just
substitute guanine for adenine in “A” TP]. GTP ATP
Remaining 4-C molecule.
Figure 9.12-6
1
Acetyl CoA
CitrateIsocitrate
-Ketoglutarate
SuccinylCoA
Succinate
Fumarate
Citricacidcycle
NADH
NADH
FADH2
ATP
+ H
+ H
NAD
NAD
H2O
ADP
GTP GDP
P i
FAD
3
2
4
5
6
CoA-SH
CO2
CoA-SH
CoA-SH
CO2
Oxaloacetate
Step Six: Third Oxidation Four carbon molecule
is oxidized. FAD+ + FOUR e- and
TWO H+ FADH2. FAD+ is an integral
part of mitochondria membrane.
FADH2 can contribute e- to ETS.
Figure 9.12-8
NADH
1
Acetyl CoA
CitrateIsocitrate
-Ketoglutarate
SuccinylCoA
Succinate
Fumarate
Malate
Citricacidcycle
NAD
NADH
NADH
FADH2
ATP
+ H
+ H
+ H
NAD
NAD
H2O
H2O
ADP
GTP GDP
P i
FAD
3
2
4
5
6
7
8
CoA-SH
CO2
CoA-SH
CoA-SH
CO2
Oxaloacetate
Step Seven and Eight: Regeneration Water is added to
oxidized 4-C molecule.
After water is added, compound becomes oxidized again (2 e- are released) NAD+ + the 2 e-
NADH Ready to start again!
Figure 9.11
Pyruvate
NAD
NADH
+ HAcetyl CoA
CO2
CoA
CoA
CoA
2 CO2
ADP + P i
FADH2
FAD
ATP
3 NADH
3 NAD
Citricacidcycle
+ 3 H
Beating a dead horse!
Substrate Level
Phosphorylation
Oxidation
Glycolysis 2 ATP 2 NADH
Oxidation of Pyruvate
-------- 2NADH
Krebs 2 ATP 6 NADH + 2 FADH2
TOTAL 4 ATP 10 NADH + 2 FADH2
Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis
• Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food.
• These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation.
The Pathway of Electron Transport
The electron transport chain is in the inner membrane (cristae) of the mitochondrion.
Most of the chain’s components are proteins, which exist in multiprotein complexes.
The carriers alternate reduced and oxidized states as they accept and donate electrons.
Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O.
Figure 9.13
NADH
FADH2
2 H + 1/2 O2
2 e
2 e
2 e
H2O
NAD
Multiproteincomplexes
(originally from NADH or FADH2)
III
III
IV
50
40
30
20
10
0
Fre
e e
ner
gy
(G)
rela
tiv
e to
O2 (
kcal
/mo
l)
FMN
FeS FeS
FAD
Q
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
FeS
Electrons are transferred from NADH or FADH2 to the electron transport chain.
Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to oxygen.
The electron transport chain generates no ATP directly.
It breaks the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts.
Chemiosmosis: The Energy-Coupling Mechanism
Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space.
H+ then moves back across the membrane, passing through the protein, ATP synthase.
ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ADP.
This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work.
Figure 9.14
INTERMEMBRANE SPACE
Rotor
StatorH
Internalrod
Catalyticknob
ADP+P i ATP
MITOCHONDRIAL MATRIX
Figure 9.15
Proteincomplexof electroncarriers
(carrying electronsfrom food)
Electron transport chain
Oxidative phosphorylation
Chemiosmosis
ATPsynth-ase
I
II
III
IVQ
Cyt c
FADFADH2
NADH ADP P i
NAD
H
2 H + 1/2O2
H
HH
21
H
H2O
ATP
The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis.
The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work.
An Accounting of ATP Production by Cellular Respiration
During cellular respiration, most energy flows in this sequence: glucose NADH electron transport chain proton-motive force ATP
About 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 32 ATP.
There are several reasons why the number of ATP is not known exactly.
Figure 9.16
Electron shuttlesspan membrane
MITOCHONDRION2 NADH
2 NADH 2 NADH 6 NADH
2 FADH2
2 FADH2
or
2 ATP 2 ATP about 26 or 28 ATP
Glycolysis
Glucose 2 Pyruvate
Pyruvate oxidation
2 Acetyl CoACitricacidcycle
Oxidativephosphorylation:electron transport
andchemiosmosis
CYTOSOL
Maximum per glucose:About
30 or 32 ATP