Overview of Citric Acid Cycle
• The citric acid cycle operates
under aerobic conditions only
• The two-carbon acetyl group in
acetyl CoA is oxidized to CO2
• It produces reduced coenzymes
NADH and FADH2 and one
ATP directly
• In the citric acid cycle:
- acetyl (2C) bonds to
oxaloacetate (4C) to form
citrate (6C)
- oxidation and decarboxylation
convert citrate to oxaloacetate
- oxaloacetate bonds with
another acetyl to repeat the
cycle
Reaction 1: Formation of Citrate
• Oxaloacetate combines with the two-carbon acetyl group
to form citrate
-
COO
COO-
O
C O + CH3 C SCoA
CH2
acetyl CoA
COOoxaloacetate
CH2
HO C COO
CH2
COOcitrate
+ HS-CoA + H+
Reaction 2: Isomerization to Isocitrate
• Citrate isomerizes to isocitrate
• The tertiary –OH group in citrate is converted to a
secondary –OH that can be oxidized
COO-
COO
COO
H2O
CH2
-
HO C COO
CH2
-
-
H2O
CH2
-
-
Aconitase
C COO
CH
CH2
Aconitase
H C COO
HO C H
COO
COO
COO-
Citrate
Aconitate
Isocitrate
-
-
Reaction 3: Oxidative Decarboxylation 1
• A decarboxylation removes a carbon as CO2 from
isocitrate
• The –OH group is oxidized to a ketone releasing H+ and
2e- that form reduced coenzyme NADH
-
-
COO
COO
CH2
H C COO
HO C H
COOIsocitrate
Isocitrate
dehydrogenase
+
+ NAD
CH2
H C H
C O + CO2 + NADH
COO-
Reaction 4: Oxidative Decarboxylation 2
• In a second decarboxylation, a carbon is removed as CO2
from -ketoglutarate
• The 4-carbon compound bonds to coenzyme A providing
H+ and 2e- to form NADH
-
COO
COO-
CH2
CH2
CH2 + NAD+ + CoASH
CH2 + CO2 + NADH
C O
C O
COO-
S
-Ketoglutarate
Succinyl CoA
CoA
Reaction 5: Hydrolysis of Succinyl CoA
• The hydrolysis of the thioester bond releases energy to add
phosphate to GDP and form GTP, a high energy compound
COOCH2
CH2 + GDP + Pi
C O
S
CoA
Succinyl CoA
-
Succinyl CoA COO
synthetase
CH
2
+ GTP + CoA-SH
CH2
COOSuccinate
Reaction 6: Dehydrogenation of Succinate
• In this oxidation, two H are removed from succinate to
form a double bond in fumarate
• FAD is reduced to FADH2
COO-
-
Succinate
dehydrogenase
CH2
+ FAD
CH2
-
COO
Succinate
COO
C H
+ FADH2
H C
-
COO
Fumarate
Reaction 7: Hydration of Fumarate
• Water is added to the double bond in fumarate to form
malate
COO-
COO-
C H
H C
+
COOFumarate
Fumarase
H2O
HO C H
H C H
COOMalate
Reaction 8: Dehydration of Malate
• Another oxidation forms a C=O double bond
• The hydrogens from the oxidation form NADH + H+
COOHO C H
H C H
-
COO
Malate
+ NAD
+
Malate
dehydrogenase
-
COO
+
C O + NADH + H
CH2
-
COO
Oxaloacetate
Summary of Products from Citric Acid Cycle
In one turn of the citric acid cycle:
• Two decarboxylations remove two carbons as 2CO2
• Four oxidations provide hydrogen for 3NADH and one FADH2
• A direct phosphorylation forms GTP which is used to form ATP
• Overall reaction of citric acid cycle:
Acetyl CoA + 3NAD+ + FAD + GDP + Pi + 2H2O
2CO2 + 3NADH + 2H+ + FADH2 + HS-CoA + GTP
Regulation of the Citric Acid Cycle
The citric acid cycle:
• Increases its reaction rate
when low levels of ATP
or NAD+ activate
isocitrate dehydrogenase
to formation of acetyl
CoA for the citric acid
cycle
• Slows when high levels
of ATP or NADH inhibit
citrate synthetase (first
step in cycle), decreasing
the formation of acetyl
CoA
Electron Carriers
• The electron transport chain
consists of electron carriers that
accept H+ ions and electrons from
the reduced coenzymes NADH
and FADH2
• The H+ ions and electrons are
passed down a chain of carriers
until in the last step they combine
with oxygen to form H2O
• Oxidative phosphorylation is the
process by which the energy from
transport is used to synthesize ATP
Oxidation and Reduction of Electron Carriers
• Electron carriers are continuously oxidized and reduced as
hydrogen and/or electrons are transferred from one to the
next
• The energy produced from these redox reactions is used to
synthesize ATP
electron carrier AH2(reduced)
electron carrier A(oxidized)
electron carrier B(oxidized)
electron carrier BH2(reduced)
FMN (Flavin Mononucleotide)
• FMN coenzyme is derived from riboflavin (vitamin B2)
- it contains flavin, ribitol,and a phosphate
- it accepts 2H+ + 2e- to form reduced coenzyme FMNH2
Iron-Sulfur (Fe-S) Clusters
• Fe-S clusters are groups of proteins containing iron ions
and sulfide
• They accept electrons to reduce Fe3+ to Fe2+, and lose
electrons to re-oxidize Fe2+ to Fe3+
Coenzyme Q (CoQ or Q)
• Coenzyme Q (Q or CoQ) is a mobile electron carrier
derived from quinone
• It is reduced when the keto groups accept 2H+ and 2e-
Cytochromes (Cyt)
• Cytochromes (cyt) are proteins containing heme groups with
iron ions.
• In a cytochrome, Fe3+ accepts an electron to form Fe2+
(reduction), and the Fe2+ is oxidized back to Fe3+ when it
passes an electron to the next carrier: Fe3+ + e-  Fe2+
• They are abbreviated as cyt a, cyt a3, cyt b, cyt c, and cyt c1
Electron Transport System
• The electron carriers in the electron transport
system are attached to the inner membrane of the
mitochondrion
 They are organized into four protein complexes:
Complex I
NADH dehydrogenase
Complex II
Succinate dehydrogenase
Complex III
CoQ-Cytochrome c reductase
Complex IV
Cytochrome c Oxidase
Electron Transport Chain
Complex I: NADH Dehydrogenase
• At Complex I, hydrogen and electrons are transferred:
- from NADH to FMN:
FMN + NADH + H+ 
FMNH2 + NAD+
- from FMNH2 to Fe-S clusters and Q, which reduces Q to
QH2 and regenerates FMN
Q + FMNH2

QH2 + FMN
- to complex I to Complex III by Q (QH2), a mobile carrier
Complex II: Succinate Dehydrogenase
• At Complex II, hydrogen and electrons are transferred:
- from FADH2 to Complex II, which is at a lower energy
level than Complex I
- from FADH2 to coenzyme Q, which reduces Q and
regenerates FAD
Q + FADH2

QH2 + FAD
- from complex II to Complex III by Q(QH2), a mobile
carrier
Complex III: Coenzyme Q-Cytochrome c Reductase
• At Complex III, electrons are transferred:
- from QH2 to two Cyt b, which reduces Cyt b and
regenerates Q
2Cyt b (Fe3+) + QH2
 2Cyt b (Fe2+) + Q + 2H+
- from Cyt b to Fe-S clusters and to Cyt c, the second
mobile carrier
2Cyt c (Fe3+) + 2Cyt b (Fe2+)  2Cyt c (Fe2+) + 2Cyt b (Fe3+)
Complex IV: Cytochrome c Oxidase
• At Complex IV, electrons are transferred:
- from Cyt c to Cyt a
2Cyt a (Fe3+) + 2Cyt c (Fe2+)  2Cyt a (Fe2+) + 2Cyt c (Fe3+)
- from Cyt a to Cyt a3, which provides the electrons to
combine H+ and oxygen to form water
4H+ + O2 + 4e- (from Cyt a3)  2H2O
Oxidative Phosphorylation and the Chemiosmotic Model
• In the chemiosmotic model, complexes I, III, and IV pump
protons into the intermembrane space, creating a proton gradient
• Protons must pass through ATP synthase to return to the matrix
• The flow of protons through ATP synthase provides the energy for
ATP synthesis (oxidative phosphorylation):
ADP + Pi + Energy  ATP
ATP Synthase
• In ATP synthase protons flow back to the matrix through a
channel in the F0 complex
• Proton flow provides the energy that drives ATP synthesis
by the F1 complex
ATP Synthase F1 Complex
• In the F1 complex of ATP synthase, a center subunit () is surrounded
by three protein subunits: loose (L), tight (T), and open (O)
• Energy from the proton flow through F0 turns the center subunit (),
which changes the shape (conformation) of the three subunits
• As ADP and Pi enter the loose L site, the center subunit turns,
changing the L site to a tight T conformation
• ATP is formed in the T site where it remains strongly bound
• Energy from proton flow turns the center subunit, changing the T site
to an open O site, which releases the ATP
Electron Transport and ATP Synthesis
• In electron transport, the energy level decreases for electrons:
• Oxidation of NADH (Complex I) provides sufficient energy
for 3ATPs
NADH + 3ADP + 3Pi  NAD+ + 3ATP
• Oxidation of FADH2 (Complex II), which enters the chain as
a lower energy, provides sufficient energy for only 2ATPs
FADH2 + 2ADP + 2Pi  FAD
+ 2ATP
ATP from and Regulation of Electron Transport
• Low levels of ADP, Pi, oxygen, and NADH decrease electron
transport activity
• High levels of ADP activate electron transport
• As the electrons flow through decreasing energy levels, three
of the transfers provide enough energy for ATP synthesis
ATP from Glucose
• The complete oxidation of glucose yields 6CO2, 6H2O, and 36 ATP
ATP Regulation
• ATP levels are maintained through control of glucose metabolism