ELECTRON TRANSPORT CHAIN
(oxidative phosphorylation)
Oxidative Phosphorylation
• Culmination of all aerobic-energy yielding
metabolism
• Energy from fuel oxidation is converted to the high
energy bonds of ATP, via a chain of electron
carriers
Q: The high energy molecules that transfer their
energy to ATP are _______
NADH and ______
FADH2
Q: The metabolic pathways where NADH and FADH2
are generated include __________
Glycolysis and
Kreb Cycle
___________
Carbohydrates, fatty acids, amino acids
NAD+
FAD
METABOLISM
NADH + H+
FADH2
CO2 and H2O
O2
ADP + Pi
OXIDATIVE
PHOSPHORYLATION
NADH + H+
FADH2
NAD+
FAD
ATP
H2O
Location of the ETC
Q: Where in the mitochondria is the ETC located?
What is the composition of the mitochondrial
matrix
A. The inner membrane. 50% proteins/enzymes
involved in oxidative metabolism (Kreb cycle,
oxidation of fatty acids, amino acids) , NAD+
and FAD, ADP and inorganic phosphate
Mitochondria
Organization of the ETC
Q. Which one of the following is not protein in nature?
• COMPLEX I (NADH dehydrogenase)
• COMPLEX II (Succinate dehydrogenase)
• COMPLEX III (cytochrome b-c1 complex)
• COMPLEX IV (cytochrome a1 + a3 complex)
• COMPLEX V (ATP synthase complex)
And
• Coenzyme Q
• Cytochrome C
COMPLEX I
NADH Dehydrogenase
Q: The structural features of Complex I that
allow it to accept 2H+ and 2e-s from
FMN
NADH is _____________
and to pass the
centers
e-s on to CoQ is Fe-S
____________
COMPLEX I
NADH Dehydrogenase
• Protein complex that spans the cell membrane. It
contains
–
–
–
–
a NADH binding site
a molecule of Flavin mononucleotide (FMN)
Iron–sulfur (Fe-S) centers
A binding site for CoQ
• Transfers two H atoms (2e- and 2H+) from NADH
(and H+) to FMN which become FMNH2. FMNH2
passes the electrons to the Fe-S centers which
transfer electrons to CoQ.
• Pumps four protons from the matrix into the
intermembrane space
Coenzyme Q (ubiquinone)
• Only component of the ETC that does not contain protein
• Quinone derivative with a long hydrophobic isoprenoid tail
• Transfers e-s from complex I and other flavoproteins to
Complex III
• Three other flavoproteins including complex II (succinate
dehydrogenase), glycerol 3- phosphate- and fatty acyl
CoA dehydrogenase shuttle e-s via FADH2
Coenzyme Q (ubiquinone)
Q. What is the function of the isoprenoid
tail found in CoQ?
A. The isoprenoid tail makes CoQ
lipophilic -allowing it to diffuse through
the mitochondrial membrane
COMPLEX II
Succinate Dehydrogenase
Q. How does complex II differ from the
other complexes of the ETC (at least 2
differences)
A:
It is a part of the Kreb cycle
It is not a transmembrane protein and
It does not pump protons
COMPLEX II
Succinate Dehydrogenase
• Part of the TCA cycle (Kreb cycle)
– Oxidation of succinate to fumarate, reduction
of FAD+ to FADH2
• Does not span the membrane, present
towards the matrix
• Transfers electrons from FADH2 to CoQ
• No proton pumping action
Flavoproteins
• FMN and FAD contain the water soluble
vitamin Riboflavin (B2).
• Although rare, a dietary deficiency of
riboflavin can impair the function of these
proteins and thereby the ETC
The Cytochromes
Q: What structural feature of cytochromes
allow it to accept/donate electrons:
• Cytochromes are proteins that contain a bound
heme group (an iron bound to a porphyrin ring
similar to heme in hemoglobin)
• The iron is in the form of Fe+++ rather than Fe++
(as in heme from hemogblobin)
• Accept electrons (Fe+++ gets
reduced to F++) and pass them
on (Fe++ is oxidized back to F+++)
COMPLEX III
Cytochrome bc1
• Accepts electrons from CoQ passes them on
to cytochrome C
• Three protons are pumped from the matrix
during this reaction
Cytochrome
c
COM
• Cytochrome c is loosely bound to the outer
face of the inner membrane
• Shuttles e-s from complex III to Complex IV
COMPLEX IV
Cytochrome a+a3
Q. How does cytochrome a+a3 differ from
the other complexes of the ETC (at
least 2 differences)
• reacts directly with O2
• Contains Cu atoms
COMPLEX IV
Cytochrome a+a3
• Only component of the ETC that reacts
directly with O2
• Reduces O2 to H2O by bringing together
the transported e-s, O2 and protons from
the matrix
O2 + 4e- + 4H+
2H2O
• Bound Cu atoms facilitate this reaction.
• Four protons are pumped out during this
reaction
Redox pairs
• Oxidation of a compound is always coupled
with the reduction of another compound.
• Q: Give an example of a redox pair:
NADH + H+
NAD+
FMN
FMNH2
Standard Reduction Potential
The tendency of a redox pair to lose electrons can be
specified by a constant Eo (the standard reduction
potential)
greater the potential
Q. More negative the Eo _______________
to lose electrons
• More positive the Eo greater the potential to accept
electrons
• Electrons therefore flow from the pair with the more
negative Eo to the most positive one
• The order of the complexes in the ETC is from the
more negative to more positive
Organization of the ETC according to Eo
Redox pair
NAD+/NADH
FMN/FMNH2
Cytochrome c Fe+++/Fe++
1/2O2/H2O
Eo
-0.32
-0.22
+0.22
+0.82
The order of the complexes in the ETC is from the
more negative to more positive
Free Energy of ATP
• Transport of e-s down the ETC from NADH
to O2 produces 52.8 kcal of energy
• Converting one ADP to ATP requires 7.3
Kcal/mol
• __
3 ATPs are produced/molecule of NADH
oxidized in the ETC
2 ATPS /mol of FADH2 oxidized
• __
• The remaining energy is lost as heat or
used for ancillary reactions
No ATP generated so far!!
The flow of electrons from NADH to oxygen
does not directly result in ATP synthesis
Chemiosmotic Theory
• Originally proposed by Peter
Mitchell
– was awarded the Nobel Prize
in Chemistry 1978
– when proposed, chemiosmosis
was a very radical idea and
was not well received by other
scientists!
Chemiosmotic Theory
Q: What is the Chemiosmotic Theory
• The pumping of H+ from the mitochondrial
matrix into the intermembrane space by
the complexes I, III and IV results in an
electrochemical gradient.
• The H+ can go back to the matrix only
through the ATP synthase molecules.
• This exergonic flow of H+ is used by the
enzyme to generate ATP.
COMPLEX V
ATP Synthase
Binding-mechanism of ATP
synthase
It take at least 3
protons to
produce 1 ATP
Vignette 6
Introduction: A 68-year-old female in a hypertensive crisis was being
treated in the intensive care unit (ICU) with intravenous nitroprusside
for 48 hours. The patient’s blood pressure was brought back down to
normal levels.
Presenting complaint: However, she started complaining of a burning
sensation in her throat and mouth which was followed by nausea and
vomiting, diaphoresis, agitation, and dyspnea.
On Examination: The nurse noticed an almond-like smell in her breath.
Lab investigations: An arterial blood gas revealed a significant
metabolic acidosis. A serum test suggests a metabolite of
nitroprusside, thiocyanate,is at toxic levels.
Diagnosis: Cyanide poisoning from toxic dose of nitroprusside
Treatment: Supportive therapy, gastrointestinal (GI) decontamination,
oxygen, and antidotal therapy with amyl nitrite, sodium nitrite, and
sodium thiosulfate.
Inhibitors of the ETC
Q: Cyanide poisoning affects the ETC by
complex IV (a+a3)
inhibiting ________________
Inhibitors of the ETC
Rotenone
Malonate
CO
Cyanide
Antimycin A Sodium Azide Oligomycin
INHIBITORS
Inhibitors of the ETC
Q. What would be the oxidation/reduction
status of complex I, II, III and IV incase
of Antimycin A poisoning?
• I, II, III would be reduced
• IV would be oxidized
Uncouplers of Oxidative
Phosphorylation
Q: What is meant by uncoupling of
Oxidative Phosphorylation?
• Electron flow through the ETC without
ATP synthesis.
Uncouplers of Oxidative
Phosphorylation
Uncoupling Proteins
• UCP1-5
• UCP1, also know as thermogenin is
exclusively found in brown adipose tissue
• Brown adipose tissue is abundant in
newborns and some adult mammals
• Provides body heat during cold stress in
babies and to hibernating mammals
• Uncouple the proton gradient, generating
energy in the form of heat rather than ATP
Vignette 7
An unresponsive 25-year old woman was carried to the ER by her
family. Her family members revealed that she had taken three doses of
“weight loss” pills. She developed headache, fever, chest pain, profuse
sweating and weakness soon afterwards. Initial findings were:
temperature 105.5oF, pulse 151 beats per minute, blood pressure
40/10. She died within 15 minutes. After death, rigor mortis set in
after 10 minutes and her temperature rose to 115oF after another 10
min. Among her personal effects a plastic bottle containing the weight
loss pills were found, which on analysis proved to contain
2,4,dinitrophenol.
Q. Why does DNP cause weight loss?
Decreased ATP and proton gradient
result in increased electron flow by
increasing fuel oxidation (i.e., more
carbohydrates and fats (calories) will
be consumed)
Inherited defects in the ETC
• 13 of the ~120 polypeptides required for
oxidative phosphorylation are encoded in
the mitochondrial genome
• rRNA and tRNAs required for protein
synthesis are also encoded by the
mitochondrial genome
• Mutations in these any of these genes can
cause OXPHOS diseases
THE END!