ELECTRON TRANSPORT CHAIN (oxidative phosphorylation)

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ELECTRON TRANSPORT CHAIN(oxidative
phosphorylation)
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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 _______ and ______
• Q The metabolic pathways where NADH and FADH2
  are generated include __________ and ___________

NADH FADH2
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Carbohydrates, fatty acids, amino acids
NAD
FAD
METABOLISM
NADH H
FADH2
CO2 and H2O
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Electron Transport Chain

• Briefly
• present in the mitochondria
• electrons derived NADH and FADH2 flow through a
  series of electron carriers to oxygen
• Each carrier gets reduced as it accepts electrons
  and oxidized as it passes them on to the next
  carrier
• The energy released by the oxidation-reduction
  reactions is used to pump protons from the matrix
  to the intermembrance space forming a proton
  gradient used to produce ATP (the chemiosmotic
  hypothesis)

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Location of the ETC

• Q Where in the mitochondria is the ETC located?
  What is the composition of the mitochondrial
  matrix
• 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

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Mitochondria
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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

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COMPLEX I NADH Dehydrogenase

• Q The structural features of Complex I that
  allow it to accept 2H and 2e-s from NADH is
  _____________ and to pass the e-s on to CoQ is
  ____________

FMN
Fe-S centers
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COMPLEX I NADH Dehydrogenase

• Protein complex that spans the cell membrane. It
  contains
• a NADH binding site
• a molecule of Flavin mononucleotide (FMN)
• Ironsulfur (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

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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

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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

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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

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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

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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

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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)

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COMPLEX IIICytochrome bc1

• Accepts electrons from CoQ passes them on to
  cytochrome C
• Three protons are pumped from the matrix during
  this reaction

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COM
Cytochrome c

• Cytochrome c is loosely bound to the outer face
  of the inner membrane
• Shuttles e-s from complex III to Complex IV

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COMPLEX IVCytochrome aa3

• Q. How does cytochrome aa3 differ from the other
  complexes of the ETC (at least 2 differences)
• reacts directly with O2
• Contains Cu atoms

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COMPLEX IVCytochrome aa3

• 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

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Redox pairs

• Oxidation of a compound is always coupled with
  the reduction of another compound.
• Q Give an example of a redox pair

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Standard Reduction Potential

• The tendency of a redox pair to lose electrons
  can be specified by a constant Eo (the standard
  reduction potential)
• Q. More negative the Eo _______________ the
  potential 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

greater
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Organization of the ETC according to Eo

• Redox pair Eo
• NAD/NADH -0.32
• FMN/FMNH2 -0.22
• Cytochrome c Fe/Fe 0.22
• 1/2O2/H2O 0.82

The order of the complexes in the ETC is from the
more negative to more positive
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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
• __ ATPs are produced/molecule of NADH oxidized in
  the ETC
• __ ATPS /mol of FADH2 oxidized
• The remaining energy is lost as heat or used for
  ancillary reactions

3
2
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• No ATP generated so far!!

The flow of electrons from NADH to oxygen does
not directly result in ATP synthesis
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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!

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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.

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COMPLEX VATP Synthase
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Binding-mechanism of ATP synthase
It take at least 3 protons to produce 1 ATP
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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 patients 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.
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Inhibitors of the ETC

• Q Cyanide poisoning affects the ETC by
  inhibiting ________________

complex IV (aa3)
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Inhibitors of the ETC
CO Cyanide Sodium Azide
Rotenone
Malonate
Antimycin A
Oligomycin
INHIBITORS
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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

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Uncouplers of Oxidative Phosphorylation

• Q What is meant by uncoupling of Oxidative
  Phosphorylation?
• Electron flow through the ETC without ATP
  synthesis.

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Uncouplers of Oxidative Phosphorylation
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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

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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.
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• 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)

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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

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THE END!