Electron Transport and
Oxidative Phosphorylation
Background
!Oxidation-reduction reactions occur thru
the transfer of electrons from a donor
(the reductant) to an acceptor (the
oxidant)
!Some reactions involve only electron
transfer (e.g. between cytochromes),
some electrons and protons (e.g.
between nucleotide cofactors)
Red-Ox Reactions
! Comprise two half-cells: transfer of electrons
from one half-cell to another
! Potential of the transfer to occur is referred to
as electrode potential calculated from
Nernst equation
! Usual convention is kcal/mol
! NADH + 0.5 O2 + H+ " NAD+ + H2O = -52.6kcal/mol
! ADP + Pi " ATP = +7.3kcal/mol
! Coupled together the oxidation of NADH
yields enough energy for ~3 moles of ATP
Process Occurs in Mitochondria
!Machinery located in inner membrane
and matrix
!Inner membrane essentially
impermeable to nucleotides, CoA,
proteins
!Different transporter complexes reside
in inner membrane
Mitochondrial Transporters
! Monocarboxylate
! Pyruvate for OH-
! Dicarboxylate
! Phosphate for malate
! Tricarboxylate
! Malate for citrate
! Phosphate
! Phosphate for OH-
! Adenine dinucleotide
! ADP for ATP
Reduced Electron Carriers
fuel for ATP production
!Carbohydrates
!Glucose oxidation to pyruvate then acetylCoA oxidation in TCA cycle
!Triglycerides
!Lipolysis and oxidation yields acetyl-CoA
for oxidation in TCA cycle
!Proteins
!Deamination and oxidation yield acetylCoA for oxidation in TCA cycle
2 Shuttle Mechanisms
for cytosolic NADH from glycolysis
!Malate-Aspartate
!Glycerol Phosphate
Malate-Aspartate Shuttle
(1/2) Glucose
Malate
+
NAD
Malate
MDH
Glyceraldehyde 3-phosphate
Pi
+
NAD
MDH
G3PDH
αKG
αKG
Asp
NADH
Asp
NADH
OAA
OAA
1,3-BPG
Glu
Cytosol
Pyruvate
Glu
Inner mito
membrane
copyright M.W.King 1996
Glycerol Phosphate Shuttle
(1/2) Glucose
Glyceraldehyde 3-phosphate
Pi
Glycerol-3-P
NAD+
G3PDH
Glycerol-3PDH
NADH
FAD
Glycerol-3PDH
DHAP
FADH
2
1,3-BPG
Pyruvate
Cytosol
Inner mito
membrane
copyright 1996 M.W.King
Mitochondria-derived Reduced
Electron Carriers
• Pyruvate dehydrogenase
• TCA cycle: IDH, α-KGDH, SDH, MDH
• Fatty acid oxidation: NADH and FADH2
Components of Mitochondrial
Electron Transport
! NAD-linked dehydrogenases
! Heme iron
! Flavin-linked dehydrogenases
! Heme iron
! Iron-sulfur proteins
! Iron bound to cysteine
! Cytochromes
! Iron in heme bound/associated with the protein
(heme a in cytochrome a, heme c in cytochrome c)
! Coenzyme Q
! Mobile component of the pathway
Coenzyme Q
OH
CH3
CH3O
CH3
CH3O
[CH2 CH C CH2]n H
OH
In mammals n=10 thus designated Q10
Complexes of Transport
!Complex I: NADH dehydrogenase
!contains FMN, iron-sulfur proteins
!Complex II: succinate dehydrogenase
!FAD, iron-sulfur proteins, cytochrome b560
!Complex III: CoQ-cytochrome reductase
!cytochromes b and c1, iron-sulfur protein
!Complex IV: cytochrome oxidase
!cytochromes a1 and a3, copper ions
Source of Electrons
!Complex I – gets NADH electrons from
MDH, α-KGDH, IDH, PDH, as well as
from fatty acid oxidation and from
cytosolic sources such as glycolysis
!Complex II – gets FADH2 from SDH or
the associated CoQ gets them from
glycerol phosphate shuttle or from fatty
acid oxidation
Flow of Electrons
!NADH oxidized by CoQ at complex I
!FADH2 oxidized by CoQ at complex II
!CoQ oxidized by cytochrome c at
complex III
!Cytochrome c oxidized by O2 at
complex IV
Chemiosmotic Coupling
! As electrons flow down electrochemical
potential, protons are pumped into the
intramembrane space
! Protons pumped out at complexes I, III and IV
! Creates a pH gradient that is relieved by
pumping protons back thru F0F1-ATP
synthase, the process is thus coupled to ATP
synthesis
Inhibition of Electron Transport
! Rotenone and the barbiturate amytal inhibit
NADH dehydrogenase in complex I
! Antimycin A (an antibiotic) inhibits
cytochrome b of complex III
! CO, azide and cyanide inhibit cytochrome
oxidase (complex IV)
! Oligomycin (a streptomyces antibiotic) inhibits
ATP synthase
! Certain uncouplers (e.g. DNP) act by
discharging the proton gradient
Use nitrites (NO2, NO3) to
convert oxyhemoglobin (Fe2+) to
methemoglobin (Fe3+)
Competes with cytochrome a,a3
for CNAlso use thiosulfate, (S2O32-)
Hormonal Uncoupling
in Brown Fat
!Dissipation of the H+ gradient generated
from electron transport, which is
uncoupled from ATP synthesis,
generates heat
!This is the physiological function of
brown adipose tissue
!Brown because of the abundance of
cytochrome containing mitochondria
!Newborns contain brown fat in neck and
upper back, acts as biological heating
pad
!Heat generation due to regulated
uncoupling thru action of thermogenin,
also called uncoupling protein (UCP),
which is a proton channel
!Hormone induced release of fatty acids
from triglycerides in brown fat leads to
activation of thermogenin
Control of ATP Production
!Principal controls are NADH/NAD+ and
ATP/ADP ratios