Tema 4. Electron Transportacademic.uprm.edu/~lrios/4368/Tema 4BIOL4368.pdf · Tema 4. Electron...
Transcript of Tema 4. Electron Transportacademic.uprm.edu/~lrios/4368/Tema 4BIOL4368.pdf · Tema 4. Electron...
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Tema 4. Electron Transport
Cap. 4 pages 120 -145
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Aox
Ared
INOUT
-n∆Eh = y∆p
# protons extruded -e
The generation of energy for growth-related physiological processes in respiringprokaryotes is by coupling the flow of electrons in membranes to the creation
of an electrochemical proton gradient.
Bred
Box
-n∆Eh = y∆p
# e transfered
yH+yH+
Coupling site
-e
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Who is responsible for the movement of the electrons through the membrane?
The electron carriers are either proteins or lipids!
1) Flavoproteins which carry a hydrogen and an electron
2) Quinones are lipids which carry a hydrogen and an electron
3) Iron-sulfur proteins which carry only electrons
4) Cytochromes which carry only electrons
The electrons are not carried in the protein, but in a non-protein portion called
Prosthetic group
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Prosthetic groups
1) Flavoproteins is a flavin, either flavin adenine dinucleotide (FAD) orflavin mononucleotide (FMN)
2) Iron-sulfur proteins is a cluster of iron-sulfur (FeS)
3) Cytochromes is heme.
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Flavoproteins (Fp)
Flavin comes from the Latin flavius, which means yellow.
The flavins (FAD, FMN) are synthesizedby cells from the vitamin riboflavin (B2).
FMN
FMN
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Quinones
Their hydrophobic lipid nature allow them to transfer electrons to carries that are notmobile.
UQ coenzyme Q
n = 6 to 10 The isoprenoid side chains contribute the hydrophobicity.
menaquinone
Menaquinones are derivatives of vitamin Kand differ from UQ in being a naphthoquinone
They also have a lower Ehand are used predominatelyduring anaerobic respiration
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Iron-sulfur proteinsThey contain non-heme iron andUsually acid-labile sulfur.
These proteins cover a wide rangeof potentials -400mV to +350 mV.
Example:ferredoxin
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Cytochromes
The iron is the electron carrier and is oxidizedto ferric or reduced to ferrous ion during e- transport.
Different types of heme:a, b, c, d, and o.
D and o have only been found inprokaryotic cytochrome oxidases
They can usually be identified byspectrophotometry.
Cytochrome b
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In Mitochondria
Organization of the electron carriers
Coupling sitesProton extrusion
NADH fp FeS UQ b c1c aa3
O2
E´o-320 mV
E´o+ 815 mV
Complex I Complex III Complex IV
succinate fp
FeS
Complex II
NADH dehydrogenase
Succinate dehydrogenase
Ubiquinol-cytochrome c oxidoreductase
Cytochrome aa3 oxidase
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In bacteria
AH2 dehydrogenase quinone
bc1c aa3 O2
b
o
Aerobic respiration
AH2 dehydrogenase quinone Yreductase
Anaerobic respiration
This creates flexibility, oxidases with differentaffinity to oxygen, they could differ in ∆p.
Adaptability to environmental conditions: E. colican use a reductase or an oxidase depending onoxygen availability.
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How to identify a coupling site?
NADH fp FeS
Complex I
UQ b c1
Complex III
c aa3
Complex IV
O2
succinate fp
FeS
Complex II
E´o-320 mV
E´o+ 815 mV
The # of ATP’s made for every 2e- transfer to oxygen is called the P/O ratio. It is equal to the # of ATP formed per oxygen consumed.
You could use P/2e- when the e- acceptor is not O2
NADH O2 P/O ratio = 3
succinate O2 P/O ratio = 2
succinateComplex II
Each coupling site is characterizedby a drop in midpoint potential of about 200 mV.
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The Q loop
InOut
fp
FeS
NADH + H+
NAD+2H+Site 1
Note that the electron carriersalternate between those that carry both H2 and e- and thosethat only carry e-.
2 X [e-]
QQH2 2H+
Cytochrome
oxidase
2H+
½ O2 + 2H+
2H+ 2H+H2O
Site 2
Site 3
that only carry e-.
2 X [e-]
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Q cycle ( explains observations of e- transfer in mitochondria,chloroplasts, and many bacteria)
Out
UQH
UQH2 UQUQ-O2
1e-
bL
2 H+
P site
1e-
In
UQH2
UQ- UQ
bH
UQH2
2 H+
N site
1e-
P site can by inhibited by myxothiazol and stigmatellin
N site is inhibited by antimycin.
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E. coli
NADH dehydrogenase NDH1UQ
MQ
DMQ
NO3
70%
30%
fumarate
74%
16%
10%
O2
60%
37%
3%
NADH dehydrogenase NDH2
DMQ 70% 16%37%
b562 o b558 d
O2
High affinity to O2
NDH1 and bo 8H+ NDH2 and bd 2H+
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NADH dehydrogenase NDH1 UQ bc1 c aa3O2
cyt, bb3
dehydrogenasemethanolNitrate reductase NO3
Paracoccus denitrificansNon-fermenting G-, facultative anaerobe
Nitrite reductase NO2
Nitric oxide reductase
Nitrous oxide reductase
NO
N2O
½ N2
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Wolinella succinogenesG-, anaerobe isolated from the rumen.
cty bFeSNi
H2
2 H+fumarate
+ 2H+
INOut
MK
cty bFeSMO
MO
cty b FeS
FAD
HCO2
H+ + CO2
2H+
succinate