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TRANSCRIPT
Tema 4. Electron Transport
Cap. 4 pages 120 -145
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
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
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.
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
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
Iron-sulfur proteinsThey contain non-heme iron andUsually acid-labile sulfur.
These proteins cover a wide rangeof potentials -400mV to +350 mV.
Example:ferredoxin
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
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
In bacteria
AH2 dehydrogenase quinone
bc1
c 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.
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.
The Q loop
InOut
fp
FeS
NADH + H+
NAD+
2H+Site 1Note 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-]
Q cycle ( explains observations of e- transfer in mitochondria,chloroplasts, and many bacteria)
Out
UQH
UQH2 UQUQ- O21e-
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.
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+
NADH dehydrogenase NDH1 UQ bc1c aa3
O2
cyt, bb3
dehydrogenasemethanolNitrate reductase NO3
Paracoccus denitrificansNon-fermenting G-, facultative anaerobe
Nitrite reductase NO2
Nitric oxide reductase
Nitrous oxide reductase
NO
N2O
½ N2
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