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Electron transport chain
Final stage of aerobic oxidation! _________________________________
• Also known as:
-oxidative phosphorylation (when coupled to ATP synthase)
-respiration (when coupled to ATP synthase)
• Purpose:
-Recycle reduced molecules
-Convert the energy gained from ________________ into ATP
• This occurs by:
-Oxidizing NADH and FADH2
-The e- gained from the above reactions are transported through a membrane bound electron
transport system
-This generates a membrane gradient (potential)
-This potential energy allows for the phosphorylation of ADP (to generate ATP)
Recycling
The amount of cellular adenine is constant.
- It exists as either ATP, ADP, or AMP (the concentration of these vary)
- But [Adenine] remains constant.- But [Adenine] remains constant.
- So we recycle the adenine to whatever energy we need.
- Table 10.2
Same is true for NAD and FAD; constant amount
-It exist as either NAD or NADH (FAD or FADH2)
-But [FAD] or [NAD] remains constant
- So we recycle the NAD and FAD to the desired form.
glycolysis:
glucose + 2NAD+ + 2 Pi + 2 ADP
2 pyruvate + 2 ATP + 2 NADH + H+
Pyruvate dehydrogenase & TCA:
pyruvate + 4 NAD+ + FAD + GDP + 2 H2O
3 CO2 + 4 NADH + 4 H+ + Pi + GTP + FADH2
NAD+ and FAD are in short supply, so we need to recycle. A process has developed to generate
energy from this recycling!
Chemistry C483 Fall 2009 Prof Jill Paterson 33-1
• We have covered much of the background
14.2: ETC occurs in mitochondria
Know details about mitochondria
ETC proteins are present in the inner mitochondrial membrane
Transport occurs from the matrix to the intermembrane space
14.3: Membrane gradients
Just remember to switch equation to [from]/[to]
May use ‘ln’ as we have done,
or as book does, ‘log’, by putting constant of 2.303
into our equation
Use of log allows easier movement to pH, which is a
measure of H+
We may discuss decouplers later
The ETC is:
• A complex of integral membrane proteins, located in the inner mitochondrial membrane
• These proteins oxidize NADH and FADH2 by passing electrons on to O2, leading to the
production of H2O
• The energy from oxidation creates a proton gradient (protons pumped from the matrix to the • The energy from oxidation creates a proton gradient (protons pumped from the matrix to the
intermembrane space)
• This gradient provides energy for production of ATP from ADP (ATP synthase)
NADH + H+ + ½ O2 + ADP + Pi NAD+ + H2O + ATP
FADH2 + ½ O2 + ADP + Pi FAD + H2O + ATP
Chemistry C483 Fall 2009 Prof Jill Paterson 33-2
This whole system is a _______________________
Electrons move from a reducing agent to an oxidizing agent
(thus a series of redox reactions)
NADH is the strongest reducing agent in biochemistryNADH is the strongest reducing agent in biochemistry
O2 is the strongest oxidizing agent in biochemistry
Electrons flow from a negative voltage to a positive voltage
(high energy) (low energy)
This flow of e- is spontaneous and thermodynamically favorable!
Chemistry C483 Fall 2009 Prof Jill Paterson 33-3
ETC components
The ETC is composed of 4 complexes and 2 mobile electron carriers.
Complexes:
Complex I
Complex II
Complex III
Complex IV
Carriers:
Ubiquinone (Q)
Cytochrome c
Chain:
I or II Q III cyto c IV O2 H2O
See why FADH2 produces less energy??
Complex I
• NADH donates its electrons to Complex I
• Complex I has 34-46 subunits!
• NADH-ubiquinone oxidoreductase
• Contains FMN and proteins with Fe-S clusters
This is where redox occurs
• NADH transfers _______________to FMN
• FMN transfers e- to Fe-S clusters, releases H+ to matrix
• Fe-S transfers e- to coenzyme Q
• ~ 4 H+ are transported for every 2 electrons transferred to Q
Chemistry C483 Fall 2009 Prof Jill Paterson 33-4
Complex II
• FADH2 donates its electrons to Complex II
• Complex II has 3 multisubunit enzymes
• Succinate dehydrogenase
• Contains FAD and proteins with Fe-S clusters
This is where redox occurs
• FADH2 transfers _____________________
• FAD transfers e- to Fe-S clusters, releases H+ to matrix
• Fe-S transfers e- to coenzyme Q
• Complex II does not contribute to H+ transport across membrane!
Ubiquinone (Coenzyme Q)
• Not a protein!
• Membrane soluble, low molecular weight
molecule
• Contains a long hydrophobic tail that keeps
Q in the inner mitochondrial membrane
• Accepts e- one at a time (not as a hydride)
• Shuttles e- from complex I OR complex II to
complex III
Complex III
• QH2 donates its electrons to Complex III
• Complex III has 3 main subunits
• Ubiquinol-cytochrome c oxidoreductase
• Contains several cytochromes and Fe-S clusters
This is where redox occurs
• QH2 transfers _______________________
• Complex III transfers e- to cytochrome c
• ~ 4 H+ are transported for every 1 QH2 oxidized
(2 from matrix & 2 from QH2)
Chemistry C483 Fall 2009 Prof Jill Paterson 33-5
Cytochrome
Cytochrome c
• Peripheral membrane protein
• Shuttles e- (and H+) from Complex III to Complex IV
• Another heme containing protein
Complex IV
• Cytochrome c donates e to Complex IV
• Cytochrome c oxidase
• 10 subunits
• Contains 2 cytochromes (a & a3) and proteins
with Cu or Fe
This is where redox occurs
• Transfers 4 electrons to O2, to reduce O2 to H2O
• 4 H+ are transported for every O2 reduced (2 e / O
atom)atom)
BUT it takes 2 NADH to reduce 1 O2.
So for every NADH, we only get 2 H+
transported at this step
Summary of ETC
1. NADH is oxidized by Complex
I (Complex I is reduced)
2. Complex I is oxidized by Q (Q
is reduced to QH2)
1. FADH2 is oxidized by Complex
II (Complex II is reduced)
2. Complex II is oxidized by Q (Q
is reduced to QH2)
1. Succinate is oxidized by Complex
II (Complex II is reduced)
2. Complex II is oxidized by Q (Q is
reduced to QH2)
1. QH2 is oxidized by Complex III (complex III is reduced)
2. Complex III is oxidized by cytochrome c (cytochrome c is reduced)
1. Cytochrome c is oxidized by Complex IV (Complex IV is reduced)
2. Complex IV is oxidized by O2 (O2 is reduced to form H2O)
Chemistry C483 Fall 2009 Prof Jill Paterson 33-6
Generation of a membrane gradient
1. Oxidation of NADH, succinate, and FADH2 produces energy that allows H+ to be pumped
across the membrane
2. This generates an electrical AND a chemical (concentration) gradient
(electrochemical gradient)
3. End result is the matrix is negative, while the intermembrane space is positive
Linkage of gradient to ATP production
• Chemiosmotic hypothesis (Nobel Prize, 1978)
• The proton gradient IS the energy source for ATP generation
• The proton gradient is also referred to as the protonmotice force (∆p)
• H+ flow out of the matrix due to the ETC
• H+ flow back in via ATP synthase
ATP synthesis is driven by the H+ gradient
This is how NADH and FADH2 provide additional ATP
How does cytoplasmic NADH get into the mitochondria?
Chemistry C483 Fall 2009 Prof Jill Paterson 33-7
Chemistry C483 Fall 2009 Prof Jill Paterson 33-8