7. Electron Transport Chain Flashcards
Oxidation-Reduction (Redox) Reactions
Transfer of electrons from an donor (reductant) to an acceptor (oxidant)
Reducing equivalent = (proton + electron) Reversible rxns NADH + H+ + pyruvate -> NAD+ + lactate NADH=reductant (gets oxidized) Pyruvate=oxidant (gets reduced)
NADH + H+ -> NAD+ + 2H+ + 2e- (oxidation half-rxn)
Pyruvate + 2H+ + 2e- -> lactate (reduction half-rxn)
NADH-NAD+ and pyruvate-lactate are redox pairs
Standard Redox Potential (E0′)
It is a measure of a redox pair’s affinity for electrons under standard conditions (1M each of oxidant and reductant, pH 7)
It’s units are volts (V)
Reversible rxns
H+ + e- -> ½ H2
E0’ = 0.0 V
Pyruvate + 2H+ + 2e- -> lactate
E0’ = -0.19 V
A negative standard redox potential means that the test reductant has a lower affinity for electrons than hydrogen
Change in Redox Potential (ΔE0′)
- Low E0′ means low affinity for electrons
- High E0′ means high affinity for electrons
*Electrons move from a carrier with lower standard redox potential to one with a higher standard redox potential
ΔE0′ is the difference between the standard redox potentials of an electron acceptor and an electron donor
ΔE0′ = E0′(acceptor) − E0’(donor)
ΔE0′ is Related to ΔG0′
Standard redox potential change can be used to calculate the standard free energy change associated with the electron transfer
ΔG0′ = - nFΔE0′
Where, ΔG0′ = Standard free energy change n = number of electrons transferred in the reaction F = Faraday constant (23.06 kcal/V) ΔE0′ = Standard redox potential change
Key Points:
Electron-transfer reactions are a source of free energy (ΔG)!
Low redox potential = Low affinity for electrons
The difference in redox potential (ΔE) between acceptor and donor determines the magnitude of ΔG
*(the more positive the ΔE, the more negative the ΔG)
Electron transfer reactions can be linked together in series in order of increasing redox potential (analogy: electrical circuit)
Mitochondria
The major energy-generating compartment in most eukaryotic cells
Compartments: a permeable outer membrane, semipermeable inner membrane folded into cristae, soluble matrix, intermembrane space
Reducing equivalents generated in the matrix, electron transport and ATP synthesis in inner membrane
Contain their own DNA genome (mtDNA is maternally inherited and in humans encodes 13 proteins of ETC) and ribosomes
Stages in Catabolism
Stage I:
- fats -> fatty acids, glycerol
- polysaccharides -> glc, other sugars
- proteins -> AA’s
Stage II:
-FA’s, glycerol, glc, other sugars, AA’s -> acetyl CoA
Stage III:
Acetyl CoA gives off CoA as it enters citric acid cycle, which gives off CO2 and 8 e-
8e- enters oxidative phosphorylation, which generates ATP (also, O2 -> H2O)
Carriers of Reducing Equivalents– NAD+ and FAD
Examples of “Reducing Energy”
NAD – a dissociable coenzyme
FAD, FMN – tightly bound prosthetic groups
Energy Yield of NADH Oxidation
(pic) NADH + H+ + ½O2 -> NAD+ + H2O ΔE0′ = 1.14 V ΔG0′ = (2)X(23.06)X(1.14) = − 52.6 kcal ΔG0′ of ATP is -7.3 kcal/mol
(sufficient energy to produce 7 ATP, if 100% efficient)
Overview of Electron Transport in Mitochondria
(2 pics)
Beg to end
1. Free E per e- dec (left y-axis)
2. Direc of e- flow inc (x-axis)
3, redox potential inc (right y-axis)
Substrate (reduced) -> product (oxidized)
Complex I- NADH dehydrogenase- prod FMNH2
Complex II- succinate dehydrogenase & CoQ (Coenzyme Q) = UQ (Ubiquinone)- succinate -> fumarate, prod FAD
Complex III- cytochrome b-c1- prod Fe2+
Complex IV- cytochrome c oxidase- prod Fe3+, H2O
ΔE0′ of ETC is Used to Pump Protons Out
A total of 10 H+ pumped out from the matrix to the intermembrane space
Leaves matrix with a negative charge (N-side) and intermembrane space with a positive charge (P-side)
Complex I: 4H+ (NADH + H+ -> NAD+)
II- succinate -> fumarate; II -> Q -> III
III- 4H+
IV: 2H+ (cyt c enters IV -> 1/2 O2 + 2H+ -> H2O)
Oxidative Phosphorylation
Influx of protons forms ATP in negative (N) matrix
Oxidation of fuels pumps protons out into positive (P) intermembrane space
NADH + 11 H+N + ½O2 -> NAD+ + H2O + 10 H+P (ΔG0΄= − 52.6 kcal)
3 ADP + 3Pi + n H+P -> 3 ATP + H2O + n H+N (ΔG0΄ = + 21.9 kcal)
ATP syn driven by proton-motive force: results from:
- Chemical potential (change in pH) (inside is alkaline)
- Electrical potential (change in psi) (inside is negative)
F0 in inner membrane brings H+ from intermembrane space (btwn inner and outer membranes) into matrix
F1 in matrix (connected to F0) converts ADP + P1 -> ATP
ATP Synthase
(pic)
Inner mito membrane- ATP synthase- 2 regions- F0, F1- creates pathway for proton movement across membrane
F0- cause rotation of F1- made of c-ring and subunits a,b,d,F6
-mainly hydrophobic- eight subunits and transmembrane ring
F1- made of alpha, beta, gamma, delta subunits
-hydrophilic, water soluble part hydrolyzes ATP
-alpha and beta subunits make hexamer w 6 binding sites (3 are catalytically inactive and bind ADP) (3 catalyze ATP syn)
-gamma, delta (part of the peripheral stalk that holds the F1 complex alpha3beta3 catalytic core stationary against the torque of the rotating central stalk), epsilon subunits part of rotational motor mechanism
»gamma allows beta to go through conformational changes i.e closed, half open and open states allows for ATP to be bound and released once synthesized
-large, seen in the transmission electron microscope by negative staining (alpha and beta hexamer unit spans 100 nm)
Energy Yield from ETC
Transport of 2e- from NADH through ETC results in 10 H+ being pumped out of matrix
One ATP is produced for each 3 H+ that come back into matrix through ATP synthase
Thus, oxidation of one NADH yields 3 ATP molecules
Electrons from FADH2 enter ETC at Complex II, causing only 6 H+ to be pumped out
Thus oxidation of one FADH2 yields 2 ATP molecules
Inhibitors Block ETC; Uncouplers Block ATP Synthesis
(pic)
Inhibitors w location- ATP synthase- oligomycin Complex I- rotenone Complex II- thenoyl-trifluroacetone Complex III- antimycin Complex IV- cyanide, carbon monoxide
Uncoupler w location-
Btwn ATP synthase and complex I: 2,4-dinitrophenol
Clicker- ETC can func to produce 2 ATPs from oxidation of succinate even in presence of which of following agents?
A: rotenone (complex I, and complex 2 acts indptly of I (2 -> 3 -> 4)
Clicker- inhibition of ETC by which of following inhibitors is relieved by 2,4-dinitrophenol? Oligomycin
uncoupler- can let you do one w.o other, proton gradient dissipated so no ATP formed
couple=ETC couple w ATP syn;
oxphos: oxi red (e transport), phos- ATP phos
H+ can only come back into matrix w ATP-shuttles H+ back and forth across membrane
Clicker- acetylsalicylate (aspirin) is antipyretic, but overdose can cause hyperthermia. Which mechanism explains inc thermogenesis w high dose aspirin?
A: electron transport is uncoupled from ATP syn
Comparison of Inhibitors and Uncouplers
Inhibitors of ETC (eg Rotenone, antimycin, CO)
- dec O2 consumption (not relieved by uncouplers)
- dec ATP prod
Inhibitors of ATP synthase (eg oligomycin)
- dec O2 consumption (relieved by uncouplers)
- dec ATP prod
Uncouplers (eg DNP)
- inc O2 consumption
- dec ATP prod
