Electron transport Chain

Oxidation of carbs and fats is the major energy source for aerobes

transfers of electrons from compounds where electrons have high energy to one where electrons have low energy

high energy electrons removed from nutrients are eventually transferred to oxygen to produce water

energy difference is captured and converted to ATP

the tendency for a species to gain electrons is its reduction potential

electrons flow spontaneously from reductant (more negative E) to oxidant (more positive E)

carbs and fats have low E (reductant), oxygen has high E (oxidant)

=Eoxidation - Ereduction

= -nFdeltaE

so large positive E gives large negative G; spontaneous reaction that releases lots of energ

more C-O bonds or fewer C-H bonds

carbs are more easily oxidized that fats, so less energy is released from oxidation of a gram of carbohydrate than from a gram of fat.

major source of energy for cells under aerobic conditions

NADH and FADH2 pass electrons through several steps to oxygen which is reduced to water

released energy is used to pump H+ out of mitochondria

electron transport along the inner mitochrondrial membrane coupled to proton transport across the membrane

charge separation

electrical potential

stored energy derived from oxygen reduction by NADH and FADH2

cells use this stored energy to synthesize ATP via:
proton transfer back into the mitochondrion coupled to ATP synthase (ATPase) enzyme

stepwise electron transfer from low potential to higher potential carriers
(NADH to UQ to Cyt. C to O2)

more efficient capture of energy released by redox reactions

energy released at each step used to move H+ from inside (matrix) to outside (intermembrane space)

electron transfer complexes is from low-potential cofactors to successively higher potential cofactors

FAD and FMN: enzyme bound 2 electron carriers
dont diffuse away from the enzyme they stay tightly bound

Iron-Sulfur clusters: enzyme-bound 1-electron carriers, carry only 1 e at a time, carbon based compounds dont like having an odd number of electrons, FMN has 1 eletron left after it donates it to FeS making it highly reactive with oxygen

UQ is a diffusable 2-electron carrier that is localized to the mambrane

electron transfer from NADH to CoQ

more than 40-45 protein subunits - mass of 850 kD

PATH:
NADH to FMN to Fe-S to UQ

delta G = -80 kJ/mol, -4 protons transferred

aka succinate dehydrogenase (TCA cycle)

four subunits, including 2 Fe-S proteins

PATH:
succinate to FADH2 to 2FE2+ to UQH2

NET REACTION:
succinate + UQ to Fumarate +UQH2

delta G = -6kJ/mol, no protons transferred (not enough E to pump H+ out across membrane, exisiting in steady-state)

4Fe-4S, 3Fe-4S, 2Fe-2S

complex III uses UQH2 produced by complex I, complex II, and several other dehydrogenases to reduce cytochrome C

passes electrons from UQH2 to cyt C (and pumps H+) in a unique redox cycle known as the Q cycle

QH2 is a lipid solube electron carrier

cyt c is a water-soluble electron carrier

delta G = -35 kJ/mole, 2 H+ are translocated

the b cytochrome - with hemes bL and bH

redox potentials of heme b, c, and a differ, but depend greatly on environment on the inside of the protein

The two electrons of a bound QH2 are transferred, one to cytochrome c and the other to a bound Q to form the semiquinone Q•-. The newly formed Q dissociates and is replaced by a second QH2, which also gives up its electrons, one to a second molecule of cytochrome c and the other to reduce Q•- to QH2. This second electron transfer results in the uptake of two protons from the matrix.

electrons from cyt C are used in a four-electron reduction of O2 to produce 2 H2O

oxygen is the terminal acceptor of electrons in the electron transport pathway

cyt c oxidase utilizes 2 hemes (a and a3) and 2 copper sites (4 levels of electron carriers)
O2 only binds to Fe2+ (reduced form)

delta G = -100 kJ/mol

transports 4H+

proton translocatoin by electron-transfer complexes generates H+ gradient

stored energy

delta G = 2.3 RT*deltapH + zFdeltaV

in respiring mitochondria
delta pH = 0.75 - 1.0 and delta V = 0.15 - .2 V

to remove 1 H from the matrix to the IMS delta G = 19 - 25 kJ/mol

proton transport down electrochemical gradient (IMS to matrix) provides energy to ATP synthase

electron transfer is coupled to ATP syntheisis

no oxygen uptake or electron transfer in the absence of ADP

block NADH to O2

removes respiratory control

NADH to O2 occurs in the absence of ADP

they are membrane-permeable weak acids or ionophores
weak acids carry protons across the membranes
dissipate the proton gradient

ionophores such as valinomycin
carries ion such as K+ across the membrane (from positive side to negative side) dissipate the membrane potential

F1 is the catalytic site
three alphas and three beta subunits
three binding sites for adenine nucleotides on beta subunits

F0 is a proton channel
allows proton movement through membrane
proton movement coupled to ATP synthesis

proton movements cause the release of tightly bound ATP
movement of central gamma subunit alters conformation of beta subunits
tight to open to losse

i.e. how many ATP made per electron pair

electron transport chain yields ~10 H+ pumped out per electron pair from NADH to oxygen
4H+ flow back into matrix per ATP to cytosol
10/4 = 2.5 fpr electrons entering as NADH

for electrons entering as succinate (FADH2), ~6 H+ pumped per electron per electron pair to oxygen
6/4 = 1.5 for electrons of succinate

3 H+ to make ATP in mito and 1 more H+ to get back into cyto

will vary with energy state of the cell (non-equilibrium thermodynamics)
for redox couple: ox + ne to red
E=Eknot + RT/nF*ln([ox]/[red])
actual potential (E) depends on the ([ox]/[red]) ratio

for NAD+/NADH couple: E = -320 mV but since [NAD]/[NADH] = 50, E= -280 mV

Since [O2] = 200 mM, E = 720 mV for O2/H2o couple

delta E for NADH to O2 = 1 V and delta G = -180 kJ/mol

if cell gets 3 ATP per NADH then the efficient is ~100%

actualy yield is 2 - 2.5 ATP per NADH, depending on energy state of cell

ither transport process deplete H= gradient as well