Mitochondrial Electron Transport

Mitochondria

First let's review mitochondrial structure. Recall that the inner membrane is very tight - that is the passage of polar molecules and ions is prevented without a specific transport vehicle.

Energy and Reduction Potentials

The electron transport system involves a variety of redox reactions, so it is useful to review some electrochemical relationships. First, the free energy of a reaction is related to the reduction potential for electron transfer by the equation:

G°' = - nFE°'

where n= moles of electrons transferred, and F is Faraday's constant (96,485 J V-1mol-1).

The standard reduction potential for a reaction can be found from the difference between the reduction potentials of the electron acceptors and donors:

E°'= E°'e- acceptor- E°'e- donor

(Table 10.5, p 318, in your text lists biologically useful reduction potentials. Note that the higher the positive value of the reduction potential, the greater the tendency to pick up electrons: that is, electrons flow from more negative to more positive reduction potentials. The potentials are all relative to the potential of the SHE, or Standard Hydrogen Electrode, with a defined potential of Zero V.) The Nernst equation, which describes the reduction potential for an electrochemical reaction,

E= E° '- (RT / nF) lnQ

is very similar to the free energy equation,

G = G°' + RTlnQ,

while the equation for the reduction potential for an equilibrium system,

E°' = (RT / nF) lnKeq,

reminds one of the free energy/equilibrium relationship,

G°' = - RTlnKeq.

Note the standard reduction potentials and resultant standard free energies: NADH: -0.315 V; O2: +0.815 V. So for the reaction:

we have 0.815 - (- 0.315) = 1.130 V, which, using the relationship between free energy and potential gives -218 kJ/mol. The free energy of hydrolysis of ATP is -30.5 kJ/mol, so if three ATP are made/pair of electrons flowing through ETS 91.5 kJ are captured out of 218 kJ available, or 42% . This is of course under "Standard Conditions." It is thought by some that under physiological conditions this may actually be closer to 70%.

The Electron Transport System

The inner mitochondrial membrane is protein rich. If carefully broken down we find it is very rich in five protein complexes: I -IV are large protein complexes involved in electron transport, while V is the ATP synthatase driven by proton gradients. It turns out that if you grind up mitochondria carefully the four complexes can be isolated from the inner membrane that are involved in electron transport. These complexes appear to be independently "floating" in the inner membrane. Some of the properties for mammalian complexes are listed in the table:
 Complex MW Subunits Redox Components I. NADH:ubiquinone (CoQ) oxidoreductase 900+ kD ( 'L' shaped complex) 42 or 43 subunits of unknown stoichiometry (25-26 types) 1 FMN; 7 or 8 different Fe-S centers, covalently bound lipid, 3 or more bound quinols. II. Succinate:ubiquinone (CoQ) reductase 127 kD (anchored to membrane by b cytochrome) 4 1 FAD; 3 Fe-S clusters; 1 cytochrome b 560 III. CoQ:cytochrome c reductase (Cytochrome bc1) 280 kD 11 (only 3 of which have redox centers and bacterial homologs) 2 cytochrome b; 1 Fe-S cluster; 1 cytochrome c 1 IV. Cytochrome c oxidase 400 kD dimer (13 chains each) 1 cytochrome a ; 1 CuA; 1 cytochrome a 3; 1 CuB; bound phospholipids

Electron flow through the ETS is summarized below. Only major electron carriers are shown within complexes (enclosed in brackets):

NADH [FMN Fe-S] Q [Fe-S/Cyt b Cyt c1] Cyt c [Cyt a Cyt a3] O2

Diagramatically:

Notice the central position of Coenzyme Q (aka ubiquinone, CoQ10). The pool of CoQ forms a reservoir where reducing equivalents may be stored between the various inputs to the ETS, such as Complex I or Complex II and Complex III [overhead]:

Fatty acyl-CoA Fatty acyl-CoA DHsoluble [FAD Fe-S] Q

Let's look at the structures of the electron transport components we have not seen: Coenzyme Q/CoQ2 (ubiquinone, see below);

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CoQ is a quinone with a long hydrocarbon tail. It exists as a pool of CoQ molecules dissolved in and diffusing through the lipid bilayer of the inner membrane. Both the quinone (oxidized) and quinol (reduced) form of the cofactor can "flip" in the membrane, thus the quinone ring can freely cross. CoQ can pick up electrons one at a time from either Complex I or II, then diffuse through the bilayer until collision with complex III allows it to pass them singly to that complex.

FeS clusters (Note that the common Fe4S4 cluster seen in redox proteins has 4 links to Cys-S's [Figure 7.3, p 7.3], and Cytochrome C [Figure 7.38, p 222].

With these electron transport components we can create a path for the transfer of electrons from substrate to oxygen, as above, but we want more than a wire: want to use this flow of electrons to run a couple of "motors" to pump protons across the inner mitohondrial membrane, forming an electrochemical gradient, as shown in Figure 14.6, p 424. Note that on this diagram the heights of the boxes indicates the change in standard potential or energy. From this we can see that complexes I and IV obviously have enough energy change to support the phosphorylation of ADP to ATP, while complex III is marginal and complex II obviously does not have sufficient energy change.

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But how is this energy captured? Mitochondria appear to be using the energy of moving the electrons through this potential gradient to pump protons across the inner mitochondrial membrane. The resulting proton motive gradient can then be used to make ATP as we shall see later.

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