Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 438

Introductory Biochemistry

Spring 2007

Lecture Notes: 9 April

© R. Paselk 2006
 
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Mitochondrial Electron Transport

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 14.1 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 negative to 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:

1/2 O2 + NADH H2O + NAD+

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 sythatase 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:
 

modified from Moran & Scrimgeour Biochemistry 2nd ed. (1994) p 18.8 updated with Matti Saraste "Oxidative Phosphorylation at the fin de siécle " Science 283 (5 March 1999) pp 1488-1492.

 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 (see also Figure 14.6). [overhead] 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]:

Succinate [FAD Fe-S] Q

Glycerol-P [FAD Fe-S] Q

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

Let's review the structures of the electron transport components: NAD+/NADH; FAD/FADH2 ; Coenzyme Q/CoQ2; and FeS clusters, (Note that the common Fe4 S4 cluster seen in redox proteins has 4 links to Cys-S's.) Heme; Cytochrome c [overheads]

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. [overhead, lower] 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.

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. (Figure 14.1) [overhead] The resulting proton motive gradient can then be used to make ATP as we shall see later. Lets look at the four complexes and what seems to be occurring in each.

Complex I: [Figure 14.7, overhead] Note that the electrons are received as a pair (a hydride ion) on NADH, but most carriers can only handle single electrons so FMN acts as a transducer, picking up a pair of electrons, but passing them on singly to the FeS centers in this complex. The electrons are then passed singly on to CoQ, which, like FMN can carry either pairs or single electrons. Four H+ are pumped across the inner mitochondrial membrane by complex I. (Note that formally the protons from NADH and H+ can be considered as going to FMNH2 and the UQH2.)

Complex II: [Figure 14.9, overhead] An electron pair enters this complex via FAD going to FADH2. Of course FAD has the same active portion as FMN so it can again carry electrons single or in pairs. FADH2 then passes its electrons on singly to FeS centers, which then pass them on to CoQ. No protons are pumped by this complex. Thus we see the difference in the number of ATPs which are produced by NADH vs. FADH2.

Coenzyme Q (ubiquinone): As we've seen before 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.

Complex III: [Figure 14.11, overhead] This complex picks up single electrons from the CoQ pool, taking the quinol form in two steps to the quinone form of the coenzyme. One electron is passed on through FeS centers to Cytochrome c, while a second is recycled via cytochrome b (b566 & b560) to reduce Q to Q- in the Q-cycle (Figure 14.11) [overhead] . A second QH2 repeats the cycle and provides the second electron for the first recycled Q. The net result is the oxidation of one QH2 and the transport of four H+ for each pair of electrons flowing through the complex.

Cytochrome c: This cytochrome is a mobile protein carrier attached to the outer side of the inner mitochondrial membrane (a peripheral membrane protein). It transports single electrons from complex III to complex IV.

Complex IV: [Figure 14.13, overhead] This complex picks up electrons singly from cytochrome c and transfers them via cytochrome a-CuA to cytochrome a3-CuB where they are passed on to oxygen. Note that four electrons are needed for each oxygen molecule, and that the equivalent of two H+ are transferred out of the matrix for each pair of electrons (4 protons/oxygen). The high DG for this complex, which would allow a significantly larger number of protons to be pumped, assures the completion of the overall reaction of oxygen and NADH to give water.

(For an insider's review and evidence on ETS and OxPhos see M. Saraste Science 283 (5 March 1999) pp 1488-93)


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Last modified 9 April 2007