Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 431	Biochemistry	Fall 2001
Lecture Notes:: 26 November	© R. Paselk 2001
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Mitochondrial Electron Transport

Mitochondria: First let's review mitochondrial structure (V&V Figure 17.2 & 17.3) [overhead 18.2 P]. Recall that the inner membrane is very tight - that is the passage of polar molecules and ions is prevented without a specific transport vehicle. The inner membrane is also 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.

 

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:

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:

(Note that there is a table of biologically useful reduction potentials on pg. 501 of V&V. Recall also 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,

is very similar to the free energy equation,

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

reminds one of the free energy/equilibrium relationship,

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

It turns out that if you grind up mitochondria carefully 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 17.7, p 499 V&V). [overhead 18.8 P, top; 21.4, G&G] Only major electron carriers are shown within complexes (enclosed in brackets):

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:

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]

Pathway Diagrams

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Last modified 26 November 2001