2. The Electron Transport Chain

Reduction Potential

Note from the previous lectures, Universal Electron Acceptors could also be called Universal Electron Donors (UED). This is what drives the flow of electrons through ETC. So, during the CAC they accept electrons  during the ETC they donate them. For these reactions, NAD+ and FAD hang out in the Mitrochondrial Matrix.

Reduction Potential (RP)

The flow of electrons between two molecules depends on their relative affinity for electrons. The RP measures an molecule’s affinity for electrons.

Electrons move to an acceptor with a higher RP than the donor: from low to high! Published tables list the standard RP for various molecules.

Electron Affinity

The standard Reduction Potential is measured at pH of 7, T = 25C, [species] = 1 mole

Spontaneity in RP

Direction of spontaneous electron flow depends on the difference of the oxidation reduction, similar to ΔG. This is an oxido-reduction (biochemical reaction) so it makes sense to change to ΔFE.

ΔE == change in Reduction Potential during transfer of electrons between two molecules in proportion to the ΔG.

ΔG = -n F ΔE, n is the number of electrons transferred, F = Faraday constant = 96.5 kJ/Vmol

ΔG ́° = -n F ΔE ́°, at standard conditions

=> ΔE > 0 => ΔG <0 => spontaneous

Conclusion: If ΔE > 0, then the reaction is spontaneous

Example:

The ΔG ́° for a redox reaction during alcoholic fermentation. The sum of two half reactions, both as reductions.

Acetaldehyde + NADH + H+ —-> Ethanol + NAD+

1) Acetaldehyde + 2H+ + 2 e —> Ethanol, E‘0 = -0.197 V

2) NAD+  + 2H+ + 2 e —> NADH +H+, E‘0 = -0.320 V

ΔE ́°  = (E‘0 electron acceptor) – (E‘0 electron donor) = +0.132V

ΔG ́° = -n F ΔE ́°  = -2(96.5 kJ/Vmol)(0.123) = -23.7 kJ/mol < 0 => spontaneous

Calculating E under non-standard conditions

RP = RPstandard + (RT/nF) * ln([acceptor]/[donor])

Example:

Acetaldehyde + NADH + H+ —-> Ethanol + NAD+

1) Acetaldehyde + 2H+ + 2e —> Ethanol

2) NAD+  + 2H+ + 2 e- —> NADH + H+

But here assume non-standard concentrations:

[acetaldehyde], [NADH]=1M
[ethanol], [NAD+] = 0.1M

Eacetald = -0.197 V + (RT/nF) * ln([acetaldehyde]/[ethanol]) = -0.167V

ENAD+ = -0.320 V + (RT/nF) * ln([NAD+]/[NADH]) = -0.350 V

Then apply the same difference as before:

ΔE  = (E electron acceptor) – (E electron donor)

= -0.167 V – (-0.350 V) = 0.183 V > 0 => spontaneous

Types of Electron Carriers in the ETC

Q, the first type

Ubiquinone (Q), a.k.a., Coenzyme Q, a.k.a., CoQ10. It has an enormously long tail. C59H90O4. Extremely hydrophobic. Lives inside the inner membrane of mitochondria. This is the fully oxidized form of this molecule.

Accepts one electron and one proton to form the semiquinone (QH*). Semi-reduced.

Accepts another e and p to form the fully reduced Ubiquinol (QH2).

The second type are Cytochromes

Classified by the Heme bound to this protein.

lecture slide from HarvardX class. All rights reserved.
lecture slide from HarvardX class. All rights reserved.

cytochrome A contains a Heme (Heme A) that has a long tail.

The Heme in cytochrome B (Heme B) is similar to the Heme found in Hemoglobin. Both cytochrome A and B are integral membrane proteins found inside the inner mitochondrial membrane.

The Heme in cytochrome C (Heme C), which is covalently attached to the protein. This is a peripheral membrane protein that has electrostatic interactions with the surface on the inner membrane in the inner membrane space (IMS)

The third type are Iron-Sulfur Centers

Each is associated with different types of proteins.

The sulfur are either free or are belong to the side chain of Cystine residues

Both Type 2 and Type 3 transfer one electron at a time.

The sequence of electron flow

Table of electron transfer values
lecture slide from HarvardX MCB63x. All rights reserved.

The electrons should (will and do) flow in the direction down the table, from low RP to high RP, from NAD+ to O2.

This can be validated by experiments with drugs that block certain of these carriers.

This is exactly why cyanide is so lethal…

ETC blockers
lecture slide from HarvardX MCB63x. All rights reserved

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