18. Electron Transport & ATP Synthesis

Overview of Oxidative Phosphorylation

Oxidative phosphorylation is the final stage of aerobic respiration and the process by which the vast majority of cellular ATP is produced. It takes place at the inner mitochondrial membrane, which is folded into cristae to maximize surface area. The process couples two phenomena: (1) the transfer of electrons from NADH and $\text{FADH}_2$ through a series of membrane-bound protein complexes to molecular oxygen, and (2) the use of the resulting proton gradient to drive ATP synthesis.

The electrons flow through four major complexes (I–IV) in order of increasing reduction potential, ultimately reducing $\text{O}_2$ to $\text{H}_2\text{O}$. The free energy released during electron transfer is used to pump protons ($\text{H}^+$) from the matrix to the intermembrane space, establishing an electrochemical gradient called the proton-motive force. ATP synthase (Complex V) then harnesses the flow of protons back down this gradient to catalyze the phosphorylation of ADP to ATP.

The overall reaction for electron transfer from NADH to $\text{O}_2$ is:

This enormous free energy release ($-218.5$ kJ/mol) is captured in stages across the three proton-pumping complexes, rather than being released all at once. This stepwise energy extraction is the hallmark of biological elegance in energy transduction.

Electron Transport Chain Complexes

The electron transport chain (ETC) consists of four large multisubunit complexes (I–IV) embedded in the inner mitochondrial membrane, plus two mobile electron carriers: ubiquinone (coenzyme Q, CoQ) and cytochrome c. Each complex contains multiple redox-active prosthetic groups including flavins, iron-sulfur clusters, hemes, and copper centers.

Redox Potentials and Free Energy

The driving force for electron transport is the difference in standard reduction potential ($\Delta E^{\circ\prime}$) between donor and acceptor redox couples. The total redox span from NADH to $\text{O}_2$ is:

The free energy available from each electron transfer step is related to the redox potential difference by the Nernst-related equation:

where $n$ is the number of electrons transferred (2 for each NADH or $\text{FADH}_2$) and $F$ is the Faraday constant (96,485 C/mol). For each complex, the free energy captured is:

The sum of the free energies ($-70.4 - 36.7 - 112.1 = -219.2$ kJ/mol) closely matches the total $\Delta G^{\circ\prime} = -218.5$ kJ/mol, confirming that nearly all the available energy is partitioned among the three proton-pumping complexes. Complex II, which does not pump protons, captures a much smaller fraction of the total redox potential.

Chemiosmotic Theory

The chemiosmotic hypothesis, proposed by Peter Mitchell in 1961 (Nobel Prize in Chemistry, 1978), explains how the energy from electron transport is coupled to ATP synthesis. Mitchell's revolutionary insight was that the free energy of electron transfer is not used to directly drive a chemical reaction, but instead creates an electrochemical proton gradient across the inner mitochondrial membrane.

The proton-motive force (pmf, or $\Delta p$) has two components: a chemical gradient ($\Delta\text{pH}$) and an electrical potential ($\Delta\psi$):

At physiological temperature (37$^\circ$C, T = 310.15 K), the factor $2.303\,RT/F \approx 61.5$ mV, giving:

In mitochondria, the membrane potential ($\Delta\psi \approx 150\text{--}180$ mV, matrix negative) dominates over the pH gradient ($\Delta\text{pH} \approx 0.5\text{--}1.0$ units, matrix alkaline). Typical values give a total proton-motive force of approximately 180–220 mV, corresponding to a free energy of about $-21$ kJ/mol per proton translocated.

ATP Synthase (Complex V)

ATP synthase is a remarkable molecular machine — a rotary motor that converts the proton-motive force into the chemical energy of ATP. It consists of two functionally distinct domains:

Paul Boyer's binding change mechanism (Nobel Prize 1997, shared with John Walker) describes how the three $\beta$-subunits cycle through three conformational states:

Each 120$^\circ$ rotation of the $\gamma$-subunit converts one $\beta$-subunit from T $\rightarrow$ O (releasing ATP), one from O $\rightarrow$ L (binding substrates), and one from L $\rightarrow$ T (forming ATP). A full 360$^\circ$ rotation produces 3 ATP molecules. The number of $\text{H}^+$ required per full rotation equals the number of c-subunits in the c-ring.

For a c-ring with 10 subunits (yeast/mammalian consensus), approximately 10/3 $\approx$ 3.3 $\text{H}^+$ flow through F$_0$ per ATP synthesized. Including the 1 $\text{H}^+$ consumed by the phosphate carrier (which imports P$_i$ in symport with $\text{H}^+$), the total cost is approximately 4 $\text{H}^+$ per ATP. This gives the P/O ratios:

Total ATP Yield from Glucose

We can now calculate the complete ATP yield from the aerobic oxidation of one glucose molecule through glycolysis, pyruvate dehydrogenase, the TCA cycle, and oxidative phosphorylation:

*The yield from glycolytic NADH depends on which shuttle system transports the electrons into the mitochondria, since the inner mitochondrial membrane is impermeable to NADH:

Fortran: ETC Redox Potential Calculator

The following Fortran program computes the complete energetics of the electron transport chain using double-precision arithmetic. It calculates the redox potential drop ($\Delta E^{\circ\prime}$) and free energy change ($\Delta G^{\circ\prime} = -nF\Delta E^{\circ\prime}$) for each complex, the proton-motive force at 37$^\circ$C, P/O ratios, and the total ATP balance sheet for complete glucose oxidation.

Key Concepts