Electron Transport Chain

Overview of the electron transport chain

Electrons are passed between 4 protein complexes embedded in the mitochondrial membrane. In the process H+’s are pumped into the intermembrane space creating a proton gradient. Complex V converts this chemiosmotic gradient into ATP

What happens to NADH and FADH2

are oxidized, passing electrons through a series of carriers, and ultimately to O2

Fe4S4

4 Fe’s complexed with the 4 sulfurs of 4 Cys and 4 sulfides (inorganic sulfur)

FADH or FADH2

carries 1 or 2 electrons

Coenzyme Q

very lipophilic so it diffuses in the membrane

Oxidation and reduction of coenzyme Q occurs one electron at a time, like the flavins

heme from cytochrome b c and c1

Is the same as found in hemoglobin and myoglobin.

The heme is attached to 2 Cys residues of the protein.

Found in Complex II and III

The heme from cytochrome a and a3

has 2 modified side chains

Found in Complex IV

Electrons are passed from

high reduction potential carriers to lower reduction potential carriers

Complex I

NADH-Ubiquinone Oxidoreductase

Prosthetic groups: FMN, Fe-S on the enzyme.

2 electrons from NADH are transferred to FMN, which transfers them one at a time to a series of iron-sulfur clusters, and finally to CoQH2.

In the process, 4 protons are pumped from the matrix to the intermembrane space through conformational changes that change the pKa’s of side chains

Complex II

Succinate-Ubiquinone Oxidoreductase

Adds electrons but no H+’s. Prosthetic groups: FAD, Fe-S

In the TCA cycle, 2 electrons are extracted from succinate to FADH2 These electrons are transferred through a series of iron-sulfur clusters to CoQ

Electron Transferring flavoprotein

ETF-Q oxidoreductase receives electrons from over 12 FAD-containing dehydrogenases from fatty acid and amino acid oxidation

Complex III

Ubiquinol-Cytochrome c Oxidoreductase

Transfers electrons from CoQH2to cytochrome c

Prosthetic groups: Fe-S, cytochrome bH, cytochrome bL, cytochrome c1

Dimer with each monomer of the dimer composed of 10-11 polypeptides

4 protons are pumped into intermembrane space (IMS), from QH2

The Q-cycle

Path of electrons through Complex III. Electrons are transferred from the 2-electron donor, CoQH2, to cytochromes which are one electron acceptors

Complex IV

Cytochrome c Oxidase
Cytochrome c transfers electrons to Complex IV

Composed of 13 polypeptides, 28 transmembrane helices

Prosthetic groups: Cytochrome a, Cytochrome a3, CuA, CuB 4 protons pumped into intermembrane space for every O2 reduced

Complex IV net reaction

Cytochrome c mechanism

4 electrons from 4 cytochrome c molecules are used to reduce O2 to H2O

Cytochome c step 1

2 molecules of cyt c sequentially transfer e- to CuB and hemea3

Cytochome c step 2

Reduced CuB and Fe in heme a3 bind O2, forming a peroxide bridge

Cytochome c step 3

2 more molecules of cyt c add 2 more e and 2 H+, cleaving the peroxide bridge

Cytochome c step 4

2 more H+ are added from matrix, generating H2O