NADH-Coenzyme Q oxidoreductase (Complex I)

The first protein complex in the electron transport chain is named NADH-Coenzyme Q oxidoreductase and is commonly labeled Complex I. It is also called NADH dehydrogenase. It is a large L-shaped multiunit protein complex which accepts the high energy electrons from NADH. The NADH molecule donates two electrons to an acceptor group flavin mononucleotide (FMN) which is found on the vertical arm of the complex as shown below. The FMN is reduced to the form FMNH₂.

From this point the electrons move along a series of iron-sulfur groups (about eight of them) and are transferred to the associated coenzyme Q (ubiquinone). The ubiquinone also extracts two protons from the matrix to form the fully reduced ubiquinol (QH₂). As the electrons are moving through the series of FeS clusters, they use the provided electrical energy to pump 4 H⁺ ions out of the mitochondrial matrix and into the intermembrane space to provide them for the production of the high energy molecule ATP in the oxidative phosphorylation process.

Flavin Mononucleotide (FMN)

Flavin mononucleotide (FMN) is a biomolecule produced from riboflavin (vitamin B2) by the enzyme riboflavin kinase and functions as the prosthetic group of various oxidoreductases, including NADH dehydrogenase. Prosthetic groups are cofactors which take an active part in enzyme catalytic processes. FMN acts as an electron carrier in Complex1 and delivers the electrons from NADH to the Fe-S for further transport.

Iron-Sulfur Clusters

Iron-sulfur proteins play a part in oxidation-reduction reactions in the electron transport chain in mitochondria and chloroplasts. Both Complex I and Complex II have multiple Fe-S clusters for electron transport.

Coenzyme Q (Ubiquinone)

Ubiquinone is often just indicated by Q in discussions of the electron transport chain. It is used to collect electrons from both Complex I and Complex II, and collects protons to form the fully reduced form ubiquinol, indicated as QH2. In the QH2 form it enters Complex III to transfer electrons to cytochrome c and pump protons into the intermembrane space of the mitochondrian.

Coenzyme Q, also known as ubiquinone, is a coenzyme family that is ubiquitous in animals and most bacteria (hence the name ubiquinone). It is a 1,4-benzoquinone, where Q refers to the quinone chemical group and 10 refers to the number of isoprenyl chemical subunits in its tail. In natural ubiquinones, the number can be anywhere from 6 to 10. It is present in all respiring eukaryotic cells, primarily in the mitochondria. It is a component of the electron transport chain and participates in aerobic cellular respiration, which generates energy in the form of ATP. Ninety-five percent of the human body's energy is generated this way (Wikipedia).

The ubiquinol form QH₂ is a two-electron carrier, but part of its role in Complex III is to transfer electrons to cytochrome c, which can only accept one electron. This difficulty is overcome partly with the use of the fact that coenzyme Q can also take the form ubisemiquinone with the capacity to transfer just one electron. This is central to its role in the electron transport through iron-sulfur complexes which can accept only one electron.

Another useful biological function of coenzyme Q is it's capacity to act as an antioxidant by scavenging free radicals.