Fundamentals of Microbiology

3.1.7 Electron transport chain

An electron transport chain consists of a sequence of carrier molecules that are capable of oxidation and reduction. As electrons are passed through the chain, there is a step wise release of energy, which is used to drive the chemiosmotic generation of ATP.

There are three classes of carrier molecules in electron transport chains. The first are flavoproteins. These proteins contain flavin, a coenzyme derived from riboflavin (vitamin B₂), and are capable of performing alternating oxidations and reductions. One important flavin coenzyme is flavin mononucleotide (FMN). The second class of carrier molecules are cytochromes, proteins with an iron containing group (heme) capable of existing alternately as a reduced form (Fe²⁺) and an oxidized form (Fe³⁺). The cytochromes involved in electron transport chains include cytochrome b (cyt b), cytochrome c₁ (cyt c₁), cytochrome c (cyt c), cytochrome a (cyt a), and cytochrome a₃ (cyt a₃). The third class is known as ubiquinones (or coenzyme A), symbolized Q, these are small nonprotein carriers.

The electron transport chains of bacteria are somewhat diverse, in that the particular carriers used by a bacterium and the order in which they act may differ from those of other bacteria and from those of eukaryotic mitochondrial systems.

An important feature of the electron transport chain is the presence of some carriers, such as FMN and Q that accept and release protons as well as electrons, and other carriers, such as cytochromes, that transfer electrons only. Electron flow down the chain is accompanied at several points by the active transport (pumping) of protons from the matrix side of the inner mitochondrial membrane to the opposite side of the membrane. The result is a buildup of protons on one side of the membrane.

This build up of protons provides energy for the generation of ATP by the chemiosmotic mechanism.