Electron transport system

The electron transport system is a coordinated series of reactions that operate in eukaryotic organisms and in prokaryotic microorganisms , which enables electrons to be passed from one protein to another. The purpose of the electron transport system is to pump hydrogen ions to an enzyme that utilizes the energy from the ions to manufacture the molecule known as adenine triphosphate (ATP). ATP is essentially the fuel or energy source for cellular reactions, providing the power to accomplish the many varied reactions necessary for life.

The reactions of the electron transport system can also be termed oxidative phosphorylation.

In microorganisms such as bacteria the machinery of the electron transport complex is housed in the single membrane of Gram-positive bacteria or in the outer membrane of Gram-negative bacteria. The electron transport process is initiated by the active, energy-requiring movement of protons (which are hydrogen ions) from the interior gel-like cytoplasm of the bacterium to a protein designated NADH. This protein accepts the hydrogen ion and shuttles the ion to the exterior. In doing so, the NADH is converted to NAD, with the consequent release of an electron. The released electron then begins a journey that moves it sequentially to a series of electron acceptors positioned in the membrane. Each component of the chain is able to first accept and then release an electron. Upon the electron release, the protein is ready to accept another electron. The electron transport chain can be envisioned as a coordinated and continual series of switches of its constituents from electron acceptance to electron release mode.

The energy of the electron transport system decreases as the electrons move "down" the chain. The effect is somewhat analogous to water running down a slope from a higher energy state to a lower energy state. The flow of electrons ends at the final compound in the chain, which is called ATP synthase.

The movement of electrons through the series of reactions causes the release of hydrogen to the exterior, and an increased concentration of OH ^– ions (hydroxyl ions) in the interior of the bacterium.

The proteins that participate in the flow of electrons are the flavoproteins and the cytochromes. These proteins are ubiquitous to virtually all prokaryotes and eukaryotes that have been studied.

The ATP synthase attempts to restore the equilibrium of the hydrogen and hydronium ions by pumping a hydrogen ion back into the cell for each electron that is accepted. The energy supplied by the hydrogen ion is used to add a phosphate group to a molecule called adenine diphosphate (ADP), generating ATP.

In aerobic bacteria, which require the presence of oxygen for survival, the final electron acceptor is an atom of oxygen. If oxygen is absent, the electron transport process halts. Some bacteria have an alternate process by which energy can be generated. But, for many aerobic bacteria, the energy produced in the absence of oxygen cannot sustain bacterial survival for an extended period of time. Besides the lack of oxygen, compounds such as cyanide block the electron transport chain. Cyanide accomplishes this by binding to one of the cytochrome components of the chain. The blockage halts ATP production.

The flow of hydrogen atoms back through the membrane of bacteria and the mitochondrial membrane of eukaryotic cells acts to couple the electron transport system with the formation of ATP. Peter Mitchell, English chemist (1920–1992), proposed this linkage in 1961. He termed this the chemiosmotic theory. The verification of the mechanism proposed in the chemiosmotic theory earned Mitchell a 1978 Nobel Prize.

See also Bacterial membranes and cell wall; Bacterial ultrastructure; Biochemistry; Cell membrane transport