Electron transport

Mitochondrial Electron Transport Chain

Electron transport chains (also called electron transfer chains) are biochemical reactions that produce ATP, which is the energy currency of life. Only two sources of energy are available to living organisms: oxidation-reduction (redox) reactions and sunlight (photosynthesis). Organisms that use redox reactions to produce ATP are called chemotrophs. Organisms that use sunlight are called phototrophs. Both chemotrophs and phototrophs use electron transport chains to convert energy into ATP.

Background

ATP is made by an enzyme called ATP synthase. The structure of this enzyme and its underlying genetic code is remarkably similar in all known forms of life.

ATP synthase is powered by a transmembrane electrochemical potential gradient, usually in the form of a proton gradient. The function of the electron transport chain is to produce this gradient. In all living organisms, a series of redox reactions is used to produce a transmembrane electrochemical potential gradient.

Redox reactions are chemical reactions in which electrons are transferred from a donor molecule to an acceptor molecule. The underlying force driving these reactions is the Gibbs free energy of the reactants and products. The Gibbs free energy is the energy available (“free”) to do work. Any reaction that decreases the overall Gibbs free energy of a system will proceed spontaneously.

The transfer of electrons from a high-energy molecule (the donor) to a lower-energy molecule (the acceptor) can be spatially separated into a series of intermediate redox reactions. This is an electron transport chain.

The fact that a reaction is thermodynamically possible does not mean that it will actually occur. A mixture of hydrogen gas and oxygen gas does not spontaneously ignite. It is necessary either to supply an activation energy, or to lower the intrinsic activation energy of the system, in order to make most biochemical reactions proceed at a useful rate. Living systems use complex macromolecular structures (enzymes) to lower the activation energies of biochemical reactions.

It is possible to couple a thermodynamically favorable reaction (a transition from a high-energy state to a lower-energy state) to a thermodynamically unfavorable reaction (such as a separation of charges, or the creation of an osmotic gradient), in such a way that the overall free energy of the system decreases (making it thermodynamically possible), while useful work is done at the same time. Biological macromolecules that catalyze a thermodynamically favorable reaction if and only if a thermodynamically unfavorable reaction occurs simultaneously underlie all known forms of life.

Electron transport chains produce energy in the form of a transmembrane electrochemical potential gradient. This energy is used to do useful work. The gradient can be used to transport molecules across membranes. It can be used to do mechanical work, such as rotating bacterial flagella. It can be used to produce ATP and NADH, high-energy molecules that are necessary for growth.

A small amount of ATP is available from substrate-level phosphorylation (for example, in glycolysis). Some organisms can obtain ATP exclusively by fermentation. In most organisms, however, the majority of ATP is generated by electron transport chains.

Electron transport chains in mitochondria

The cells of all eukaryotes (all animals, plants, fungi, algae – in other words, all living things except bacteria and archaea) contain intracellular organelles called mitochondria that produce ATP. Energy sources such as glucose are initially metabolized in the cytoplasm. The products are imported into mitochondria. Mitochondria continue the process of catabolism using metabolic pathways including the Krebs cycle, fatty acid oxidation and amino acid oxidation.

The end result of these pathways is the production of two energy-rich electron donors, NADH and FADH2. Electrons from these donors are passed through an electron transport chain to oxygen, which is reduced to water. This is a multi-step redox process that occurs on the mitochondrial inner membrane. The enzymes that catalyze these reactions have the remarkable ability to simultaneously create a proton gradient across the membrane, producing a thermodynamically unlikely high-energy state with the potential to do work.

The similarity between intracellular mitochondria and free-living bacteria is striking. The known structural, functional and DNA similarities between mitochondria and bacteria provide strong evidence that mitochondria evolved from intracellular prokaryotic symbionts that took up residence in primitive eukaryotic cells.

Mitochondrial redox carriers

Four membrane-bound complexes have been identified in mitochondria. Each is an extremely complex transmembrane structure that is embedded in the inner membrane. Three of them are proton pumps. The structures are electrically connected by lipid-soluble electron carriers and water-soluble electron carriers. The overall electron transport chain is:

Complex I

Complex I (NADH dehydrogenase) removes two electrons from NADH and transfers them to a lipid-soluble carrier, ubiquinone (Q). The reduced product, ubiquinol (QH2) is free to diffuse within the membrane. At the same time, Complex I moves four protons (H+) across the membrane, producing a proton gradient.

Complex II

Complex II (succinate dehydrogenase) is not a proton pump. It serves to funnel additional electrons into the quinone pool (Q) by removing electrons from succinate and transferring them (via FAD) to Q. Other electron donors (e.g. fatty acids and glycerol 3-phosphate) also funnel electrons into Q (via FAD), again without producing a proton gradient.

Complex III

Complex III (cytochrome bc1 complex) removes in a stepwise fashion two electrons from QH2 and transfers them to two molecules of cytochrome c, a water-soluble electron carrier located on the outer surface of the membrane. At the same time, it moves four protons across the membrane, producing a proton gradient.

Complex IV

Complex IV (cytochrome c oxidase) removes two electrons from two molecules of cytochrome c and transfers them to molecular oxygen, producing H2O. At the same time, it moves two protons across the membrane, producing a proton gradient.

Summary

The mitochondrial electron transport chain removes electrons from an electron donor (NADH or FADH2) and passes them to a terminal electron acceptor (O2) via a series of redox reactions. These reactions are coupled to the creation of a proton gradient across the mitochondrial inner membrane. There are three proton pumps: I, III and IV. The resulting transmembrane proton gradient is used to make ATP via ATP synthase.

The reactions catalyzed by Complex I and Complex III exist roughly at equilibrium. The steady-state concentrations of the reactants and products are approximately equal. This means that these reactions are readily reversible, simply by increasing the concentration of the products relative to the concentration of the reactants (for example, by increasing the proton gradient). ATP synthase is also readily reversible. Thus ATP can be used to make a proton gradient, which in turn can be used to make NADH. This process of reverse electron transport is important in many prokaryotic electron transport chains.

Summary

Electron transport chains are the source of energy for all known forms of life. They are redox reactions that transfer electrons from an electron donor to an electron acceptor. The transfer of electrons is coupled to the translocation of protons across a membrane, producing a proton gradient. The proton gradient is used to produce useful work.

The coupling of thermodynamically favorable to thermodynamically unfavorable biochemical reactions by biological macromolecules is an example of an emergent property – a property that could not have been predicted, even given full knowledge of the primitive geochemical systems from which these macromolecules evolved. It is an open question whether such emergent properties evolve only by chance, or whether they necessarily evolve in any large biogeochemical system, given the underlying laws of physics.

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