The concept of “Proteobacteria”

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A review the basics of electron transport.

Electron transport is carried out by a series of electron carriers in an 'electron transport chain' in the cell membrane. This allows the oxidation half-reaction (electron donation) and the reduction half-reaction (electron accepting) to be physically separated, so that the energy released by the reaction can be captured.


Some electron carriers in the electron transport chain really do carry just electrons, others carry hydrogens (electrons + protons). When an electron carrier transfers an electron to a hydrogen carrier, the hydrogen carrier must capture a proton from solution; when it then transfers the hydrogen to an electron carrier, it releases the proton to solution.

The carriers in the electron transport chain are physically organized in such a way that when a carrier needs to capture a proton (when an electron carrier is donating to a hydrogen acceptor), it gets it from the cytoplasmic side of the membrane. When a carrier needs to release a proton (when a hydrogen carrier is donating to an electron acceptor), it does so at the outside surface of the membrane. The result, then, of electron transport down the chain is that protons are collected from inside and released to the outside of the membrane. In this way, the chemical energy of the oxidation/reduction reaction mediated by the chain is captured in the form of a proton gradient, rather than being entirely lost to heat as would occur if the reaction occurred in solution.

The energy in this proton gradient is in two forms: a chemical gradient (high protons outside, low protons inside) and an electrical potential (positive outside, negative inside). The energy in the proton gradient is converted back to chemical energy, which the cell can use, by ATPase. ATPase (named for the reverse reaction) is a membrane protein that leaks the protons back into the cell, using the energy of the proton gradient to phosphorylate ADP to make ATP.


The Proteobacteria

The 'purple bacteria & relatives' really are 'Proteobacteria'; they seem to have to change readily (in evolutionary terms) between sulfur oxidation or reduction, photosynthesis, heterotrophy, nitrogen oxidation or reduction, &c, &c. All of these lifestyles are based on the same electron transport chain, all that's changed are the inputs and outputs, i.e. the electron donors and acceptors of the oxidation/reduction reaction from which they derive energy.

Most heterotrophs oxidize organic compounds into CO2 to generate NADH, which serves as the electron donor for electron transport (and ultimately ATP synthesis), using O2 as the terminal electron acceptor. Sulfur oxidizing autotrophs use H2S as an electron donor (converting it to sulfate, if completely oxidized) and oxygen as the electron acceptor (generating water). Other electron donors are thiosulfate, elemental sulfur, activated photosystem chlorophylls, hydrogen, methane, and ammonia. Other electron acceptors are sulfate, sulfite, sulfur, nitrite, nitrate, ferric ion (and many other oxidized metal ions), and reduced photosystem chlorophylls. For the production of a proton gradient for ATP synthesis, the electron flow is in the 'forward' direction. Reverse electron flow is generally reserved for the synthesis of NADH by autotrophs (who don't use organic carbon to make NADH) for reducing CO2 to organic carbon (see Chapter 9).

Because all of these metabolisms are base on the same electron transport change, all that’s needed to change metabolic phenotype is to change either the enzyme that catalyses the oxidation reaction (feeding electrons into the beginning of electron transport chain) or the reduction reaction (transferring electrons from the end of the electron transport chain to the terminal electron acceptor) or both. For example, the acquisition of a single polypetide, nitrate reductase, could convert a heterotroph from obligate aerobe into an anaerobic nitrate reducer.

The proteobacteria seem to be particular good at changing the inputs and outputs of the electron transport chain, thus the latin ‘changeable Bacteria’. The genes for these enzymes feeding into and out of the electron transport chain are presumably most frequently acquired by horizontal transfer. The proteobacteria may be particularly good at acquiring such useful genes from other sources, and perhaps they are more readily able to accommodate such foreign enzymes into electron transport chain function.

In any case, the metabolic phenotype of an organism can clearly be a superficial trait, and is certainly not a reliable guide to phylogenetic relationships.