Microbes and Metabolism 31Electron transport chains: oxidative phosphorylation and methanogenesis
As described in the previous section, NADH and other reduced cofactors may
be reoxidised by the reduction of organic receptors such as pyruvate. This is the
fermentation route.
Alternatively, the reducing agent (or reductant) can transfer the electrons to an
electron transport chain which ultimately donates them to an inorganic receptor
(the oxidising agent or oxidant). In aerobic respiration, this receptor is oxygen.
However, some bacteria have electron transport chains which use other electron
sinks such as nitrate, sulphate, carbon dioxide and some metals, with respiration
being described as anaerobic in these cases. The use of nitrate in this role leads
to the process of denitrification, which plays an important part in many aspects
of the applications of environmental biotechnology.
A number of events occur during the flow of electrons along the chain
which have been observed and clearly described for a number of organisms
and organelles, most especially the mitochondria of eukaryotic cells. These are
fully discussed in many biochemistry textbooks, an excellent example being
Lehninger (1975), the gist of which is outlined in this section. The details of
exactly how these phenomena combine to drive the synthesis of ATP is still
unclear but various models have been proposed.
The chemiosmotic model, proposed by Peter Mitchell in 1961, states that the
proton, or hydrogen ion, gradient which develops across an intact membrane
during biological oxidations is the energy store for the subsequent synthesis
of ATP. This model somewhat revolutionised the then current thinking on the
energy source for many cellular processes, as the principles of energy storage
and availability according to the chemiosmotic theory were applicable to many
energy-demanding cellular phenomena including photosynthetic phosphorylation
and some cross-membrane transport systems. It could even account for the move-
ment of flagellae which propel those bacteria possessing them, through a liquid
medium. The chemiosmotic theory accounts for the coupling of the transmem-
brane proton gradient to ATP synthesis. It implies that during oxidation, the
electrons flow down from high to low energy using that energy to drive protons
across a membrane against a high concentration, thus developing the proton gra-
dient. When the electron flow stops, the protons migrate down the concentration
gradient, simultaneously releasing energy to drive the synthesis of ATP through
membrane-associated proteins. The model system described first is that of mito-
chondria and, later in this chapter, comparisons with bacterial systems associated
with oxidative phosphorylation and those systems associated with methanogenesis
will be made.
Electron transport chains comprise cytochrome molecules which trap electrons,
and enzymes which transfer electrons from a cytochrome to its neighbour. The
quantity of energy released during this transfer is sufficient to drive the synthesis
of approximately one ATP molecule by the enzymeATP synthetase. The whole
system is located in a membrane which is an essential requirement of any electron