Microbes and Metabolism 33ATP from ADP and phosphate, as a result of electron transfer from one com-
plex to its neighbour. These are designated site I between NADH and coenzyme
Q, site II betweencytochromes bandc, and site III betweencytochrome aand
free oxygen. Site III occurs within complex IV, the final complex which may
also be referred to ascytochrome oxidase.Its overall function is to transfer elec-
trons from cytochrome c to cytochrome a, then to a 3 and finally to molecular
oxygen. It is this final stage which is blocked by the action of cyanide and by
carbon monoxide. Associated with the electron flow, is the ejection of hydrogen
ions from inside the mitochondrion, across the membrane, and in complex IV,
the reduction of the oxygen molecule with two hydrogen ions originating from
inside the mitochondrion. If all three sites were involved, the amount of energy
released is sufficient to drive the synthesis of two and a half molecules of ATP
for each pair of electrons transported. If the first site was omitted, the number
falls to one and a half. In neither case is it a complete integer because there is not
a direct mole for mole relationship between electron transport and ATP synthesis
but as described earlier, it is part of a much more complicated process described
above as the chemiosmotic theory.
Bacterial electron transport systems and oxidative phosphorylation
Bacterial electron transport chains have fundamentally the same function as that
described for mitochondrial electron transport chains but with several notable
differences in their structure. For example, the cytochrome oxidase which is the
final complex nearest the oxygen in mitochondria, is not present in all bacteria.
The presence or absence of this complex is the basis of the ‘oxidase’ test for
the identification of bacteria. In these organisms, cytochrome oxidase is replaced
by a different set of cytochromes. An interesting example isEscherichia coli,
an enteric bacterium and coliform, which is commonly found in sewage. It has
replaced the electron carriers of cytochrome oxidase with a different set including
cytochromes b 558 ,b 595 ,b 562 , d and o, which are organised in response to the level
of oxygen in the local environment. Unlike the mitochondrial chain, the bacterial
systems may be highly branched and may have many more points for the entry
of electrons into the chain and exit of electrons to the final electron acceptor.
Bacterial electron transport systems, denitrification and methanogenesis
As previously mentioned, the term respiration is applied to many processes. With-
out further specification it is usually used to mean the consumption of molecular
oxygen, by reduction to water in the case of the electron transport discussed
above, or by oxidation of an organic molecule to produce carbon dioxide and ser-
ine in the case of photorespiration, discussed later in this chapter. Thus the term
anaerobic respiration seems a contradiction. It does, however, describe funda-
mentally the same process of electron transfer to a final acceptor which although
inorganic, in this case is not oxygen. An example of such an electron acceptor