Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 33

varies from cell type to cell type, however the concentration of the cytochrome a
complex per unit area of inner membrane stays fairly constant. What changes
from cell type to cell type is the degree of infolding of the inner membrane,
such that cells requiring a large amount of energy have mitochondria which have
a very large surface area of inner membrane, which is highly convoluted thus
providing a high capacity for electron transport. The process which couples ATP
synthesis to electron transport in mitochondria and which still evades a complete
description, is oxidative phosphorylation or more accurately, respiratory-chain
phosphorylation. There are three sites within the mitochondrial chain which
span the interaction between two neighbouring complexes, which on the basis
of energy calculations are thought to witness a release of energy sufficient to
synthesise almost one molecule of ATP from ADP and phosphate, as a result of
electron transfer from one complex to its neighbour. These are designated site I
between NADH and coenzyme Q, site II between cytochromes b and c and site
III between cytochrome a and free oxygen. Site three occurs within complex
IV, the final complex which may also be referred to as cytochrome oxidase .Its
overall function is to transfer electrons 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 is Escherichia 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.