MICROBIAL METABOLISM CONTD..
The mitochondrial electron transport chain is composed of a series
of electron carriers that operate together to transfer electrons
from donors, like NADH and FADH2, to acceptors, such as O2.
The electrons flow from carriers with more negative
reduction potentials to those with more positive potentials and
eventually combine with O2 and H to form water. This pattern
of electron flow is exactly the same as seen in the electron tower
that was described in chapter 8 (see figure 8.7). The electrons
move down this potential gradient much like water flowing down
a series of rapids. The difference in reduction potentials between
O2 and NADH is large, about 1.14 volts, and makes possible the
release of a great deal of energy. The potential changes at several
points in the chain are large enough to provide sufficient energy
for ATP production, much like the energy from waterfalls can be
harnessed by waterwheels and used to generate electricity. The
electron transport chain breaks up the large overall energy release
into small steps. Some of the liberated energy is trapped in the
form of ATP. As will be seen shortly, electron transport at these points may generate proton and electrical gradients. These gradients
can then drive ATP synthesis.
The electron transport chain carriers reside within the inner
membrane of the mitochondrion or in the bacterial plasma membrane.
The mitochondrial system is arranged into four complexes
of carriers, each capable of transporting electrons part of the way
to O2 . Coenzyme Q and cytochrome c connect the complexes with each other.
The process by which energy from electron transport is used
to make ATP is called oxidative phosphorylation. Thus as many
as three ATP molecules may be synthesized from ADP and Pi
when a pair of electrons pass from NADH to an atom of O2. This
is the same thing as saying that the phosphorus to oxygen (P/O)
ratio is equal to 3. Because electrons from FADH2 only pass two
oxidative phosphorylation points, the maximum P/O ratio for
FADH2 is 2. The actual P/O ratios may be less than 3.0 and 2.0 in eucaryotic mitochondria.
The preceding discussion has focused on the eucaryotic mitochondrial
electron transport chain. Although some bacterial
chains resemble the mitochondrial chain, they are frequently very
different. They vary in their electron carriers (e.g., in their cytochromes) and may be extensively branched. Electrons often can
enter at several points and leave through several terminal oxidases.
Bacterial chains also may be shorter and have lower P/O
ratios than mitochondrial transport chains. Thus procaryotic and
eucaryotic electron transport chains differ in details of construction
although they operate using the same fundamental principles.
The electron transport chains ofi Escherichia coli and Paracoccus
denitrificans will serve as examples of these differences. A simplified
view of the E. coli transport chain is shown in figure.
Although it transports electrons from NADH to acceptors and
moves protons across the plasma membrane, the E. coli chain is
quite different from the mitochondrial chain. For example, it is
branched and contains a quite different array of cytochromes. Coenzyme
Q or ubiquinol donates electrons to both branches, but they
operate under different growth conditions. The cytochrome d
branch has very high affinity for oxygen and functions at low oxygen
levels. It is not as efficient as the cytochrome o branch because
it does not actively pump protons. The cytochrome o branch has
moderately high affinity for oxygen, is a proton pump, and operates at higher oxygen concentrations.
Fig: ETC In Mitochondria of cell