Electron transport system WORLD OF MICROBIOLOGY AND IMMUNOLOGY182•
EElectron transport systemLECTRON TRANSPORT SYSTEM
The electron transport system is a coordinated series of reac-
tions that operate in eukaryotic organisms and in prokaryotic
microorganisms, which enables electrons to be passed from
one protein to another. The purpose of the electron transport
system is to pump hydrogen ions to an enzyme that utilizes the
energy from the ions to manufacture the molecule known as
adenine triphosphate (ATP). ATP is essentially the fuel or
energy source for cellular reactions, providing the power to
accomplish the many varied reactions necessary for life.
The reactions of the electron transport system can also
be termed oxidative phosphorylation.
In microorganisms such as bacteriathe machinery of
the electron transport complex is housed in the single mem-
brane of Gram-positive bacteria or in the outer membrane of
Gram-negative bacteria. The electron transport process is ini-
tiated by the active, energy-requiring movement of protons
(which are hydrogen ions) from the interior gel-like cytoplasm
of the bacterium to a protein designated NADH. This protein
accepts the hydrogen ion and shuttles the ion to the exterior. In
doing so, the NADH is converted to NAD, with the conse-
quent release of an electron. The released electron then begins
a journey that moves it sequentially to a series of electron
acceptors positioned in the membrane. Each component of the
chain is able to first accept and then release an electron. Upon
the electron release, the protein is ready to accept another elec-
tron. The electron transport chain can be envisioned as a coor-
dinated and continual series of switches of its constituents
from electron acceptance to electron release mode.
The energy of the electron transport system decreases as
the electrons move “down” the chain. The effect is somewhat
analogous to water running down a slope from a higher energy
state to a lower energy state. The flow of electrons ends at the
final compound in the chain, which is called ATP synthase.
The movement of electrons through the series of reac-
tions causes the release of hydrogen to the exterior, and an
increased concentration of OH–ions (hydroxyl ions) in the
interior of the bacterium.
The proteins that participate in the flow of electrons are
the flavoproteins and the cytochromes. These proteins are
ubiquitous to virtually all prokaryotes and eukaryotesthat
have been studied.
The ATP synthase attempts to restore the equilibrium of
the hydrogen and hydronium ions by pumping a hydrogen ion
back into the cell for each electron that is accepted. The
energy supplied by the hydrogen ion is used to add a phos-
phate group to a molecule called adenine diphosphate (ADP),
generating ATP.
In aerobic bacteria, which require the presence of oxy-
gen for survival, the final electron acceptor is an atom of oxy-
gen. If oxygen is absent, the electron transport process halts.
Some bacteria have an alternate process by which energy can
be generated. But, for many aerobic bacteria, the energy pro-
duced in the absence of oxygen cannot sustain bacterial sur-
vival for an extended period of time. Besides the lack of
oxygen, compounds such as cyanide block the electron trans-
port chain. Cyanide accomplishes this by binding to one of thecytochrome components of the chain. The blockage halts ATP
production.
The flow of hydrogen atoms back through the mem-
brane of bacteria and the mitochondrial membrane of eukary-
otic cells acts to couple the electron transport system with
the formation of ATP. Peter Mitchell, English chemist
(1920–1992), proposed this linkage in 1961. He termed this
the chemiosmotic theory. The verification of the mechanism
proposed in the chemiosmotic theory earned Mitchell a 1978
Nobel Prize.See alsoBacterial membranes and cell wall; Bacterial ultra-
structure; Biochemistry; Cell membrane transportEElectrophoresisLECTROPHORESIS
Protein electrophoresis is a sensitive analytical form of chro-
matography that allows the separation of charged molecules in
a solution medium under the influence of an electric field. A
wide range of molecules may be separated by electrophoresis,
including, but not limited to DNA, RNA,and protein molecules.
The degree of separation and rate of molecular migra-
tion of mixtures of molecules depends upon the size and shape
of the molecules, the respective molecular charges, the
strength of the electric field, the type of medium used (e.g.,
cellulose acetate, starch gels, paper, agarose, polyacrylamide
gel, etc.) and the conditions of the medium (e.g., electrolyte
concentration, pH, ionic strength, viscosity, temperature, etc.).
Some mediums (also known as support matrices) are
porous gels that can also act as a physical sieve for macro-
molecules.
In general, the medium is mixed with buffers needed to
carry the electric charge applied to the system. The
medium/buffer matrix is placed in a tray. Samples of mole-
cules to be separated are loaded into wells at one end of the
matrix. As electrical current is applied to the tray, the matrix
takes on this charge and develops positively and negatively
charged ends. As a result, molecules such as DNA and RNA
that are negatively charged, are pulled toward the positive end
of the gel.
Because molecules have differing shapes, sizes, and
charges they are pulled through the matrix at different rates
and this, in turn, causes a separation of the molecules.
Generally, the smaller and more charged a molecule, the faster
the molecule moves through the matrix.
When DNA is subjected to electrophoresis, the DNA is
first broken by what are termed restriction enzymesthat act to
cut the DNA is selected places. After being subjected to
restriction enzymes, DNA molecules appear as bands (com-
posed of similar length DNA molecules) in the electrophoresis
matrix. Because nucleic acids always carry a negative charge,
separation of nucleic acids occurs strictly by molecular size.
Proteins have net charges determined by charged groups
of amino acids from which they are constructed. Proteins can
also be amphoteric compounds, meaning they can take on a
negative or positive charge depending on the surrounding con-
ditions. A protein in one solution might carry a positive chargewomi_E 5/6/03 2:12 PM Page 182