BERKELEY -- In the short-term, the announcement is of consequence
only to those cell biologists whose focus is on "mitochondria,"
the subcellular structures which generate energy for living
cells. In the long-term, however, the announcement may be
recorded as a milestone on the road to discovering a way
to slow down the aging process, or a means of treating Alzheimer's
disease or any of a number of other potentially devastating
degenerative disorders.
An international team of researchers which includes scientists
with the U.S. Department of Energy's Lawrence Berkeley National
Laboratory has determined the complete crystal structure
of one of the four protein complexes in the mitochondrial
respiratory chain. Called cytochrome bc1 or complex III,
this enzyme plays a critical role in the relay of electrons
for producing energy that sustains the health of tissues,
organs, and the body as a whole.
Bing Jap, a biophysicist with Berkeley Lab's Life Sciences
Division, led the effort which took approximately eight
years. Using x-ray crystallography, Jap and his colleagues
have produced structural images of the entire 11 subunits
of cytochrome bc1 at a resolution of approximately 3 angstroms.
Images of the cytochrome bc1 complexes, which came from
cow heart cells, provide the most detailed and complete
pictures of the complex ever reported.
Every cell in the body contains hundreds of mitochondria
which, together, provide about 90-percent of the energy
that a cell needs to carry out its life processes. This
energy is produced by the transfer of electrons from molecules
of food through the respiratory chain and into the production
of ATP, the molecules that served as traveling battery packs,
delivering energy throughout the cell.
Numerous studies of the mitochondria have shown that anything
that impedes the flow of electrons through the respiratory
chain causes a decline in energy production. If uncorrected,
this energy drop-off eventually begins to debilitate the
normal operations of cells which, in turn, poses mounting
problems for tissues and organs. Impeded electron flow also
promotes the production of oxygen free-radical molecules
which can attack all components of a cell and cause mutations
in both nuclear and mitochondrial DNA.
As determined by Jap and his colleagues, the structure
of cytochrome bc1 is that of a dimer, a complex of two neighboring
molecular chains called monomers. The shape of the dimer
is considered essential to its role in the electron-transfer
process which is carried out in a hollow between its two
monomers. This work has provided structural confirmation
of the hollow's existence.
The cytochrome bc1 dimer is a non-crystalline protein embedded
in the lipids of the mitochondrial inner membrane. Coaxing
the protein into forming a crystal, a pre-requisite for
x-ray imaging, was a major challenge.
"To crystallize cytochrome bc1 we had to first purify the
protein by breaking the membrane using just the right proportion
of a specific detergent," says Jap. "It is one of the most
difficult of all crystallization techniques to carry out,
but we now have one of the finest collections of (cytochrome
bc1) crystals known. Without these high-quality crystals,
we could not have achieved success."
With the solving of the cytochrome bc1 structure, scientists
now have more than half of the mitochondrial respiratory
chain filled in. What remains is the small complex II protein,
and the very large complex I protein. Jap and his group
will join the effort underway to solve complex I. For this,
he plans to try a new strategy, one in which he will combine
x-ray and electron crystallography techniques. This approach
will include the use of multiple crystal averaging and phase
information from both heavy atom derivatives and electron
micrographs. Despite the small size of the complex II protein,
its structure will also have to be solved.
As Jap explains, "Scientists need to have the resources
to evaluate the entire structural picture of all the protein
complexes in the mitochondrial respiratory chain in order
to understand exactly what is going on. Otherwise, it is
like trying to construct a map of Berkeley by visiting only
half the city."
Collaborating with Jap on his work from Berkeley Lab were
Joong Lee and John Kyongwon Lee, both with the Life Sciences
Division. Other collaborators came from Sweden, Germany,
and France. Their results were published in the July 3,
1998 issue of the journal Science (Vol. 281, pp. 64-71).
Berkeley Lab is a U.S. Department of Energy national laboratory
located in Berkeley, California. It conducts unclassified
scientific research and is managed by the University of
California.
