For years, scientists studying photosynthesis
have noted that electrons in photosynthetic bacteria always choose one
of two identical pathways of electron transport in the reaction center
(RC) protein, which is the factory for photosynthesis. The electrons
always go to one pigment , sometimes called the "right" side, shunning
the left. The molecule-to-molecule movement of electrons stimulated by
sunlight is called charge separation. It's the basic modus operandi of
photosynthesis, whereby plants and some bacteria use sunlight to
produce chemical energy. The reaction center protein is like a forest
with two roads. The chemists got the electrons to take the path not
traveled.
Now a husband-wife physical chemistry team at
Washington University in St. Louis and their collaborators at Stanford
University have created a mutant photosynthetic reaction center that
passes electrons along "the road not taken." And they've done so like
gangbusters. Dewey Holten, Ph.D., professor of chemistry, and
Christine Kirmaier, Ph.D., research associate professor of chemistry
in Arts & Sciences at Washington University in St. Louis, first got
bacteria to use the other side in 1995 and got a 15 percent yield, and
did so again in 1996, producing a 30 percent yield.
The find advances the understanding of
photosynthesis, which is the tool plants incorporated from bacteria to
evolve on Earth. Many other kinds of proteins, critical for human and
other life, transfer electrons, and the findings should help shed
light on how they work, among other basic issues. It also advances
multi-step electron transfer processes, which could have an impact in
solar energy conversions.
'Lazarus' protein
Holten and Kirmaier, of WUSTL, and Steven G. Boxer,
Ph.D., professor of chemistry at Stanford and his graduate student
Jessica I. Chuang, made just three changes in the reaction center
protein of the Rhodobacter capsulatus DLL mutant to make the electrons
travel to the other side with unorthodox abandon.
The results were published in the March 28, 2006
issue of Biochemistry. It was funded by the National Science
Foundation. Biochemistry highlighted the paper as one of
extreme interest, and C&E News reported on it.
In the laboratory, the mutant bacteria are kept in
the dark, fed sugar, and use respiration to make energy. Their oxygen
exposure is kept minimal. Nonetheless, they make the reaction center
protein, even though they don't use it when growing by respiration.
"This trick allows us to study RC mutations that
would be fatal to the organism if it had to live by photosynthesis, "
Kirmaier explained.
To get the electrons to go the other side, Holten
and Kirmaier altered three amino acids. In doing so, they were a bit
like shade tree mechanics puzzling over a dead car trying to make it
start.
"Even though we've been able to make electrons use
the other side before, this was different because of the high yield
and the fact that we started with something absolutely dead and made
it work from first principles," Holten said.
"It was fun," Kirmaier said. "It's a very
satisfying thing to take something that simply doesn't work and apply
the knowledge you've gained over 26 years and get results."
From here the chemists intend to make other
alterations in the reaction center and perhaps do some
retro-engineering to make even fewer changes and achieve the same, if
not higher, outcomes.
"Really, just one of the changes in redox should
have been enough," said Kirmaier. "You like to keep the native system
intact as much as possible to find out minimally what you have to do."
