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The structure of the L and M
subunits of the photosynthetic reaction center from Rhodobacter
sphaeroides (based on PDB entry 1PCR). The protein is represented
in purple, the cofactors are represented in red, blue, black and
yellow.
Image by Professor Neal Woodbury,
Biodesign Institute at ASU
Haiyu Wang, Neal Woodbury (l-r)
and colleagues at the Biodesign Institute at ASU have provided new
insight into the basic mechanisms of photosynthesis, a biological
process that supports all life on earth. The biodesign team has
shown that it is protein motion that determines the dynamics of
the initial electron transfer reactions of photosynthesis, rather
than static interactions between cofactors. The Biodesign
researches use an ultrafast laser facility to capture 'snapshots'
of the electron transfer at femtosecond rates, or a fifteen
millionth of a second.
Photo by Barb Backes, Biodesign
Institute at Arizona State University |
"The studies that led up to this work
initiated 20 years ago when Jim Allen and I looked at one of our
mutants and thought our spectrometer was broken," Woodbury said. "That
mutant turned out to be the first of a long series of mutations that
systematically altered the energy of the initial reaction." Since then,
Woodbury and colleagues have managed to shed light on an amazing
process that provides earth's primary power source.
To get a closer look at what was happening during
photosynthesis, the team used a well studied purple photosynthetic
bacterium called Rhodobacter sphaeroides. This type of organism was
likely one of the earliest photosynthetic bacteria to evolve. The
researchers focused their efforts by studying the center stage of
photosynthesis, the reaction center, where light energy is funneled
into specialized chlorophyll binding proteins.
The textbook picture of photosynthesis represents
the reaction center proteins as a scaffold, holding chlorophyll
molecules at a highly optimized distance and orientation so that
electrons can hop from one chlorophyll to another. With the
chlorophylls in just the right position, any systematic protein
movement was thought to be merely a side product of electrons
shuttling between chlorophyll molecules.
Woodbury and his colleagues tried to uncover more
of the physical mechanism driving photosynthesis by creating mutants
that would theoretically tweak the electron transfer relationships
between molecules in the reaction center.
"After years of failure trying to break the system
by changing the energetics, we were left with the nagging question of
how it continued to work so well," said Woodbury, ASU professor of
Chemistry and Biochemistry and director of Biodesign's Center for
Bio-Optical Nanotechnology.
The researchers started to inch closer to an answer
when Wang, a postdoctoral research associate in Woodbury's lab,
noticed something in common with all of the different mutants. When
using a new model based on reaction-diffusion kinetics, Wang saw that
the curves representing how fast electrons moved in the reaction
center had a similar shape. "He decided that there must be some sort
of underlying physical principle involved," Woodbury said.
Not many research groups are equipped to measure
the early events in photosynthesis because of the extremely short
timescale –similar to the amount of time it takes a supercomputer to
carry out a single flop. Wang was able to use the ultrafast laser
facility (funded by the National Science Foundation), which acts like
a high-speed motion picture camera that can capture data from these
lightning-fast reactions.
"He tried a really hard experiment, and he was
actually able to measure the protein motion and match it to electron
transfer," Woodbury said. This discovery helped the researchers
understand why changing the energetics didn't knock out photosynthesis.
The movement of the reaction center proteins during
photosynthesis allows the plant or bacteria to harness light energy
efficiently even if conditions aren't optimal. So, while Woodbury and
colleagues made it difficult for photosynthesis to work, the proteins
were able to compensate by moving and energetically guiding the
electrons through their biological circuit.
According to Woodbury, the reaction center proteins
work for electrons in a way similar to how a slow moving elevator with
no doors would work for people. The electrons are able to get off at
the spot that they need to because the protein motion adjusts the
energetics until it is just right. Even if the elevator starts a
little too high or low (initial energies are not optimal), the people
(electrons) can still get off on the right floor.
This way of representing the electron transfer
process successfully captured the contribution of the protein
movements to the rate of the reaction. The scientists were then able
to quantitatively model the effect of the mutations on the initial
rate of photosynthetic electron transfer and answer questions that had
been haunting them for 20 years.
The answers may be good news for the development of
organic solar cells, which have been of commercial interest due to
their relatively low cost compared to traditional silicon solar cells.
"Some of the problems that you have with the organic photovoltaics
arise from the fact that they don't work under all of the conditions
you want them to," Woodbury said.
The robustness of the natural system may offer some
useful lessons for engineers trying to improve on current technologies.
Woodbury proposed that there might be a way to increase the
flexibility of the system used in organic solar cells by incorporating
solvents that move on a variety of time scales that could "tune" the
molecules to work in a wider variety of conditions.
Woodbury also expects that this new research will
help move the study of photosynthesis forward. "It's changed the way I
look at how photosynthesis works and has opened up a whole set of new
questions," he said.
"One of the areas that we're particularly
interested in is how the absorption of light starts protein movement,"
Woodbury said. The researchers are also looking for future experiments
to help explain what sort of protein movements may be occurring in the
reaction center and then try to match these findings with current
computer models of protein movement. |
