Chemists discover why photosynthetic light-harvesting is so efficient
The disorganized arrangement of the proteins in light-harvesting
complexes is the key to their extreme efficiency

Date:
July 3, 2023
Source:
Massachusetts Institute of Technology
Summary:
Chemists have measured the energy transfer between photosynthetic
light- harvesting proteins. They discovered that the disorganized
arrangement of light-harvesting proteins boosts the efficiency of
the energy transduction.


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When photosynthetic cells absorb light from the sun, packets of energy
called photons leap between a series of light-harvesting proteins until
they reach the photosynthetic reaction center. There, cells convert
the energy into electrons, which eventually power the production of
sugar molecules.

This transfer of energy through the light-harvesting complex occurs with extremely high efficiency: Nearly every photon of light absorbed generates
an electron, a phenomenon known as near-unity quantum efficiency.

A new study from MIT chemists offers a potential explanation for how
proteins of the light-harvesting complex, also called the antenna,
achieve that high efficiency. For the first time, the researchers were
able to measure the energy transfer between light-harvesting proteins,
allowing them to discover that the disorganized arrangement of these
proteins boosts the efficiency of the energy transduction.

"In order for that antenna to work, you need long-distance energy
transduction.

Our key finding is that the disordered organization of the
light-harvesting proteins enhances the efficiency of that long-distance
energy transduction," says Gabriela Schlau-Cohen, an associate professor
of chemistry at MIT and the senior author of the new study.

MIT postdocs Dihao Wang and Dvir Harris and former MIT graduate student
Olivia Fiebig PhD '22 are the lead authors of the paper, which will
appear in the Proceedings of the National Academy of Sciences. Jianshu
Cao, an MIT professor of chemistry, is also an author of the paper.

Energy capture For this study, the MIT team focused on purple bacteria,
which are often found in oxygen-poor aquatic environments and are commonly
used as a model for studies of photosynthetic light-harvesting.

Within these cells, captured photons travel through light-harvesting
complexes consisting of proteins and light-absorbing pigments such
as chlorophyll. Using ultrafast spectroscopy, a technique that uses
extremely short laser pulses to study events that happen on timescales
of femtoseconds to nanoseconds, scientists have been able to study how
energy moves within a single one of these proteins. However, studying how energy travels between these proteins has proven much more challenging
because it requires positioning multiple proteins in a controlled way.

To create an experimental setup where they could measure how energy
travels between two proteins, the MIT team designed synthetic nanoscale membranes with a composition similar to those of naturally occurring
cell membranes. By controlling the size of these membranes, known as
nanodiscs, they were able to control the distance between two proteins
embedded within the discs.

For this study, the researchers embedded two versions of the primary
light- harvesting protein found in purple bacteria, known as LH2 and LH3,
into their nanodiscs. LH2 is the protein that is present during normal
light conditions, and LH3 is a variant that is usually expressed only
during low light conditions.

Using the cryo-electron microscope at the MIT.nano facility, the
researchers could image their membrane-embedded proteins and show that
they were positioned at distances similar to those seen in the native
membrane. They were also able to measure the distances between the light-harvesting proteins, which were on the scale of 2.5 to 3 nanometers.

Disordered is better Because LH2 and LH3 absorb slightly different
wavelengths of light, it is possible to use ultrafast spectroscopy to
observe the energy transfer between them. For proteins spaced closely
together, the researchers found that it takes about 6 picoseconds for
a photon of energy to travel between them. For proteins farther apart,
the transfer takes up to 15 picoseconds.

Faster travel translates to more efficient energy transfer, because the
longer the journey takes, the more energy is lost during the transfer.

"When a photon gets absorbed, you only have so long before that energy
gets lost through unwanted processes such as nonradiative decay,
so the faster it can get converted, the more efficient it will be," Schlau-Cohen says.

The researchers also found that proteins arranged in a lattice structure
showed less efficient energy transfer than proteins that were arranged
in randomly organized structures, as they usually are in living cells.

"Ordered organization is actually less efficient than the disordered organization of biology, which we think is really interesting because
biology tends to be disordered. This finding tells us that that may
not just be an inevitable downside of biology, but organisms may have
evolved to take advantage of it," Schlau-Cohen says.

Now that they have established the ability to measure inter-protein
energy transfer, the researchers plan to explore energy transfer between
other proteins, such as the transfer between proteins of the antenna to proteins of the reaction center. They also plan to study energy transfer between antenna proteins found in organisms other than purple bacteria,
such as green plants.

The research was funded primarily by the U.S. Department of Energy.

* RELATED_TOPICS
o Plants_&_Animals
# Cell_Biology # Molecular_Biology # Biology #
Extreme_Survival
o Matter_&_Energy
# Optics # Energy_Technology # Solar_Energy # Biochemistry
* RELATED_TERMS
o Chlorophyll o Photosynthesis o Renewable_energy o
Bioluminescence o Food_chain o Electroluminescence o Lighting
o Calorie

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Materials provided by Massachusetts_Institute_of_Technology. Original
written by Anne Trafton. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Dihao Wang, Olivia C. Fiebig, Dvir Harris, Hila Toporik, Yi Ji,
Chern
Chuang, Muath Nairat, Ashley L. Tong, John I. Ogren, Stephanie
M. Hart, Jianshu Cao, James N. Sturgis, Yuval Mazor, Gabriela
S. Schlau-Cohen.

Elucidating interprotein energy transfer dynamics within the antenna
network from purple bacteria. Proceedings of the National Academy
of Sciences, 2023; 120 (28) DOI: 10.1073/pnas.2220477120 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/07/230703160002.htm

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