Release Subtitle: Scientists define the role of a cyanobacterial protein in photosynthetic energy transfer process
Release Summary Text:
Photosynthesis is one of the most fundamental processes that support
life on earth. The mechanistic details of how the energy captured from
the sun is transferred within the cellular photosynthetic structures are
still not understood well. A group of scientists from Okayama
University, Japan, analyzed the structural and spectroscopic data of the
“IsiA-PSI” supercomplex, and have unraveled a part of the puzzle of
photosynthetic energy transfer in cyanobacteria.
Full text of release:
Cyanobacteria, commonly referred to as blue-green algae, are the first
organisms on earth that learned to extract electrons from water and
convert sunlight to usable energy through photosynthesis. Using
cyanobacteria as a model organism, the details of photosynthesis—the key
process that supports all forms of advanced lives on earth—have been
studied for many decades. And all studies, despite their differences,
reveal one thing: that it is an astonishingly precise process,
consisting of numerous small reactions run by many proteins and their
combinations. However, the molecular-level details of many of these
steps are still not understood very well.
The initial energy capture and transmission is one such step that still
holds many unanswered questions. At the start of a photosynthetic cycle,
special proteins on the membrane of cyanobacteria absorb the solar
energy and then transfer this energy to other cellular proteins. This is
the “light-harvesting” process. As excess energy can harm the cell,
some proteins take part in the dissipation of excess energy, a process
known as “energy quenching.” These are closely coordinated processes,
with energy transfer between different molecules taking place very
quickly, within a few tens of picosecond (1 picosecond = 1 × 10-12
second). In cyanobacteria, two reaction-systems, the Photosystems (PS) I
and II, work together to capture the energy from sunlight.
Now, in a new study published in Communications Biology, a team of
scientists led by Associate Professor Fusamichi Akita of Okayama
University, Japan, has investigated the structure of PSI of the
cyanobacterium Thermosynechococcus vulcanus, formed under an
iron-deficient condition, to decipher the role of a key protein called
iron-stress inducible A protein or “IsiA” in photosynthesis.
In the cyanobacterial membrane, IsiA appears in low iron level
conditions, and combines with a trimeric core of PSI to perform the
light-harvesting step. Much akin to a runner in a relay race, IsiA
“donates” or transfers the captured energy to the trimeric core of PSI
that performs the subsequent step in the photosynthesis process. It has
been believed that in addition to functioning an “energy harvester,”
IsiA also works as a “quencher” that gets rid of excess energy as heat
when light intensity is too high for the cells to grow.
Dr Akita explains what made them interested in the IsiA’s function,
stating “While IsiA has been considered as both the energy donor and
quencher for a long time, the path of these processes is not clear.
Moreover, the data accumulated so far suggests a different story,
indicating a contrasting possibility that in reality IsiA is involved
either in the energy transfer or quenching processes but not in both
steps.”
To answer the first question, the team first used a technique called
“single-particle cryogenic electron microscopy,” and determined the
structure of the “supercomplex” that IsiA forms with the trimeric PSI
core and many other molecules in the absence of iron. They found that 18
copies of IsiA come together to form a ring encircling the trimeric PSI
core. As a result of this arrangement, several possible energy transfer
pathways from IsiA to the PSI core were formed, and a pathway that had
the fastest rate of energy transfer was determined to serve as the main
route through which the energy moves from IsiA to the PSI core.
To solve the rest of the puzzle as to whether IsiA works as a quencher
too, the scientists used a spectroscopic technique called “femtosecond
time-resolved fluorescence decay.” The result of this study ruled out
the possibility of any energy-quenching taking place in the IsiA-PSI
core supercomplex, confirming IsiA’s role as an energy harvester and
donor.
Highlighting the significance of this exciting study, Dr Akita states,
“These structural and spectroscopic findings provide important insights
into the molecular arrangement and energy-transfer mechanisms in the
photosystems of cyanobacteria. A deeper understanding of how
photosynthetic energy transfer takes place will help to develop new
energy devices based on photosynthesis.”
Further studies comparing the inter-species differences in PSI systems
are required before we can generalize these findings, but for now, the
results of this study have laid to rest a polarizing debate on the
function of IsiA.
Release URL: https://www.eurekalert.org/pub_releases/2020-07/ou-tqo070520.php
Reference:
Title of original paper: Structure of a cyanobacterial photosystem
I surrounded by octadecameric IsiA antenna proteins
Journal: Communications Biology
DOI: http://dx.doi.org/10.1038/s42003-020-0949-6
Contact Person: Fusamichi Akita
E-mail: fusamichi_a(a)okayama-u.ac.jp
For inquiries, please contact us by replacing (a) with the @ mark.
https://www.okayama-u.ac.jp/eng/research_highlights/index_id105.html
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