Release Subtitle: New Study Resolves Mystery Surrounding Unique Light-Harvesting Structures in Algae
Release Summary Text:
Organisms capable of photosynthesis—a biochemical process that converts
solar energy into chemical energy—consist of special assemblies of
proteins and pigments that capture the light energy efficiently. These
assemblies are known as “light-harvesting complexes” (LHCs). They not
only capture the sunlight but also initiate a series of events wherein
energy is transmitted from one molecular complex to another, ultimately
“trapping” the energy in the form of chemical bonds in organic
compounds. Moreover, LHCs take part in the dissipation or “quenching” of
excess energy under strong light, to protect the cells from
light-induced damage, called photodamage. The conversion of light energy
to chemical energy takes place in two distinct photosystems,
photosystem I and II (PSI and PSII). The intriguing features of LHCs,
including their structures and relationship with other molecules in the
photosystems, have remained poorly understood.
Recently, researchers in Japan, led by Associate Professor Fusamichi
Akita from Okayama University, have published a promising study in
Nature Communications that revealed new structural details of the
LHCI-PSI complex of an aquatic microorganism. Dr Akita explains, “To
gain more insights into the process of photosynthesis, it is imperative
to reveal the structural details of the proteins involved.”
These scientists focused on a type of diatom, which is a photosynthetic
organism prevalent in aquatic environments. These organisms are a unique
type of algae—they produce almost 20% of Earth’s oxygen as a by-product
of their photosynthesis process and have developed different pigment
molecules as part of their LHCs to absorb sunlight in different
conditions. Diatoms are one of the major groups of the red lineage
organisms, and their LHC proteins are also called
fucoxanthin-chlorophyll a/c-binding proteins or FCPs, as they bind
pigments “fucoxanthin” and chlorophyll a/c. But, how FCPs fit into the
overall structure of the diatom PSI and take part in the energy flow is
not very clear.
The scientists focused on understanding the structure of a specific FCP,
called FCPI. To begin with, Dr Akita and his team captured
high-resolution images of the FCPI-PSI macromolecular assembly using a
technique called cryo-electron microscopy. They found that this complex
has 16 subunits of FCPI surrounding the core of PSI. This discovery was
exciting, as this was the largest number of LHC molecules ever reported
to form a complex with PSI. They also found that while 9 of the subunits
formed an inner ring that was connected to the core, 7 subunits formed
an outer ring that had no direct interaction with the core. Further
analysis of the structures and placement of the subunits led the
scientists to gain additional insight: two unique FCP subunits were
identified in the FCPI-PSI of the diatom, which had no counterpart in
other algae or higher plants. The scientists could also trace the
pathways by which energy transfers from one unit to another in the
complex. It was revealed that the pigment molecules present among FCPI
subunits not only transfer energy but also play a role in the quenching
of excess energy. Dr Akita explains, “The unique structural formation of
a large number of FCPI subunits in diatom PSI gives them special
capabilities of light harvesting and energy quenching in the aquatic
environment, where light is often limited and highly fluctuating in
their intensity."
Understanding the details of these unique photosynthetic complexes has
important implications in the development of advanced solar devices. Dr
Akita concludes, “Our research reveals the structural basis of an
efficient energy harvesting and transfer process. The detailed
mechanisms of light energy harvesting and utilization in photosynthesis
could serve as models for the development of novel, efficient
solar-energy utilization devices."
Release URL: https://www.eurekalert.org/pub_releases/2020-07/ou-nsr070520.php
Reference:
Title of original paper: Structural basis for assembly and function of a diatom photosystem I-light-harvesting supercomplex
Journal: Nature Communications
DOI: http://dx.doi.org/10.1038/s41467-020-16324-3
Contact Person: Fusamichi Akita
E-mail: nagaoryo(a)okayama-u.ac.jp, fusamichi_a(a)okayama-u.ac.jp
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