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Figure two cryo-electron microscopy images
of Rubisco complexes interacting.
If the subunits necessary for solubility are missing, individual enzyme complexes interact in this way to form a thread-like structure, the so-called fibrils
.
Under normal circumstances, however, Rubisco would not form such fibers
.
Rubisco, the core biocatalyst of photosynthesis, is the most abundant enzyme
on Earth.
By reconstructing enzymes from billions of years ago, Max Planck researchers have deciphered one of the key adaptations of
early photosynthesis.
Their findings not only provide insight into the evolution of modern photosynthesis, but also provide new impetus
for improving photosynthesis.
Life now relies entirely on photosynthetic organisms like plants and algae to capture and convertCO2.
At the heart of these processes is an enzyme called Rubisco, which can capture more than 400 billion tons ofCO2 per year
.
Living organisms today make a staggering amount of Rubisco: Rubisco on Earth has a mass that exceeds that of
all humans.
To dominate the global carbon cycle, Rubisco must constantly adapt to changing environmental conditions
.
A team at the Max Planck Institute for Terrestrial Microbiology in Marburg, Germany, in collaboration with the University of Singapore, has now succeeded in resurrecting and studying billion-year-old enzymes
in the laboratory through a combination of computational and synthetic methods.
In this process, which they call "molecular paleontology," the researchers found an entirely new component that prepares photosynthesis to accommodate rising oxygen levels, rather than a direct mutation
in the active center.
Rubisco is ancient: it appeared in primordial metabolism about 4 billion years ago, when there was no oxygen
on Earth.
However, with the invention of oxygen-producing photosynthesis and the increase in atmospheric oxygen, the enzyme began to catalyze an unwanted reaction in which it mistook O2 forCO2 producing metabolites
that were toxic to cells.
This chaotic range of substrates still scars Rubiscos to this day and limits photosynthetic efficiency
.
Although Rubiscos, which evolved in oxygenated environments, is more specific toCO2, none of them can completely escape the oxygen trapping reaction
.
The molecular determinant ofCO2 increase, namely the specificity of Rubisco, remains largely unknown
.
However, they are of great interest
to researchers working to improve photosynthesis.
Interestingly, those Rubiscos, which showed an increase inCO2, specifically recruited a new protein component
of unknown function.
The ingredient is suspected to be related to an increase inCO2, but the true reason for its emergence remains difficult to pin down because it evolved billions of years ago
.
To understand this pivotal event in the evolution of Rubiscos more specifically, collaborators at the Max Planck Institute for Terrestrial Microbiology in Marburg and Nanyang Technological University in Singapore used a statistical algorithm to recreate the form of
Rubiscos that existed billions of years ago before oxygen levels began to rise.
Researchers in Max Planck's lab resurrected these ancient proteins in the lab to study their properties
.
In particular, the scientists wondered if Rubisco's new ingredient was linked
to the evolution of higher specificity.
"We hope that the new composition will somehow directly exclude oxygen
from the Rubisco catalytic center.
" But this is not the
case.
Instead, this new subunit appears to be an evolutionary modulator: the recruitment of subunits alters the effect
of subsequent mutations on the catalyzed subunit of Rubisco.
When this new ingredient emerged, a previously inconsequential mutation suddenly had a huge impact
on specificity.
This new subunit seems to have completely changed Rubisco's evolutionary potential
.
”
The function of this "evolutionary regulator" also explains another mysterious aspect of this new protein composition: the Rubisco containing it is completely dependent on it, although other forms of Rubisco work well
without it.
The same regulatory effect explains why: When Rubisco binds to this small protein component, it becomes resistant to mutations that would otherwise cause catastrophic harm
.
As this mutation accumulates, Rubisco is effectively addicted to its new subunit
.
Together, these findings ultimately explain why Rubisco has been around
since it encountered this new protein component.
Georg Hochberg, head of Max Planck's research group, explains: "The fact that this link has only now been understood highlights the importance of
evolutionary analysis for understanding the biochemistry that drives life around us.
The history of biomolecules like Rubisco can tell us a lot about why they are what
they are today.
There are many more evolutionary histories of biochemical phenomena that we don't know
.
So it's a very exciting time for an evolutionary biochemist: almost the entire molecular history of the cell is still waiting to be discovered
.
”
The study also has important implications for how to improve photosynthesis, with Max Planck Director Tobias Erb saying: "Our study tells us that the traditional attempt to improve Rubisco may have been in the wrong place: for years, research has focused only on changing Rubisco's own amino acids to improve it
.
Our work now suggests that adding entirely new protein components to enzymes may be more effective and may open up otherwise impossible evolutionary pathways
.
Evolution of increased complexity and specificity at the dawn of form I Rubiscos
