Repairing the photosynthetic enzyme Rubisco

Date:
October 20, 2020
Source:
Max-Planck-Gesellschaft
Summary:
Researchers have elucidated how Rubisco activase works. As the name
indicates, this enzyme is critical for repairing Rubisco once it
has lost its activity.



FULL STORY ========================================================================== Manajit Hayer-Hartl, head of the research group "Chaperonin-assisted
Protein Folding," has a long-standing interest in the central enzyme of photosynthesis called Rubisco. Her team has already reported on many of
the interacting partners of Rubisco that are required for the folding
and assembly of this highly abundant protein. In the current study,
they have elucidated how Rubisco activase works. As the name indicates,
this enzyme is critical for repairing Rubisco once it has lost its
activity. The study was published in Cell.


==========================================================================
The enzyme Rubisco catalyzes the assimilation of CO2 from the atmosphere
into organic matter. This is the central step in photosynthesis
that generates sugar molecules for the production of essentially all
biomass. Despite its pivotal role, Rubisco works relatively slowly and is easily inhibited by sugar products. By improving the function of Rubisco Hayer-Hartl hopes to be able to boost the process of photosynthesis. The
goal is to address the growing global demand for food and reduce the
current greenhouse gas-induced climate change.

The enzyme Rubisco activase, Rca, is present in plants, algae and
certain cyanobacteria. Rca is a ring-shaped complex of six subunits with
a central pore. How exactly Rca interacts with the inhibited Rubisco
and releases the bound sugar from the active site pocket of Rubisco,
restoring its CO2 fixing activity, was unclear until now. With the help of biochemistry, crystallography and cryo-electron microscopy, Hayer-Hartl & colleagues have now succeeded in deciphering the molecular mechanism of
a cyanobacterial Rca.

They discovered that the Rca grabs the N-terminal tail of Rubisco
and by pulling and pushing actions, using the energy of ATP, opens
the active site pocket. This results in the release of the inhibitory
sugar molecule. In cyanobacteria Rubisco is packaged into specialized micro-compartments called carboxysomes, in which a high concentration
of CO2 is generated to facilitate the function of Rubisco.

In an earlier study, Hayer-Hartl showed how Rubisco is recruited into carboxysomes via interactions with the SSUL domains of the scaffolding
protein CcmM. Interestingly, the researchers now found that Rca is
recruited into carboxysomes using a very similar trick. The Rca hexamer
also contains SSUL domains that dock onto Rubisco during carboxysome
formation. This makes sure that enough Rca is present inside carboxysomes
to perform its essential repair function. Thus, Rca not only functions
in Rubisco activation but also mediates its own recruitment into
carboxysomes.

Manajit Hayer-Hartl concludes: "Rca is absolutely required for Rubisco
to function optimally. Deciphering its mechanism and dual function in cyanobacteria will further help us to make photosynthesis more effective
in the future. Hopefully, this will get us closer to our ultimate goal,
to increase agricultural productivity."

========================================================================== Story Source: Materials provided by Max-Planck-Gesellschaft. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Mirkko Flecken, Huping Wang, Leonhard Popilka, F. Ulrich Hartl,
Andreas
Bracher, Manajit Hayer-Hartl. Dual Functions of a Rubisco Activase
in Metabolic Repair and Recruitment to Carboxysomes. Cell, 2020;
183 (2): 457 DOI: 10.1016/j.cell.2020.09.010 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201020150512.htm

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