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Molecular monogamy to protect the climate

How a vital plant protein finds its form to fix CO2

Munich, 01/20/2010

Plants remove carbon dioxide (CO2) from the air by using solar energy to combine it with water to form sugars. Rubisco, the enzyme responsible for this carbon fixation reaction is made up of only two types of protein, but the active form contains no fewer than eight copies of each. Its correct assembly requires the participation of accessory proteins called chaperonins, but details of the process have remained obscure. In a new study published in today’s issue of Nature, teams from several institutions including the LMU have used a combination of structural analysis and biochemical experimentation to dissect how the active complex is put together. Rubisco is reputed to be the most abundant enzyme in the biosphere, but it is not particularly efficient. “However, our new results should facilitate efforts to improve the enzyme’s performance,” says LMU biologist Professor Roland Beckmann, one of the responsible authors of the study. “This could lead to significant increases in plant productivity and crop yields, while enhancing the capacity of plants to take up CO2.” (Nature, 14. January 2010)

The potentially disastrous long-term effects of climate change owing to increasing levels of carbon dioxide make it imperative to find ways of removing the gas from the atmosphere. Carbon dioxide can be sequestered in sedimentary rocks, but plants and photosynthetic microorganisms also remove it from the air by combining it with water to form sugars. This carbon fixation reaction is catalyzed by the enzyme Rubisco which is made up of 16 subunits that must fold into their correct shapes and interact properly to form the functional enzyme. This process is known to require other proteins called chaperonins (so called because they accompany their initially immature charges and ensure that they behave appropriately!), but details have been hard to come by. The appearance of the new structural and biochemical information changes this situation radically.

The new study set out to assemble one specific and active version of Rubisco from the cyanobacterium Synechococcus in the test tube. When the biochemists around Beckmann included the specific chaperonin RbcX in the reaction they observed that a stable complex of the 16 subunits was formed – in the desired shape. “Actually RbcX has a collar-like structure, and appears to act as a molecular staple, binding to the C-terminal ends of some of the subunits and inducing them to adopt an ordered structure”, adds Professor Beckmann. This action of RbcX was confirmed by crystal analysis in the group in Martinsried.

In addition to fixing carbon, Rubisco catalyzes the reverse reaction, producing CO2 from oxygen. By describing conditions in which the active Rubisco enzyme can be assembled in a cell-free system, the new findings raise hopes that it will become possible to curb this activity. An “improved” version of Rubisco might be used to enhance the efficiency of agriculture and reduce its carbon footprint. In this study Professor Roland Beckmann’s group at the LMU’s Gene Center provided some of the structural data, while his colleagues at the Max Planck Institute for Biochemistry in Martinsried and collaborators in Berlin characterized the biochemistry of protein folding and assembly. (PH/suwe)

“Coupled chaperone action in folding and assembly of hexadecameric Rubisco”
Cuimin Liu, Anna L. Young, Amanda Starling-Windhof, Andreas Bracher, Sandra Saschenbrecker, Barathi Vasudeva Rao, Karnam Vasudeva Rao, Otto Berninghausen, Thorsten Mielke, F. Ulrich Hartl, Roland Beckmann, Manajit Hayer-Hartl
Nature, Vol 463, pp 197-204, January 14, 2010
doi: 10.1038/nature08651

Dr. Roland Beckmann
Gene Center Munich
Center for Integrated Protein Science Munich (CIPSM)
Department of Chemistry and Biochemistry, LMU München
Tel.: +49 89/ 2180 76900


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