What is it about?

Earth's most abundant protein, Rubisco, can enhance its function by being part of a protein condensate; membraneless organelles with liquid-like properties. Rubisco in condensates and specialised Rubisco compartments in cyanobacteria called carboxysomes, contribute to approximately 50% of global CO₂ capture from the atmosphere annually, playing a critical role in atmospheric CO₂ balance. This work uses a mathematical model of Rubisco compartment function with a focus on protons released by Rubisco reactions when it captures CO₂ to form sugars. Our analysis shows that the release of protons inside a Rubisco compartment helps elevate the concentration of CO₂ around the enzyme, leading to an increase in enzyme reaction rate. The results also suggest that the movement of protons in and out of the compartment is driven primarily by the Rubisco reaction substrate RuBP, and the product PGA, rather than free protons, which are present in extremely small quantities. Our model also provides key insights into how these important condensates evolved over geological timescales, suggesting that periods of low atmsopheric CO₂, and environmental niches with dim light, favour their evolution. It has been proposed previously that a period of low CO₂ in the Earth's atmosphere ~300-400 million years ago may have been the evolutionary trigger which led to Rubisco compartment evolution and what are known as CO₂ concentrating mechanisms in cyanobacteria and many algae. Our results are consistent with this idea but also raise the possibility that similar Rubisco compartments may be found in organisms growing in low light environments.

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Why is it important?

Our paper provides a novel insight into the function of liquid-liquid phase condensates of enzymes. Condensates have become an increasingly apparent means by which cells can organise cellular processes and biochemistry without the use of classical organelles. The enzyme Rubisco is responsible for the bulk of CO₂ capture from the atmosphere to the biosphere and present in a large number of organisms as liquid droplets or carboxysomes, bacterial protein microcompartments in which Rubisco is sequestered via a liquid-liquid phase condensation process during biogenesis in some cases. The contribution of these compartmentalized forms of Rubisco to global CO₂ fixation means that understanding their function is of both fundamental and academic interest. There are currently a number of crop engineering programs world-wide in which attempts are being made to consruct these Rubisco compartments in plant cell chloroplasts to improve photosynthetic efficiency and therefore crop yield. Understanding the processes which enable their efficient function is critical to our progress in improving crop production over the coming decades.

Perspectives

This article may be a little dense for the average reader to consume as it dives into some detail. Nonetheless, we provide some simpkle take-home figures and concepts which we hope inspire some thought on both Rubisco condensate and carboxysome function, and more broadly the function of protein condensates containing enzyme processes.

Ben Long
Australian National University

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This page is a summary of: Rubisco proton production can drive the elevation of CO 2 within condensates and carboxysomes, Proceedings of the National Academy of Sciences, April 2021, Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2014406118.
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