Rubisco proton production can drive the elevation of CO2 within condensates and carboxysomes

Membraneless organelles containing the enzyme ribulose-1,5bisphosphate carboxylase/oxygenase (Rubisco) are a common feature of organisms utilizing CO2 concentrating mechanisms to enhance photosynthetic carbon acquisition. In cyanobacteria and proteobacteria, the Rubisco condensate is encapsulated in a proteinaceous shell, collectively termed a carboxysome, while some algae and hornworts have evolved Rubisco condensates known as pyrenoids. In both cases, CO2 fixation is enhanced compared with the free enzyme. Previous mathematical models have attributed the improved function of carboxysomes to the generation of elevated CO2 within the organelle via a colocalized carbonic anhydrase (CA) and inwardly diffusing HCO3 ?, which have accumulated in the cytoplasm via dedicated transporters. Here, we present a concept in which we consider the net of two protons produced in every Rubisco carboxylase reaction. We evaluate this in a reaction?diffusion compartment model to investigate functional advantages these protons may provide Rubisco condensates and carboxysomes, prior to the evolution of HCO3? accumulation. Our model highlights that diffusional resistance to reaction species within a condensate allows Rubisco-derived protons to drive the conversion of HCO3? to CO2 via colocalized CA, enhancing both condensate [CO2] and Rubisco rate. Protonation of Rubisco substrate (RuBP) and product (phosphoglycerate) plays an important role in modulating internal pH and CO2 generation. Application of the model to putative evolutionary ancestors, prior to contemporary cellular HCO3? accumulation, revealed photosynthetic enhancements along a logical sequence of advancements, via Rubisco condensation, to fully formed carboxysomes. Our model suggests that evolution of Rubisco condensation could be favored under low CO2 and low light environments.

Automated summary

Membraneless organelles containing Rubisco enzyme are common in organisms using CO2 concentrating mechanisms for photosynthetic carbon acquisition. The Rubisco condensates in different organisms, like carboxysomes in cyanobacteria and proteobacteria, and pyrenoids in some algae and hornworts, enhance CO2 fixation compared to free enzyme. Mathematical models suggest that the enhanced function of carboxysomes is due to elevated CO2 levels generated within the organelle via colocalized carbonic anhydrase and inwardly diffusing bicarbonate ions.