Carbon Dioxide Fixation in RuBisCO Is Protonation-State-Dependent and Irreversible

Most CO 2 from the atmosphere is assimilated into photosynthetic organisms by the ribulose 1,5-bisphosphate carboxylase-oxygenase (RuBisCO) enzyme as part of the Calvin cycle. Despite its relevance and many efforts in the last few decades, the mechanistic picture of the catalytic CO 2 fixation reaction is still under debate. Here, we combine QM/MM molecular dynamics simulations with high-level electronic structure methods and the projector-embedding approach to provide reference values for the activation and reaction free energies of the catalytic CO 2 fixation reaction. Our results show that carboxylation is protonation-state-dependent and irreversible, making the reverse reaction (decarboxylation reaction) highly unfavorable. The carbamylated lysine residue, Kcx201, coordinated to the magnesium(II) cation in the active site plays a central role shuffling protons from and to the substrate, creating the proper reactive enolate species that adds CO 2 . The emerging microscopic picture that involves several protonation equilibria and the free-energy profile of the CO 2 fixation reaction provides insights that may be used in the future to improve enzymatic efficiency in RuBisCO.