Most CO2 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
CO2 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
CO2 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
CO2. The emerging microscopic picture that involves several
protonation equilibria and the free-energy profile of the CO2 fixation reaction provides insights that may be used in the future
to improve enzymatic efficiency in RuBisCO.