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Electronic structure benchmark calculations of CO2 fixing elementary chemical steps in RuBisCO using the projector‐based embedding approach

Ribulose 1,5‐bisphosphate carboxylase‐oxygenase (RuBisCO) is the main enzyme involved in atmospheric carbon dioxide (CO2) fixation in the biosphere. This enzyme catalyzes a set of five chemical steps that take place in the same active‐site within magnesium (II) coordination sphere. Here, a set of electronic structure benchmark calculations have been carried out on a reaction path proposed by Gready et al. by means of the projector‐based embedding approach. Activation and reaction energies for all main steps catalyzed by RuBisCO have been calculated at the MP2, SCS‐MP2, CCSD, and CCSD(T)/aug‐cc‐pVDZ and cc‐pVDZ levels of theory. The treatment of the magnesium cation with post‐HF methods is explored to determine the nature of its involvement in the mechanism. With the high‐level ab initio values as a reference, we tested the performance of a set of density functional theory (DFT) exchange‐correlation (xc) functionals in reproducing the reaction energetics of RuBisCO carboxylase activity on a set of model fragments. Different DFT xc‐functionals show large variation in activation and reaction energies. Activation and reaction energies computed at the B3LYP level are close to the reference SCS‐MP2 results for carboxylation, hydration and protonation reactions. However, for the carbon–carbon bond dissociation reaction, B3LYP and other functionals give results that differ significantly from the ab initio reference values. The results show the applicability of the projector‐based embedding approach to metalloenzymes. This technique removes the uncertainty associated with the selection of different DFT xc‐functionals and so can overcome some of inherent limitations of DFT calculations, complementing, and potentially adding to modeling of enzyme reaction mechanisms with DFT methods.