Theoretical studies of nonenzymatic reaction pathways for the three reaction stages of the carboxylation of ribulose-l,5-bisphosphate

Alternative reaction pathways of the nonenzymatic carboxylation of D-ribulose-l,5-bisphosphate (RuBP) have been theoretically deduced by carrying out a series of first-principle calculations on two model systems. Several favorable competing reaction pathways have been found. The optimized geometries of the transition states and intermediates and the calculated relative energies are employed to explore the nature of transition-state stabilizing factors. It has been shown that hydrogen bonding plays a key role in the transition state stabilization in the competing reaction pathways for addition of CO₂ and H₂O. Substituent effects on the calculated energy barriers are also very large for the addition of CO₂, but less important for the subsequent reaction stages. Including solvent effects, the energy barriers calculated for the addition of CO₂ become significantly lower, while the energy barriers calculated for the addition of H₂O and for the C-C bond cleavage step become significantly and slightly higher, respectively.