Theoretical studies of nonenzymatic reaction pathways for the three reaction stages of the carboxylation of ribulose-1,5-bisphosphate

Alternative reaction pathways of the nonenzymatic carboxylation of D-ribulo se-1,5-bisphosphate (RuBP) have been theoretically deduced by carrying out a series of first-principle calculations on two model systems, Several favo rable competing reaction pathways have been found. The optimized geometries of the transition states and intermediates and the calculated relative ene rgies are employed to explore the nature of transition-state stabilizing fa ctors. It has been shown that hydrogen bonding plays a key role in the tran sition state stabilization in the competing reaction pathways for addition of CO2 and H2O. Substituent effects on the calculated energy barriers are a lso very large for the addition of CO2, but less important for the subseque nt reaction stages. Including solvent effects, the energy barriers calculat ed for the addition of CO2 become significantly lower while the energy barr iers calculated for the addition of H2O and for the C-C bond cleavage step become significantly and slightly higher, respectively.