Alternative reaction mechanisms for C2-C3 bond-breaking and inversion of configuration at C2 within the carboxylation path of Rubisco substrate, D-ribulose 1,5-bisphosphate, are investigated. Hartree-Fock calculations were carried out at 3-21G and 6-31G** level, and 3- and 5-carbon molecular models used; electron correlation has been included for the smallest model at MP2/6-31G**. The new proposal has been mapped as a set of transition structures with putative intermediary species. The results agree with experiment. However, even if most of the mechanism can be mapped to in vacuo steps, inversion at C2 results in a number of alternatives. As the model offers a number of propensities we conclude that fine tuning the mechanism must be modulated by the enzyme. The C-C bond breaking appears to be related to formation of a carbonyl group at C3 irrespective of the mechanism used to transfer the H-atom from one of the hydroxyl groups initially bound to C3 in the gem-diol species.