An extended scheme of enzyme catalysis is presented. This highlights the process of binding leading to activated substrates and requires a detailed knowledge of the transition structure, which is a saddle point of index one on the energy hypersurface, characterizing the chemical interconversion step for the reaction in vacuo. The theory underlying the new scheme goes a step beyond the standard transition-state approach to rate processes. An AM1 characterization is presented of saddle points of index 1 for the enolization carboxylation and oxygenation steps in the molecular reaction mechanism of ribulose-1,5-bisphosphate carboxylase/oxygenase, using 3,4-dihydroxypentan-2-one as substrate model. It is shown that the transition structure of highest energy is for enolization. The successor complex of enolization is a fragment of the precursor complex for carboxylation and oxygenation. The former is in a singlet spin state, the latter is a triplet. Both reactions are ‘inevitable’ once enolization is accomplished in the distorted geometry the fragment has at the active site. Moreover, similar geometric structures are found for the D-ribulose-1,5-bisphosphate (RuBP) moiety in the transition structures of carboxylation and oxygenation showing that the precursor complexes correspond to highly deformed molecular species with respect to the ground-state structures in vacuo. The computed results allow for a simple explanation of Rubisco's bifunctionality.