Transition structures (TSs) are characterized as saddle points of index 1 for the intramolecular proton transfer for keto-enol interconversion in simple models related to Rubisco’s substrate, D-ribulose 1,5-bisphosphate. Ab initio analytical gradients and second derivatives at a HF level of theory and 4-31G and 6-31G** basis set and AM1 semiempirical approach are used to identify the stationary points. An AM1 potential energy surface is calculated for the smallest molecular system around the TS in its cis-like conformation; a global search was done to identify TS for intramolecular hydrogen pathways related with the conformer found in the inhibitor xylulose 1,5-bisphosphate. A simple theoretical scheme is introduced to discuss enzyme-catalyzed reactions with the help of in vacuum theoretical results. The theory gives clues to understanding enzyme catalytic properties from a point of view differing from the standard scheme used until now. Finally, the present view simplifies the current mechanism of enolization in Rubisco. The complicated set of steps for enolization that are currently advocated in the literature is replaced by one step only. The structures involved not only give clues to understand the slowness of this step but also a first explanation for the slow inactivation of the enzyme during catalysis.