Pin1 represents an enzyme that specifically catalyzes the isomerization of peptide bonds between phosphorylated threonine or serine residues and proline. Despite its relevance as molecular timer in a number of biological processes related to cancer and Alzheimer disease, a detailed understanding of the factors contributing to the catalysis is still missing. In this study, we employ extensive QM/MM molecular dynamics simulations in combination with the mean reaction force (MRF) to discern the influence of the enzyme on the reaction mechanism and the origin of the catalysis. As a recently introduced method, the MRF separates the activation free energy barrier to reach the transition state into structural and electronic contributions providing a more detailed description of the enzyme's function. As a reference, we first study the isomerization starting from the cis form in solution and obtain a free energy barrier and a reaction free energy, which are in agreement with previous studies and experiment. With the new mean reaction force method, intramolecular hydrogen bonds in the peptide were identified that stabilize the transition state and reduce the electronic contribution to the free energy barrier. To elucidate the mechanism of catalysis of Pin I, the reaction in solution and in the catalytic cavity of the enzyme were compared. Both yield the same free energy barrier for the isomerization of the cis form, but with different decomposition in structural and electronic contributions by the mean reaction force. The enzyme reduces the energy required for structural rearrangements to reach the transition state, pointing to a destabilization of the reactant, but increases the electronic contribution to the barrier through specific enzyme-peptide hydrogen bonds. In the reverse reaction, the isomerization of the trans form, the enzyme alters the energetics and the mechanism of the reaction considerably. Unfavorable enzyme-peptide interactions in the catalytic cavity during the isomerization change the reaction coordinate, resulting in two minima with small energy differences to the transition state. These small free energy barriers should in principle make the reaction feasible at room temperature once the conformer is bound in the right conformation.