In an era in which climatic change puts at risk the planet, the study and develop of alternative green chemistry which can help and improve our life can play an essential role. In this context, the use of artificial enzymes capable of substitute traditional industrial processes by environmental friendly routes is a challenge. Unfortunately, the complete understanding of the catalytic activity and selectivity of enzymes remains still elusive, thus hampering creation and development enzymatic proteins. In this paper, the molecular mechanism of the non-natural multistep retro-aldolase reaction catalysed by a de novo biocatalyst, the RA95.5-5, has been investigated by means of multiscale QM/MM methods. The design of a retro-aldolase presents the difficulty to create an enzyme being able to stabilize several transition states, maintaining low energy barriers along the overall reaction. The obtained QM/MM free energy landscape has allowed defining the rate determining step corresponding to the carbon-carbon bond scission of the substrate, which is in accordance with the experimental data. A deep analysis of the electrostatic interactions between the substrate and the different amino acid residues of the protein, as well as the estimation of the electrostatic potential generated on key atoms of the substrate, has been carried out for the key steps of the reaction. The results, compared with previous computational studies on the most efficient de novo retro-aldolase, the RA95.5-8F, explains the different activities achieved during the directed evolution process and provides insights for future developments of more efficient enzymes.