Insights into the Catalytic Mechanism and Selectivity of S-Adenosyl Methionine (SAM)-Dependent Fluorinase toward Carbon-Halogen Bond Formation

Halogenases are enzymes that incorporate halogens with high regioselectivity into biosynthetic precursors. Fluorinase, an S-adenosyl methionine (SAM)-dependent halogenase, catalyzes the formation of carbon-halogen bonds through an S_N2 reaction, converting SAM into 5′-fluoro-5′-deoxyadenosine (5′-FDA) using the fluoride ion (F^-) as substrate. Fluorinase exhibits a high degree of efficiency in forming C-F bonds, a moderate level of efficiency in forming C-Cl bonds, and no activity in forming C-Br bonds. This study presents a comprehensive quantum chemical analysis of the reaction mechanism and its selectivity for C-X (X = F^-, Cl^-, and Br^-) bond formation in fluorinase. To this end, the complete reaction pathway was obtained and characterized using physicochemical descriptors to obtain chemical insights into the inner workings of the enzyme. Our results reveal an energy barrier height of 19.5, 24.4, and 25.3 kcal/mol for the fluorination, chlorination, and bromination processes, respectively; where the electronic contributions (ΔE_int) dominate the catalytic efficiency of fluorinase. Thus, the selectivity among halogens is mainly governed by electronic work (W₂) and the orbital interactions energy (ΔE_oi). Therefore, as the proton affinity of the nucleophile decreases, the interaction between the nucleophile and SAM results in a higher reaction barrier.