Insight into Enzymatic C−F Bond Formation from QM and QM/MM Calculations

The C−F bond-forming step in the fluorinase, the only native fluorination enzyme characterized
to date, has been studied. The enzyme catalyzes the reaction between S-adenosyl-l-methionine (SAM)
and fluoride ions to form 5‘-fluoro-5‘-deoxyadenosine (5‘-FDA) and l-methionine. To obtain an insight into
the mechanism of this unusual enzymatic reaction and to elucidate the role of the enzyme in catalysis, we
have explored the conformational energetics of SAM and the intrinsic reactivity patterns of SAM and fluoride
with DFT (BP86) and continuum solvent methods, before investigating the full enzymatic system with
combined DFT/CHARMM calculations. We find that the enzymatic reaction follows an SN2 reaction
mechanism, concurring with the intrinsic reactivity preferences in solution. The formation of sulfur ylides is
thermodynamically strongly disfavored, and an alternative elimination−addition mechanism involving the
concerted anti-Markovnikov addition of HF to an enol ether is energetically viable, but kinetically prohibitive.
The SN2 activation energy is 92 (112) kJ mol-1 in solution, but only 53 (63) kJ mol-1 in the enzyme, and
the reaction energy in the enzyme is −25 (−34) kJ mol-1 (values in parentheses are B3LYP single-point
energies). The fluorinase thus lowers the barrier for C−F bond formation by 39 (49) kJ mol-1. A
decomposition analysis shows that the major role of the enzyme is in the preparation and positioning of
the substrates.