Theoretical Studies of Models of the Active Site of the Tungstoenzyme Acetylene Hydratase

Models of the tungstoenzyme acetylene hydratase (AH), which catalyzes the addition of water to acetylene through a nonredox process, are examined using density-functional theory (DFT). The relative energy of acetylene adduct formation was calculated for several tungsten- and molydbenumoxo dithiocarbamates (dtc) and dithiolates (dtl). Stronger coordination of acetylene to tungsten and dtc complexes is consistent with experimental K-eq values and attributed to the larger W 5d orbitals and the overall negative charge of the dithiolate complexes. The recently solved AH X-ray crystal structure suggests the presence of a water molecule bonded to the metal and the possibility that catalysis occurs via a non-organometallic intermediate. Models of the truncated active site are used to analyze this claim by determining the relative energies of acetylene versus water coordination. Complexation of acetylene is favored over water by 12 kcal/mol with the aquo complex formation endergonic by 7 kcal/mol (Delta G). These results suggest that water may be easily replaced by acetylene Such that catalysis occurs via an organometallic eta(2)-acetylene intermediate. The selectivity of the enzyme for tungsten is discussed in terms of these results.