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

Abstract

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 molydbenum-oxo dithiocarbamates (dtc) and dithiolates (dtl). Stronger coordination of acetylene to tungsten and dtc complexes is consistent with experimental Keq 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 (ΔG). These resul...