Recent structural studies of acetyl-coenzyme A synthase/carbon monoxide dehydrogenase from Moorella thermoacetica revealed a unique Cu-Ni-[Fe4-S4] active site. New data now indicate that active enzyme contains a Ni-Ni-[Fe4-S4] site. Novel chemistry is posited for the central Ni atom of this enzyme, which utilizes the toxic gas CO as a catalytic intermediate.