Nickel containing CO dehydrogenases and hydrogenases.
The two redox catalysts described here can generate very low potential electrons in one direction and perform chemically difficult reductions in the other. The chemical transformations occur at unusual metal clusters. Spectroscopic, crystallographic, and kinetic analyses are converging on answers to how the metals in these clusters are arranged and how they are involved in the chemical and redox steps. The first structure of CO dehydrogenase, which will appear in the next year, will help define a firm chemical basis for future mechanistic studies. In the immediate future, we hope to learn whether the hydride intermediate in hydrogenase or the carbonyl intermediate in CO dehydrogenase bind to the Ni or Fe subsites in these heterometallic clusters. Or perhaps could they be bridged to two metals? Inter- and intramolecular wires have been proposed that connect the catalytic redox machine to proximal redox centers leading eventually to the ultimate redox partners. Elucidating the pathways of electron flow is a priority for the future. There is evidence for molecular channels delivering substrates to the active sites of these enzymes. In the next few years, these channels will be better defined. The products of CO2 and proton reduction are passed to the active sites of other enzymes and, in the case of H2, even passed from one organism to another. In the future, the mechanism of gas transfer will be uncovered. General principles of how these redox reactions are catalyzed are becoming lucid as the reactions are modeled theoretically and experimentally. Proton and CO2 reduction and the generation of C-C bonds from simple precursors are important reactions in industry. H2 could be the clean fuel of the future. Hopefully, the knowledge gained from studies of hydrogenase, CO dehydrogenase, and acetyl-CoA synthase can be used to improve life on earth.