FIGURE 3.1 Schematic of hydrogen metabolism and the hydrogenase active site. (A) The cell of C. pasteurianum whose metabolism involves the oxidation of sugars and evolution of hydrogen by the iron-only hydrogenase designated as a hexagon. (B) The range of organisms that use hydrogen as a reductant and use the nickel-iron uptake hydrogenase. (C) Schematic of the iron-only hydrogenase enzyme showing paths for electron and proton transfer converging at the H center. (D) Schematic of the H center showing the six-iron cluster with a two-iron subcluster bound to five CO or CN ligands.

SOURCE: Adams, M.W.W. and E. I. Stiefel. 1998. Biological Hydrogen Production: Not So Elementary. Science 282(5395): 1842-1843.

FIGURE 3.2 Examples of two of the main iron-sulfur cluster containing hydrogenase active sites.

SOURCE: Modified from presentation of Marcetta Darensbourg, Texas A&M University, based on crystal structures derived by (A) J.W. Peters and coworkers, 1998. X-ray crystal structure of the Fe-only hydrogenase from Clostridium pasteurianum to 1.8 Angstrom Resolution. Science 282(5395): 1853-1858; and (B) J.C. Fontecilla-Camps and coworkers, 1995. Crystal structure of the nickel-iron hydrogenase from Desulfovibrio gigas. Nature 373: 580-587.

studies in conjunction with vibrational spectroscopy of diatomic ligands. This could be used to match properties of the enzyme active site with the small molecule models. The diiron organometallic molecule has several attractive features, such as the flexibility associated with the iron dithiacyclohexane ring as it switches between chair-boat forms, flipping the bridge-head carbon in the process. This feature can be monitored by nuclear magnetic resonance (NMR) spectroscopy. Darensbourg noted that additional flexibility is in the Fe(CO)3 units on each end of the diiron complex, which shows intramolecular CO site exchange also detectable in variable temperature NMR experiments. As a result of studying the fundamental properties of the molecule, Darensbourg and others found that there are still



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