Nickel-Iron-Sulfur Active Sites Hydrogenase and

NICKEL-IRON-SULFUR ACTIVE SITES HYDROGENASE AND CO DEHYDROGENASE... 

Nickel-Iron-Sulfur Active Sites Hydrogenase and CO Dehydrogenase Juan C. Fontecilla-Camps and Stephen W. Ragsdale... 

Fontecilla-Camps, J.-C., and Ragsdale, S. W., 1999, Nickel-iron-sulfur active sites hydrogenase and CO dehydrogenase. Advances in Inorganic Chemistry. A. G. Sykes and R. Cammack. San Diego, Academic Press, Inc. 47 283n333. 

Smith MC, Barclay JE, Cramer SP, Davies SC, Gu W-W, Hughes DL, Longhurst S, Evans DJ (2002) Nickel-iron-sulfur complexes approaching structural analogues of the active sites of NiFe-hydrogenase and carbon monoxide dehydrogenase/acetyl-CoA synthase. Dalton Trans. 2641-2647... 

Both hydrogenases and carbon monoxide oxidoreductases contain iron-sulfur clusters in addition to nickel. It may be noted that in addition to the Ni hydrogenases, there is another class of Fe hydrogenases, such as those in clostridia, which contain no nickel but have a specialized type of iron-sulfur cluster (28a, 28b). Therefore, it has to be established that the nickel in Ni hydrogenases is the active site as will be seen later, there is a considerable amount of circumstantial evidence for this. 

Fig. 10. Hypothetical reaction cycle for D. gigas hydrogenase, based on the EPR and redox properties of the nickel (Table II). Only the nickel center and one [4Fe-4S] cluster are shown. Step 1 enzyme, in the activated conformation and Ni(II) oxidation state, causes heterolytic cleavage of H2 to produce a Ni(II) hydride and a proton which might be associated with a ligand to the nickel or another base in the vicinity of the metal site. Step 2 intramolecular electron transfer to the iron-sulfur cluster produces a protonated Ni(I) site (giving the Ni-C signal). An alternative formulation of this species would be Ni(III) - H2. Step 3 reoxidation of the iron-sulfur cluster and release of a proton. Step 4 reoxidation of Ni and release of the other proton.

The acidification of H2 may also be involved in hydrogenase action, where H2 is beheved to bind to an Fe(II) center. Isotope exchange between H2 and D2O is catalyzed by the enzyme see Nickel Enzymes Cofactors Nickel Models of Protein Active Sites Iron-Sulfur Proteins). Similar isotope exchange can also occur in H2 complexes. Oxidative addition to give a classical dihydride is also a common reaction. [W(H2)(CO)3(PCy3)2] is in equilibrium with about 20% of the dihydride in solution. This can lead to subsequent hydrogenolysis of M-C bonds as in the case of a cyclometallated phenylpyridine complex of Ir(III). ... 

Figure 16-26 (A) Stereoscopic view of the structure of the Desulfovibrio gigas hydrogenase as an a-carbon plot. The electron density map at the high level of 8o is superimposed and consists of dark spheres representing the Fe and Ni atoms. The iron atoms of the two Fe4S4 and one Fe3S4 clusters are seen clearly forming a chain from the surface of the protein to the Ni-Fe center. (B) The structure of the active site Ni-Fe pair. The two metals are bridged by two cysteine sulfur atoms and an unidentified atom, perhaps O, and the nickel is also coordinated by two additional cysteine sulfurs. Unidentified small molecules LI, L2, and L3 are also present. From Volbeda et Courtesy of M. Frey.

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