Iron-sulfur clusters hydrogen bonding

Inspection of the citrate structure shows a total of four chemically equivalent hydrogens, but only one of these—the pro-/J H atom of the pro-i arm of citrate—is abstracted by aeonitase, which is quite stereospecific. Formation of the double bond of aconitate following proton abstraction requires departure of hydroxide ion from the C-3 position. Hydroxide is a relatively poor leaving group, and its departure is facilitated in the aeonitase reaction by coordination with an iron atom in an iron-sulfur cluster. 

The iron-sulfur cluster (1), after treatment with phenyllithium in ether, was found to hydrogenate 1-octene and trans-2-, -3- and -4-octene with little or no apparent isomerization of the double bond.3 The selectivity was dependent on the amount of phenyllithium used and the solvent. 

Some detailed comparisons of the protein environments around the HiPIP and Fd clusters have been made.769,770 It is noteworthy that the HiPIP cluster is more deeply buried (about 4.5 A) than is the case for the clusters in the other iron-sulfur proteins. All iron-sulfur proteins for which structural data are available, with the exception of the three-iron protein from Azotobacter vinelandii, have hydrogen bonding between the cysteine sulfur in the iron-sulfur cluster and the backbone peptide link. It appears that there is an approximate correlation between the number of NH S hydrogen bonds in the environment of a cluster and its redox potential. In HiPIP, these hydrogen bonds become more linear and shorten on reduction of the cluster. It is possible, therefore, that the oxidation states of the cluster may be controlled by the geometries of the hydrogen bonds.770... 

Denke, E., Merbitz-Zahradnik, T., Hatzfeld, O. M., Snyder, C. H., Link, T. A., and Trumpower, B. L., 1998, Alteration of the midpoint potential and catalytic activity of the rieske iron-sulfur protein by changes of amino acids forming hydrogen bonds to the iron-sulfur cluster, J. Biol. Chem. 273 9085n9093. 

A wide range of metal ions is present in metalloenzymes as cofactors. Copper zinc snperoxide dismntase is a metalloenzyme that nses copper and zinc to help catalyze the conversion of snperoxide anion to molecnlar oxygen and hydrogen peroxide. Thermolysin is a protease that nses a tightly bonnd zinc ion to activate a water atom, which then attacks a peptide bond. Aconitase is one of the enzymes of the citric acid cycle it contains several iron atoms bonnd in the form of iron-sulfur clusters, which participate directly in the isomerization of citrate to isocitrate. Other metal ions fonnd as cofactors in metalloenzymes include molybdenum (in nitrate rednctase), seleninm (in glutathione peroxidase), nickel (in urease), and vanadinm (in fungal chloroperoxidase). see also Catalysis and Catalysts Coenzymes Denaturation Enzymes Krebs Cycle. 

In these two Ni-functionalized CNT materials, the Ni-molecular catalyst is located at the crossroads of the three interpenetrated networks allowing percolation of protons (the Nafion membrane), hydrogen (the pores in the gas diffusion layer), and electrons (the carbon fibers of the gas diffusion layer relayed by the conducting CNTs). In a way and even if it is not as well defined as in the protein, the catalyst environment in this membrane-electrode assembly reproduces that found in the active sites of hydrogenases buried into the polypeptidic framework but connected to the surfece of the protein via a gas diffusion channel, a network of hydrogen-bonded amino acids for proton transport and the array of electrontransferring iron-sulfur clusters. 

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