Catalytic iron

The cluster is coordinated at the tip of the cluster binding subdomain. Fe" (Fe-2) is close to the surface of the protein with its histidine ligands fully exposed to the solvent, whereas Fe " (Fe-1) is buried within the protein and surrounded by the three loops forming the cluster binding subdomain. However, in NDO the histidine ligands are not solvent accessible, but buried at the interface between the Rieske domain and the catalytic domain both histidine ligands form hydrogen bonds with acidic side chains in the catalytic site close to the catalytic iron. 

What is the interaction between the Rieske-type cluster and the catalytic iron site of dioxygenases ... 

Rowley, A., Gutteridge, J.M.C., Blake, D.R, Farr, M. and Halliwell, B. (1984). Lipid peroxidation in rheumatoid arthritis thiobarbituric acid reactive material and catalytic iron salts in synovial fluid from rheumatoid patients. Clin. Sci. 66, 691-695. 

Rajasinghe, H., Jayatilleke, E. and Shaw, S. (1990). DNA cleavage during ethanol metabolism, role of superoxide radicals and catalytic iron. Life Sci. 47, 807-814. 

Shaw, D.S. and Jayatilleke, E. (1992). The role of cellular oxidases and catalytic iron in the pathogenesis of ethanol-induced liver injury. Life Sci. 50, 2045-2052. 

Saez V, Gonzalez-Garcia J, Kulandainathan MA et al (2007) Electro-deposition and stripping of catalytically iron metal nanoparticles at boron-doped diamond electrodes. Electrochem Commun 9 1127-1133... 

Reacting dinitrogen with dihydrogen on a catalytic iron surface (the Haber-Bosch process)... 

Fig. 24. X-ray crystal structure of the catalytic domain of tyrosine hydroxylase. The catalytic iron is located 10 A below the enzyme surface and is coordinated by the conserved residues His-331, His-336, and Glu-376 (PDBID 1TOH). Adapted from (494). 

It was early established that at least for PAH, which as isolated contains the catalytic iron in the ferric high-spin (S = 5/2) state, the Fe111 is reduced to Fe11 by BH4. This is termed reductive activation of PAH and in vitro this reduction is an obligate step that occurs in the pre-steady-state period (20). A midpoint potential at pH 7.25 (Em (Fein/Fen)) of +207 10 mV was calculated for the iron in hPAH, which seems to be adequate for a thermodynamically feasible electron transfer from BH4 (Em (q-BH2/ BH4) = +174mV) (40). 

Several other possible biological functions of iron-sulfur clusters have been proposed as summarized in Table 1. Let us briefly discuss this list in detail to indicate why the majority of the entries are excluded from our definition of catalytic iron-sulfur clusters. 

This leaves only the two last entries of Table 1 for our further consideration. I.C. Catalytic Iron-Sulfur Clusters... 

The existence of hydrolytic iron-sulfur enzymes is well established. Con-trarily, redox-catalytic iron-sulfur centers are presently still hypothetical at least... 

Aconitase was the first protein to be identified as containing a catalytic iron-sulfur cluster [24-26]. It was also readily established that the redox properties of the [4Fe-4S](2+ 1+) cluster do not play a role of significance in biological functioning the 1 + oxidation state has some 30% of the activity of the 2+ state [25], Since then several other enzymes have been identified or proposed to be nonredox iron-sulfur catalysts. They are listed in Table 2. It appears that all are involved in stereospecific hydration reactions. However, these proteins are considerably less well characterized than aconitase. In particular, no crystal structural information is available yet. Therefore, later we summarize structural and mechanistic information on aconitase, noting that many of the basic principles are expected to be relevant to the other enzymes of Table 2. 

Table 3 Possible Redox-Catalytic Iron-Sulfur Enzymes, Their Substrates, and the Putative Structures of Their Catalytic Center...

The next two entries to Table 3 are cited for completeness. Nitrogenase is treated in Chapter 7 and CO dehydrogenase in Chapter 9. Nitrogenase contains a very complex iron-sulfur cluster that includes another metal, molybdenum or vanadium. The crystal structure of the Mo variant has been determined. There is a third variant, alternative nitrogenase [92], whose cluster apparently does not contain any heterometal. That cluster would thus be a perfect candidate for our definition of a redox-catalytic iron-sulfur cluster. Unfortunately, this third nitrogenase has thus far been characterized to a much lesser extent than the other two forms. For all nitrogenases holds that the binding of N2 to the cluster has not been established [53] therefore, formally these enzymes have not yet been positively identified as redox iron-sulfur catalysts. 

Russell, M.J., Daniel, R.M., Hall, A.J., and Sherringham, J.A. 1994. A hydrothermically precipitated catalytic iron sulfide membrane as a first step toward life. J. Mol. Evol. 39 231-243. 

An exhaustive study has been carried out recently on the synthesis of BN nanotubes and nanowires by various CVD techniques.17 The methods examined include heating boric acid with activated carbon, multi-walled carbon nanotubes, catalytic iron particles or a mixture of activated carbon and iron particles, in the presence of ammonia. With activated carbon, BN nanowires are obtained as the primary product. However, with multi-walled carbon tubes, high yields of pure BN nanotubes are obtained as the major product. BN nanotubes with different structures were obtained on heating boric acid and iron particles in the presence of NH3. Aligned BN nanotubes are obtained when aligned multi-walled nanotubes are used as the templates (Fig. 40). Prior to this report, alignment of BN nanotubes was achieved by the synthesis of the BN nanotubule composites in the pores of the anodic alumina oxide, by the decomposition of 2,4,6-trichloroborazine at 750 °C.116 Attempts had been made earlier to align BN nanotubes by... 

Chelation of catalytic iron and free radical scavengers... 

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