Iron proteins self-oxidization

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

There are some unique examples of organic-inorganic hybrids in natural systems. Some kinds of protein possess inorganic components at their interior. Because such bio-originated hybrid structures have discrete size and composition, they can be used for preparation of fine mesoscopic structures. For example, ferritin, which is composed of 24 self-assembled peptide subunits and capable of including iron oxide, is a good candidate to fulfill this demand. Yamashita demonstrated use of a ferritin array for preparation of a mesoscopic nanodot array (Fig. 2.11 ).47 A Langmuir... 

Krische MJ, Lehn JM (2000) The Utilization of Persistent H-Bonding Motifs in the Self-Assembly of Supramolecular Architectures. 96 3-30 Krumholz P (1971) Iron(II) Diimine and Related Complexes. 9 139-174 Kuhn FE, Herrmann WA (2000) Rhenium-Oxo and Rhenium-Peroxo Complexes in Catalytic Oxidations. 97 213-236 Kubas GJ, see Ryan RR (1981) 46 47-100 Kuki A (1991) Electronic Tunneling Paths in Proteins. 75 49-84... 

Early reports on interactions between redox enzymes and ruthenium or osmium compounds prior to the biosensor burst are hidden in a bulk of chemical and biochemical literature. This does not apply to the ruthenium biochemistry of cytochromes where complexes [Ru(NH3)5L] " , [Ru(bpy)2L2], and structurally related ruthenium compounds, which have been widely used in studies of intramolecular (long-range) electron transfer in proteins (124,156-158) and biomimetic models for the photosynthetic reaction centers (159). Applications of these compounds in biosensors are rather limited. The complex [Ru(NHg)6] has the correct redox potential but its reactivity toward oxidoreductases is low reflecting a low self-exchange rate constant (see Tables I and VII). The redox potentials of complexes [Ru(bpy)3] " and [Ru(phen)3] are way too much anodic (1.25 V vs. NHE) ruling out applications in MET. The complex [Ru(bpy)3] is such a powerful oxidant that it oxidizes HRP into Compounds II and I (160). The electron-transfer from the resting state of HRP at pH <10 when the hemin iron(III) is five-coordinate generates a 7i-cation radical intermediate with the rate constant 2.5 x 10 s" (pH 10.3)... 

The defined architecture of the metalloprotein ferritin, a natural complex of iron oxide, is found in almost all domains of life and has been used as a constrained reaction vessel for the synthesis of a number of non-natural metal oxides [28, 34]. The protein ferritin consists of 24 subunits that self-assemble into a cage, consisting of a threefold hydrophilic channel coordinated to a fourfold hydrophobic channel [20, 28]. In biology, Fe(ll) is introduced into the core of the apoprotein through its hydrophiUc charmels where the ferrous ion is catalytically oxidized to a less-soluble ferric ion, Fe(lll) [20]. The ferric ion then undergoes a series of hydrolytic polymerizations to form the insoluble ferric oxyhydroxide mineral (ferrihydrite), which is physically constrained by the size of the protein cage (12 nm outer diameter, 8nm inner diameter) [35]. The enzyme ferrous oxidase is coordinated within the protein cage, the interior and exterior of which is electrostatically dissimilar, to produce spatially defined minerals. 

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