Metal-Containing Enzyme-Mediating Complexes

Although metal-catalyzed protein oxidation is undoubtedly a very effective oxidative process, the origin of free metal ions under in vivo conditions is still uncertain (see Chapter 21). However, protein oxidation can probably be initiated by metal-containing enzymes. Mukhopadhyay and Chatterjee [31] have shown that NADPH-stimulated oxidation of microsomal proteins was mediated by cytochrome P-450 and occurred in the absence of free metal ions. It is important that in contrast to metal ion-stimulated oxidation of proteins, ascorbate inhibited and not enhanced P-450-dependent protein oxidation reacting with the oxygenated P-450 complex. The following mechanism of P-450-dependent oxidation of the side chain protein amino acid residues has been proposed ... 

The incorporation of metals in multilayer thin films significantly extends the scope of useful characteristics associated with these films. By employing, for instance, polymeric Ru(II) complexes as polycationic species and poly(sodium acrylate) as polyanions in the layer-by-layer deposition process, efficient fight-emitting solid-state devices could be fabricated [91]. In another example, a ferrocene-containing redox-active polycation was combined with an enzyme to produce electrocatalyticaUy active enzyme/mediator multilayer structures [92]. Multilayers composed of poly(4-vinylpyridine) complexed with [Os(bpy)2Cl] / and poly(sodium 4-styrenesulfonate), for example, were used to accomplish the electrocatalytic reduction of nitrite [93]. 

Metal-mediated reactions involving water are essential to life and catalytic industrial processes [1-3]. In biological systems, metalloenzymes containing various divalent metal ions catalyze the hydrolysis of amide, carboxylic ester and phosphate ester bonds using both mono- and multinuclear active-site structural motifs [4—6], Mononuclear metal centers are also found within the active sites of enzymes that catalyze the hydration, or the addition of water, to CO2 [Zn(II)] and nitriles [Co(III)/Fe(III)j [7-10]. In many of these processes, formation of a metal hydroxide moiety via deprotonation of a metal-coordinated water molecule is a key proposed step in the reaction pathway. Thus, a substantial amount of research over the past several years has been directed at dehneating how the structural and electronic environments of biological metal ions influence the pKa of a metal-bound water molecule. In this regard, studies directed at the preparation, characterization and elucidation of the reactivity of discrete metal aqua and hydroxo complexes have been paramoimt [11-13]. 

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