Coupling Redox Enzymes

The thing to be noted here is that the ° values of the 02/ 02" and 02" H202 redox couples are -0.35 and 0.68 V vs Ag/AgCl at pH 7.4 and thus the SODs, for example, Cu, Zn-SOD (Cu (I/II)) with ° = 65mV can mediate both the oxidation of 02 to 02 and the reduction of 02" to H202. Such a bi-directional electromediation (electrocatalysis) by the SOD/SAM electrode is essentially based on the inherent specificity of the SOD enzyme which catalyzes the dismutation of 02 to 02 and H202 via a redox cycle of their metal complex moiety (Scheme 3). 

Moreover, it has been demonstrated that CNTs promote the direct electrochemistry of enzymes. Dong and coworkers have reported the direct electrochemistry of microperoxidase 11 (MP-11) using CNT-modified GC electrodes [101] and layer-by-layer self-assembled films of chitosan and CNTs [102], The immobilized MP-11 has retained its bioelectrocatalytic activity for the reduction of H202 and 02, which can be used in biosensors or biofuel cells. The direct electrochemistry of catalase at the CNT-modified gold and GC electrodes has also been reported [103-104], The electron transfer rate involving the heme Fe(III)/Fe(II) redox couple for catalase on the CNT-modified electrode is much faster than that on an unmodified electrode or other... 

Figure 5.9 Example of a redox titration of nickel of hydrogenase from M. marburgensis. The amplitude of the Ni EPR signal is plotted against the measured redox potential. Half of the active sites in the enzyme solution is reduced at a redox potential (midpoint potential) of — 140 mV (at pH 6).The 2H /H2 redox couple has an of —354mV at this pH.The line through the points is a theoretical line assuming a midpoint potential of — 140 mV (Coremans et al. 1989).

It has been demonstrated that Mn is the preferred substrate for MnP (13-17). The enzyme oxidizes Mn to Mn and the Mn produced, complexed with a suitable carboxylic acid ligand (12-16), diffuses from the enzyme and in turn oxidizes the organic substrates (6,8,13-17). Thus the Mn ion participates in the reaction as a diffusible redox couple (Fig. 1) rather than as an enzyme-binding activator. In support of this concept, we have demonstrated that chemically prepared Mn complexed with a carboxylic acid ligand such as malonate or lactate mimics the reactivity of the enzyme (6,8,14,15). 

Any electrochemical device using a low molecular weight redox couple to shuttle electrons from the redox center of an enzyme to the surface of an indicator electrode, thereby increasing the effectiveness of amperometry in the detection of a substrate for the particular enzyme. The internal cavities of six-, seven-, and eight-membered cyclodextrins are trapezoids of revolution with larger open mouths dimensions (/. c., respective diameters of... 

In other work, Dandliker et al. have reported the inclusion of iron porphyrins within dendrimers to serve as functional mimics of redox-based proteins (Dandliker et al., 1994, 1995, 1997). These redox-switchable porphyrins show that the Fe3+/Fe2+ redox couple can be altered by the polarity of the surrounding environment. By changing the polarity imposed by the tightly packed branches of the dendritic core, the authors have illustrated that electrochemical behavior can be controlled by slight and subtle through-space environmental factors. These mimics may potentially model a wide variety of redox-driven enzymes and possibly provide mechanistic insights into their function. 

Many proteins are exclusively involved in intra-protein electron transfer and typically function in ordered structures such as mitochondria. Under these circumstances, the redox-active centers are generally accessible on the outer surface of the protein. In contrast, the redox reactions catalyzed by oxidoreductases involve small molecules with the reaction involving two redox couples, i.e. the substrate and the co-factor or co-substrate. Because the catalytic center of the enzyme is often located... 

Fig. 17.8. Model of a particulate redox enzyme upon which the theory of electron conduction enzymes is based. Site X in the particle acts as electrode for the redox couple X and develops an equilibrium potential determined by the extent of the reduction of X. Site Y, on the opposite side, acts in the same way for Y. The potential difference causes an electronic current between sites X and Y and within the particle (from Ref. 35 with permission).

Enzyme Redox couple Redox potential (mV) References ... 

The structure and enzyme kinetics of bovine erythrocyte superoxide dismutase are reviewed. The protein has a novel imidazolate-bridged copper(II)-zinc(II) catalytic center in each of two identical subunits. Since a C /Cu1 redox couple is responsible for the dismutase activity of the enzyme, the role of zinc is of interest. Both 220-MHz NMR measurements of the exchangeable histidine protons and chemical modifications using diethylpyrocarbonate demonstrate that zinc alone can fold the protein chain in the region of the active site into a conformation resembling that of the native enzyme. Other possible roles for zinc are discussed. Synthetic, magnetic, and structural studies of soluble, imidazolate-bridged copper complexes of relevance to the 4 Cu(II) form of the enzyme have been made. 

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