Enzymes, molecular interfacing electrode surface

Fig. 10 Schematic illustration of the molecularly interfaced enzyme on the electrode surface...

In contrast to the molecular wire of molecular interface, electron mediators are covalently bound to a redox enzyme in such a manner as an electron tunneling pathway is formed within the enzyme molecule. Therefore, enzyme-bound mediators work as molecular interface between an enzyme and an electrode. Degani et al. proposed the intramolecular electron pathway of ferrocene molecules which were covalently bound to glucose oxidase [ 4 ]. However, few fabrication methods have been developed to form a monolayer of mediator-modified enzymes on the electrode surface. We have succeeded in development of a novel preparation of the electron transfer system of mediator-modified enzyme by self-assembly in a porous gold-black electrode as schematically shown in Fig.12 [14]. 

Several protein assemblies have successfully been fabricated on the solid surfaces sifter the bioinformation transduction. These include the following molecular systems molecularly interfaced redox enzymes on the electrode surfaces, calmodulin / protein hybrides, and ordered antibody array on protein A. These protein assemblies find a wider application in various fields such as biosensors, bioreactors, and intelligent materials. 

There are several molecular interfaces for redox enzymes to promote electron transfer at the electrode surface (Fig.6). 

The polypyrrole molecular interface has been electrochemically synthesized between the self-assembled protein molecules and the electrode surface for facilitating the enzyme with electron transfer to the electrode. Figure 9 illustrates the schematic procedure of the electrochemical preparation of the polypyrrole molecular interface. The electrode-bound protein monolayer is transferred in an electrolyte solution containing pyrrole. The electrode potential is controlled at a potential with a potentiostat to initiate the oxidative polymerization of pyrrole. The electrochemical polymerization should be interrupted before the protein monolayer is fully covered by the polypyrrole layer. A postulated electron transfer through the polypyrrole molecular interface is schematically presented in Fig. 10. 

Electron Transfer Type of Dehydrogenase Sensors To fabricate an enzyme sensor for fructose, we found that a molecular interface of polypyrrole was not sufficient to realize high sensitivity and stability. We thus incorporated mediators (ferricyanide and ferrocene) in the enzyme-interface for the effective and the most sensitive detection of fructose in two different ways (l) two step method first, a monolayer FDH was electrochemically adsorbed on the electrode surface by electrostatic interaction, then entrapment of mediator and electro-polymerization of pyrrole in thin membrane was simultaneously performed in a separate solution containing mediator and pyrrole, (2) one-step method co-immobilization of mediator and enzyme and polymerization of pyrrole was simultaneously done in a solution containing enzyme enzyme, mediator and pyrrole as illustrated in Fig.22. 

The need to improve the electrical communication between redox proteins and electrodes, and the understanding that the structural orientation at the molecular level of redox proteins and electroactive relay units on the conductive surfaces is a key element to facilitate ET, introduced tremendous research efforts to nano-engineer enzyme electrodes with improved ET functionalities. The present chapter addresses recent advances in the assembly of structurally aligned enzyme layers on electrodes by means of surface reconstitution and surface crosslinking of structurally oriented enzyme/cofactor complexes on electrodes. The ET properties of the nano-structured interfaces is discussed, as well as the possible application of the systems in bioelectronic devices such as biosensors, biofuel cell elements or optical and electrical switches. 

Aizawa has studied an enzyme (metal) electrode based on immobilized GOD, which is a redox enzyme [66]. Interfacing in this type of biosensor requires facilitation of electron transfer from the active site of the enzyme protein to the metal electrode. Various methods have been used, such as electron mediators, electron promoters, and molecular wires, to accomplish this purpose. For example, a conducting polymer, polypyrrole, formed by electropolymerization on the electrode surface, was used to form electron wires which can facilitate electron transfers to and from the metal electrode. 

Mediators can be polymerized on the electrode surface prior to enzyme immobilization, co-immobilized with enzyme, or simply added to the fuel solution. Common mediators used in BFC applications include low molecular weight, polymerizable, organic dyes such as methylene green, phenazines, and azure dyes, along with other redox-active compounds such as ferrocene, ferrocene derivalives, and conductive salts [14]. These mediators are often required for nicotinamide adenine dinucleotide (NAD )- and flavin adenine dinucleotide (FAD)-dependent enzymes, such as ADH, ALDH, and GOx. MET has been achieved at both cathodic and anodic interfaces through solution-phase mediators and mediators immobilized in various ways with or near the enzymes themselves [16,17]. However, these mediated systems do have drawbacks in that the species used to assist electron transfer are often not biocompatible, have short lifetimes themselves, or cause large potential losses. Table 5.1 lists common enzyme cofactors that can mediate or undergo DET with an enzyme on the electrode. 

Few redox enxymes undergo reversible electron transfer on the electrode surface primarily due to steric hindrance of the redox centers pf these enzymes. Several molecular interfaces have been designed to promote electron transfer of electrode-bound redox enzymes. 

The authors have successfully interfaced fructose dehydrogenase and alcohol dehydrogenase on the electrode surface with conducting polymer of polypyrrole, which could cause these enzymes to make an electron transfer with retaining their enzymatic activity. The molecular-interfaced redox enzymes will find their application in fabricating biosensors that respond specifically to the corresponding substrates in current. 

Molecular-interfaced alcohol dehydrogenase with NAD and its response to ethanol. It has been difficult to incorporate dehydrogenases that are coupled with NAD(P) into amperometric enzyme sensors owing to the irreversible electrochemical reaction of NAD. We have developed a molecular-interfaced alcohol dehydrogenase on the surface of an electrode on which NAD is electrochemical 1 y regenerated within a membrane matrix. 

Electronic communication of fructose dehydrogenase (FDH) with a Pt electrode was accomplished through the conducting polymer molecular interface. Electrons were reversibly transferred between the active center of FDH and the electrode surface when the electrode potential was properly controlled. The enzyme activity of the molecular-interfaced FDH was found to be modulated in the presence of D-fructose by the electrode potential. Electronic communication of alcohol dehydrogenase (ADH) with a Pt electrode was also accomplished in the... 

AIZAWA ET AL. Molecular Interfacing of Enzymes on Electrode Surface 313... 

A major advance in the construction of electrically contacted enzyme electrodes involves the structural alignment of the enzyme redox center with respect to the electrode interface in conjunction with the site-specific positioning of a redox relay component between the enzyme redox center and the electrode. The design of such electrodes promotes a new level of molecular architecture of biomolecules on surfaces, enabling us to optimize the electrical contact of the resulting enzyme elec-... 

---