FAD center

Particularly useful have been the artificial mediators that shuttle electrons between the FAD center and the surface by the following scheme ... 

Examples of surface-immobilized mediators are electropolymerized azines for electro-oxidation of The extreme form of this approach is formation of biocatalytic monolayer, comprising a surface-bound mediator species that is itself bound to a single enzyme molecule. Katz et al. report a complete cell based on novel architecture at both electrodes (Figure 7). On the anode side, the FAD center of glucose oxidase is removed from the enzyme shell and covalently attached to a pyrroloquinoline quinone (PQQ) mediator species previously immobilized on a gold surface. The GOx apoenzyme (enzyme with active center removed) is reintroduced in solution and selectively binds to FAD, resulting in a PQQ-... 

Amperometric biosensors based on flavin-containing enzymes have been studied for nearly 30 years. These sensors typically undergo several chemical or electrochemical steps which produce a measurable current that is related to the substrate concentration. In the initial step, the substrate converts the oxidized flavin adenine dinucleotide (FAD) center of the enzyme into its reduced form (FADH2). Because these redox centers are essentially electrically insulated within the enzyme molecule, direct electron transfer to the surface of a conventional electrode does not occur to a substantial degree. The classical" methods (1-4) of indirectly measuring the amount of reduced enzyme, and hence the amount of substrate present, rely on the natural enzymatic reaction ... 

Since the reconstitution of apo-GOx onto an FAD-monolayer linked to the Au-electrode did not yield an electrically contacted enzyme electrode, and since the electrocatalytic anodic current observed for the integrated electrode was observed at the redox-potential of the PQQ units, the electrical communication between the enzyme redox center and the electrode and its bioelectrocatalytic activation was attributed to the PQQ-mediated electron transfer in the system. Fig. 3-4A. The FAD centers of reconstituted GOx are reduced by glucose, and the reduced cofactors are then oxidized by the PQQ... 

Note E.C. numbers are an enzyme cla.ssification system used to identify enzymes uniquely and avoid differences in nomenclature that frequently appear in the literature.) Two electrons are transferred from glucose to the FAD centers of GOX (glucose oxidase is a dimer, and has two FAD centers). Two electrons are then transferred from the reduced centers (FADH ) to the cosubstrate, Oj, forming H,0,. 

Another method of immobilizing enzyme and mediator is the attachmait of a mediator-containing monolayer to an electrode surface. If this monolayer is also functionalized with an enzyme cofactor meant to bind the enzyme, a catalytic electrode results. For example, gold electrode surfaces ean be eovered with a PQQ monolayer. The monolayer retains the redox properties of PQQ and ean be functionalized further. If FAD eofaetors are then covalently bound, GOx apoenzyme can bind the FAD centers. A covalent molecular chain results and electrons may hop from FAD to PQQ to the electrode surface, thus mediating from the enzyme [87]. PQQ monolayers can also link to covalently bound NAD" ", for mediation to NAD" -dependent dehydrogenase enzymes [88,89]. Functionalizing with monolayers of cytochrome c has also been demonstrated [90]. 

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