Reaction bioelectrocatalytic

In DET, the enzymatic and electrode reactions are coupled by direct (mediatorless) electron transfer. In this case, the electron is transferred directly from the electrode to the substrate molecule (or vice versa) via the active site of the enzyme. In such a system, the coupled overall process is the redox transformation of the substrate(s), which can be considered as an enzyme-catalyzed electrode process. According to this mechanism, the electrode surface acts as the enzyme cosubstrate, and the enzymatic and electrode reactions cannot be considered as separate, but as formal stages of the bioelectrocatalytic reaction mechanism. The catalytic effect of the enzyme is the... 

Ferapontova EE, Castello J, Gorton L (2006) Bioelectrocatalytic properties of lignin peroxidase from Phanerochaete chrysosporium in reactions with phenols, catechols and lignin-model compounds. Biochim Biophys Acta 1760 1343-1354... 

Low-potential electron-transfer mediators such as viologens can substitute natural cofactors (particularly NADH) in some enzymatic reactions [184], The electrochemical reduction of viologens has been studied extensively [185] and they and other reductive electron mediators have been utilized to drive enzyme-catalyzed reactions [186], For instance, the electrochemical reduction of NAD(P)+ to NAD(P)H with a current efficiency of more than 97 % was achieved using alcohol dehydrogenase in the presence of acetophenone as an electron mediator [187], The addition of acetone or acetaldehyde as a substrate to the above bioelectrocatalytic system allowed the reduction of the substrate to the corresponding alcohol at alcohol dehydrogenase accompanied by the oxidation of the resulting NAD(P)H. 

A collection of redox enzymes for which efficient DET with electrodes has been observed is given in Table 2.3. Most of them are metaUoenzymes containing iron or copper. Many of these enzymes are part of electron transfer chains, i.e., have macromolecular redox partners, or react on large substrates. The evidence for DET has not always been presented by direct electrochemical measurements. In many cases the DET has been proved indirectly by measurement of a substrate dependent catalytic current. Various metabolites ranging from sugars such as fructose, cellobiose and gluconate [6], amines like methylamine and histamine [123], lactate [91],p-cresol [93] and drugs such as benzphetamine [74] can be measured with enzymes in direct contact to an electrode. The bioelectrocatalytic reaction of peroxide is one of the most important reactions not only for the determination of peroxide(s) in various media but also substrates of coupled oxidase [8] and enzyme inhibitors [130, 252]. Furthermore, enzyme immunoassays have been developed based on DET of peroxidase and laccase and electrodes [7,131,132]. 

To evaluate k at and Km values separately, bioelectrocatalysis data at higher mediator concentrations are needed. The bioelectrocatalytic reaction of bilirubin oxidase (BOD)... 

When substrates are removed from mediated bioelectrocatalytic systems, the reactions are considered to be indirect electrolysis of enzymes (or more generally proteins) ... 

The catalytic reactions of whole cells can be expressed by Eq. (28), which is of the same form as the equation for the catalytic reactions of enzymes, Eq. (2). Accordingly, the bioelectrocatalytic currents produced by whole cells can be analyzed by equations of the same form as those employed for the analyses of bioelectrocatalytic currents by enzymes. Under the conditions CgtoH > 6 mM and Cq < 50 /rM, where the current is linearly proportional to cq the steady-state limiting current is written as... 

The simplest bioelectrocatalytic method to activate the oxygen reaction is by the use of enzymes with oxidase activity and a mediator ... 

The studies of the kinetics of bioelectrocatalytic transformations show that in some systems (for instance, adsorbed laccase ) the kinetic parameters correspond to the phenomenology of electrochemical kinetics, while in other systems (for instance, lactate oxidation they fit the phenomenology of enzymatic catalysis. In the latter case, we observe a hyperbolic dependence of anode current on the substrate concentration, as expected from the Michaelis-Menten equation. The absence of a general theory of bioelectrocatalysis does not permit us to examine the kinetics of electrochemical reactions in the presence of enzymes under different conditions. At present we can only try to estimate the scope of possible accelerations of electrochemical reactions by making some simple assumptions. 

In order to screen mutants with improved direct electron transfer, it is necessary to use an electrochemical screening system. Currently, only a few electrochemical screening methods were described in literature such as the system developed by the Bartlett group used to screen NADH electro-oxidation. This system uses a multichannel potentiostat with sixty electrodes to screen zinc(n) or ruthenium(ii) complexes bearing the redox phenidione as a mediator for NADH oxidation. It allows the complete evaluation of the electrochemical kinetic constants of the mediators and the immobilization procedure. Unfortunately, this system could only be used with a single electrolyte solution for all the electrodes (e.g., when a single reaction condition or enzyme is assayed), and it requires mL-scale reaction volumes. Recently, another system was described which makes it possible to screen bioelectrocatalytic reactions on 96 independent electrodes screen-printed onto a printed-circuit-board. It showed the possibility to screen direct or mediated electron transfer between oxidoreductases and electrode by intermittent pulse amperometry at the pL-scale (Fig. 6). The direct electron transfer assay was validated with laccase and unmodified electrodes.As an example of the mediated electron transfer assay, the 96 carbon electrodes were modified by phenazines to sereen libraries of a formate dehydrogenase obtained by directed evolution. ... 

Impedance spectroscopy has been extensively used to follow changes of the interfacial properties of electrodes upon immobilization of enzymes and to characterize biocatalytic processes at enzyme-modified electrodes. Faradaic impedance spectroscopy can be used to study the kinetics of the electron transfer originating from bioelectrocatalytic reactions. It should be noted, that for characterizing redox-active biomolecules by impedance spectroscopy no additional redox probe is added to the electrolyte solution, and the measured electron-transfer process corresponds to the entire bioelectrocatalytic reaction provided by the biocatalyst. Under the condition that the enzyme is not saturated by the substrate, the electron-transfer resistance of the electrode is also controlled by the substrate concentration. Thus, the substrate concentration can be analyzed by the impedance spectroscopy following values [9]. 

Katz E, Lioubashevski O, WiUner I. Magnetic field effects on bioelectrocatalytic reactions of surface-confined enzyme systems enhanced performance of biofuel ceUs. J Am Chem Soc 2005 127 3979-3988. 

---