Advances in Biochemical Engineering, Volume 006
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Since the first bioelectrochemical studies on CDH in the late s most studies have focused on the electrocatalytical activity. CV waves are not really observable on graphite electrodes modified with CDH owing to their high capacitive current. On the other hand, non-covalent interactions between CDH from different sources and gold electrodes or SAM-modified gold electrodes, and where the enzyme was retained behind a permselective membrane, has not shown stable electrochemical CV signals [ — ].
Efforts during recent years have focused on covalently immobilising CDH on alkanethiol-modified Au electrodes to avoid the usage of the permselective membrane [ 74 , — ]. Matsumura et al. GA was used to cross-link P. The optimal ratio of the mixture for deglycosylated P. This suggests that the surrounding glycosyl residues and the radius of the CDH have an effect [ 94 ].
This immobilisation protocol was also used in a 3D mesoporous structure created by drop-casting AuNPs on the surface of Au disk electrodes based on previous work by Murata et al. The same immobilisation procedure was applied for immobilising C. The BFC had an open-circuit voltage of 0.
A screening of the influence of various metal cations on the activity of CDH was done in for the basidiomycete Schizophyllum commune in solution [ ]. A further study with immobilised P. When immobilised on the graphite electrode the DET current for M. The DET current of H. Activity assays based on cyt c were used to investigate the modulation of the IET of M.
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The inset in c shows a magnification of the response curve. In analogy to the enhancing effect of CaCl 2 , a current-increasing effect of the branched polycation polyethylenimine PEI as a promoter layer was anticipated for the construction of CDH-modified biosensors. Beneficial effects have been shown in several publications covering, for example, cyt c on carbon electrodes [ ], the detection of NADH with phenoxazine derivative modified carbon paste electrodes [ , ] and some other redox enzyme modified carbon paste electrodes [ — ] or improved electrochemistry of human sulfite oxidase on PEI-modified gold nanoparticles [ ].
In recent work the influence of PEI as a promoter layer on spectrographic graphite electrodes modified with M. The pre-modification of a graphite electrode with PEI increased the maximal catalytic current from the oxidation of 3. The modification with PEI also shifted the pH optimum from 5. Another explanation would be an increased surface loading of M.
Besides the enhancing effect of PEI on the catalytic currents, the CVs unexpectedly revealed two catalytic redox waves. Ag AgCl 0. The origin of the second catalytic wave is unknown, but has been observed before for P. However, the enhancing effect of PEI is also clearly present at potentials below the second catalytic wave. The catalytic responses of class II N. Deglycosylation of enzymes has been used to facilitate the electron transfer and even to achieve DET from deeply buried prosthetic groups of redox enzymes, even from deeply buried FAD in e.
Aspergillus niger glucose oxidase GOx [ — ]. The carbohydrate chains are believed to stabilise the tertiary protein structure and increase the solubility of the protein molecule. A voluminous carbohydrate structure on the enzyme surface will act as an insulator for enzymes attached to electrodes [ ]. However, the catalytic current densities increased two- to threefold in the presence of lactose, for deglycosylated P.
The apparent Michaelis—Menten constant for lactose was also found to decrease by two- to threefold for the deglycosylated CDHs compared to the glycosylated ones. In a different study glycosylated and deglycosylated T. Then successively alternating layers of carboxy-terminated SiNPs and a mixture of cyt c and T. Using this technique, Feifel et al. The results suggest that the increase in current density is attributed to an increase in the amount of molecules packed on the electrodes, i.
Dependence of the amperometric response on lactose concentration of a glycosylated Phanerochaete chrysosporium CDH squares and deglycosylated P. The results were obtained in a flow-injection system. Reproduced from [ 94 ] with permission from The American Chemical Society. Even though a system based on DET is, from a fundamental point of view, more interesting than one based on MET, there are limitations to the current density that can be reached with DET.
Even though it should be possible to immobilise multilayers of a redox enzyme with DET properties, it is expected that primarily only the innermost layer of enzyme molecules on the electrode surface will be able to electrochemically communicate with the electrode. However, through the use of nanostructured electrodes and with CDH oriented for improved DET [ 47 — 49 ] the current density can be largely increased compared with that obtained with conventional electrodes.
Since then mainly two directions have been followed. The first is the combination of CDH with cyt c as a mediator in an LbL approach, as discussed separately above. The second is the further development and application of osmium polymers, which were first applied together with CDH as early as [ ] on the basis of the work by Heller and co-workers [ , ]. Deposition of those polymers is achieved by local changes of the pH at the electrode surface caused by either electrolysis or change of buffer, resulting in discharged and insoluble Os-polymers that precipitate on the electrode surface.
Ag AgCl. Os-polymer-mediated electron transfer was shown for CDH, glucose oxidase and PQQ-dependent glucose dehydrogenase suitable for biofuel cell anodes and for laccase and BOx suitable for corresponding cathodes [ , ]. As CDH oxidises its substrate at the C1 it is only possible to gain two electrons per substrate molecule.
Reproduced from [ 46 ] with permission from Elsevier. However, conventional mediators such as p -benzoquinone BQ are still in use as well. This approach has been used previously [ 88 ] and was also used for the characterisation of various CDHs [ 45 ], especially for N. A biosensor for the real-time measurement of cellobiohydrolase activity was established by Cruys-Bagger and co-workers [ 54 ] using a BQ-containing carbon paste electrode modified with cross-linked P. The sensor detected cellobiose with a sensitivity of CDH was used in two ways in biosensors: either to detect the oxidation of carbohydrates or reduce quinones and catecholamines for signal amplification in an oxidative electrode setup.
Depending on the source of CDH, specific catalytic properties allow the oxidation of different substrates by different CDHs and therefore different analytes in terms of biosensor applications. But it has to be considered that these CDHs work only efficient under acidic pH conditions. In a different approach a lactose biosensor was developed by the combination of the thermometric signal from lactose oxidation with the amperometric signal of the enzymatic reaction [ ]. In contrast to the third-generation biosensor reported by Safina et al.
In another recent example, a CDH biosensor was designed to measure the cellobiohydrolase activity on insoluble cellulose [ 54 ]. The ability of CDH to oxidise cellobiose—the reaction product of cellobiohydrolase—was used to monitor the transient kinetics of cellobiohydrolase. The approach could be validation by HPLC analysis and represents an interesting real-time method to monitor cellulase activity. The discovery that class II CDHs are able to oxidise the monosaccharide glucose opened the door to designing a third-generation glucose biosensor [ 88 ]. Probably the most interesting candidate for a glucose biosensor is the CDH from C.
The third-generation glucose biosensor based on CDH showed a linear range between 0. This CDH should allow the construction of a simple glucose biosensor which does not depend on oxygen or any other artificial redox mediators and therefore represents an interesting alternative to established biosensors based on glucose oxidase and glucose dehydrogenase.
The main targets for optimising CDH-based biosensors recently focused on the increase in current densities leading to higher sensitivities by designing the interface between the electrode and the enzyme. In order to increase the current density in some of the studies, SWCNTs were also applied to increase the aspect ratio surface area to achieve higher loadings of adsorbed CDH [ 89 , 90 , 93 ].
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Mediating Os-polymers were also applied to achieve a direct contact of the electrode with the DH CDH domain aiming at higher power densities gained at lower potentials [ 89 , 91 , 93 ]. To maximise the open-circuit voltage, it is desirable to have enzyme molecules in DET communication with the electrode, in the absence of redox mediators. On the basis of the work by Murata et al. The technique was optimised and further used with a C. The performance of the biodevice was evaluated in blood and plasma. In buffer containing glucose, the open-circuit potential was 0. The performance was only slightly reduced in the physiological fluids.
The same modification was used to fabricate a device tested in lachrymal fluid tears for possible future applications on non-invasive medical devices ex vivo [ 51 ]. Reproduced from [ 51 ] with permission from Elsevier. CDH is an emerging biocatalyst for biosensors and biofuel cells. Its versatility originates from its unique molecular properties, but also from the catalytic heterogeneity of CDHs from different sources. Over 20 CDHs from different fungi have been characterised so far and recombinant expression techniques guarantee access to these enzymes.
An issue of importance with CDH is, however, the microheterogeneity of enzyme preparations, especially glycoforms. Glycosyl residues alter the IET, but also the binding to electrode surfaces and the orientation on the electrode and are therefore an important factor to watch in future experiments. Alternative expression strategies or expression hosts should be tested for better results. The development of specifically modified electrode surfaces, either by chemical modification or by nanostructures, has tremendously increased the currents achievable with CDH-modified electrodes.
A combination of the approaches from materials science and enzyme engineering will have great impact on the development of CDH-modified electrodes with high current densities in the milliampere per square centimetre range. Such high current densities will ultimately allow miniaturisation of CDH electrodes down to single carbon fibre electrodes, which can be used in miniaturised biosensors for continuous on-site or point-of-care measurements.
Finally, one should mention the exciting use of ionic liquids in combination with CDH. Ionic liquids are potential candidates as non-aqueous solvents in this context and were shown to improve the shelf-life of CDH bound to AuNPs and carbon nanoparticles [ , ]. National Center for Biotechnology Information , U. Analytical and Bioanalytical Chemistry. Anal Bioanal Chem. Published online Jan Author information Article notes Copyright and License information Disclaimer.
Lo Gorton, Email: es. Corresponding author. Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author s and the source are credited. This article has been cited by other articles in PMC.
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