Biocatalysts enzyme assemblies

Mother nature has resolved the various limitations involved in multi-electron processes. Unique assemblies composed of cofactors and enzymes provide the microscopic catalytic environments capable of activating the substrates, acting as multi-electron relay systems and inducing selectivity and specificity. Artificially tailored heterogeneous and homogeneous catalysts as well as biocatalysts (enzymes and cofactors) are, thus, essential ingredients of artificial photosynthetic devices. 

The nanoparticle assemblies can also be used to enhance the chemical reactivity of biomolecules. Au nanoparticles assembled on polyurethane nucrospheres are used as permeable high-surface supports for the immobilization of enzymes such as pepsin to provide easy access of the substrate molecules to the enzyme active centers in the multilayer enzyme assembly. Proteins immobilized in this way exhibit biocataly tic activity higher than that of the free enzyme in solution and significantly enhanced temperature and pH stability [108]. In another approach, the layer-by-layer deposition of enzymes and magnetic particles is applied to prepare a bioreactor, which allows the biocatalytic layer to be stripped out with an external magnet when it is deactivated, so that the surface could be reloaded with a new active biocatalyst layer [109]. 

Enzyme biocatalyst assemblies on electrode surfaces usually do not achieve significant electron-transfer communication between the redox center and the conductive support, mostly because of the electrical insulation of the biocatalytic site by the surrounding protein matrixes. During the past four decades, several methods have been proposed and investigated in the field of bioelectrochemical technology in an effort to establish efficient electrical communication between biocatalysts and electrodes. " In general, electron transfer is classified by two different mechanisms (see Figure 2) ... 

The electrochemical activation of enzyme electrodes results in the electrobiocata-lyzed oxidation or reduction of a substrate specific to the biocatalyst. The rate of the biotransformation is dependent on the substrate concentration, hence these assemblies provide a basis for the construction of analytical biosensors [160]. The... 

Based on these observations, Wang and Caruso [237] have described an effective method for the fabrication of robust zeolitic membranes with three-dimensional interconnected macroporous (1.2 pm in diameter) stmctures from mesoporous silica spheres previously seeded with silicalite-1 nanoparticles subjected to a conventional hydrothermal treatment. Subsequently, the zeolite membrane modification via the layer-by-layer electrostatic assembly of polyelectrolytes and catalase on the 3D macroporous stmcture results in a biomacromolecule-functionalized macroporous zeolitic membrane bioreactor suitable for biocatalysts investigations. The enzyme-modified membranes exhibit enhanced reaction stability and also display enzyme activities (for H2O2 decomposition) three orders of magnitude higher than their nonporous planar film counterparts assembled on silica substrates. Therefore, the potential of such structures as bioreactors is enormous. 

Incorporation of Biocatalysts Into Synthetic Membranes. It Is generally accepted that only a few enzymes exist In vivo as a free protein In an aqueous medium, and that most of them either are bound to membranes or to solid-state assemblies or are present In a gel-llke surrounding. Enzymes attached to synthetic membrane matrices may serve as specific heterogeneous catalysts that can be used repeatedly. If they are sufficiently stable. In comparison to natural membranes, enzyme-bound synthetic membranes possess the advantage that they are mechanically more stable. 

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