Immobilized enzyme catalysis with reaction-generated pH change

Biosensors: Electrochemical Devices-General Concepts and Performance

This review provides a general overview of different biosensors, mostly concentrating on electrochemical analytical devices, while briefly explaining general approaches to various kinds of biosensors, their construction and performance. A discussion on how all required components of biosensors are brought together to perform analytical work is offered. Different signal-transducing mechanisms are discussed, particularly addressing the immobilization of biomolecular components in the vicinity of a transducer interface and their functional integration with electronic devices. The review is mostly addressing general concepts of the biosensing processes rather than specific modern achievements in the area.

Nanocatalysts Containing Direct Electron Transfer-Capable Oxidoreductases: Recent Advances and Applications

Direct electron transfer (DET)-capable oxidoreductases are enzymes that have the ability to transfer/receive electrons directly to/from solid surfaces or nanomaterials, bypassing the need for an additional electron mediator. More than 100 enzymes are known to be capable of working in DET conditions; however, to this day, DET-capable enzymes have been mainly used in designing biofuel cells and biosensors. The rapid advance in (semi) conductive nanomaterial development provided new possibilities to create enzyme-nanoparticle catalysts utilizing properties of DET-capable enzymes and demonstrating catalytic processes never observed before. Briefly, such nanocatalysts combine several cathodic and anodic catalysis performing oxidoreductases into a single nanoparticle surface. Hereby, to the best of our knowledge, we present the first review concerning such nanocatalytic systems involving DET-capable oxidoreductases. We outlook the contemporary applications of DET-capable enzymes, present a principle of operation of nanocatalysts based on DET-capable oxidoreductases, provide a review of state-of-the-art (nano) catalytic systems that have been demonstrated using DET-capable oxidoreductases, and highlight common strategies and challenges that are usually associated with those type catalytic systems. Finally, we end this paper with the concluding discussion, where we present future perspectives and possible research directions.

Can immobilization be exploited to modify enzyme activity?

Immobilization has long been recognized as a useful tool for retaining enzymes in bioreactors and enabling the continuous operation of enzymic processes. However, the potential of immobilization for modifying enzyme activity in an advantageous, if not predictable, manner is less-well appreciated. This review summarizes selected studies that have used immobilization to tailor the catalytic properties of enzymes, and highlights the application of immobilization to the rational design of biocatalysts.

Product inhibition of immobilized Escherichia coli arising from mass transfer limitation

Mass transfer-limited removal of metabolic products led to product-inhibited growth of Escherichia coli that was immobilized in a model system. Comparison of the growth kinetics of immobilized and free-living cells revealed no further physiological differences between cells in these two modes of existence beyond those manifested in the local concentrations of substrate and product. Bacteria were retained on a microporous membrane in a dense, planar aggregate and were grown anaerobically on a glucose-based minimal medium. Radioisotope labeling of the immobilized cell mass with 35S was used to determine growth kinetic parameters. Growth rates in the immobilized cell layer were measured by an autoradiographic technique which allowed comparison of the size of the growing region with the rate of cell convection caused by growth. Immobilized cell growth rates and growth yields ranged from near maximal (0.56 h-1 and 39 g of dry cell weight/mol of glucose, respectively) to substantially reduced (0.15 h-1 and 15 g/mol). The depression of these kinetic parameters was attributed to product inhibition arising from mass transfer-limited removal of acidic waste products from the cell mass. A simple one-dimensional reaction-diffusion model, which incorporated data on the product-inhibited growth kinetics of free-living cells collected in a product-limited chemostat, satisfactorily predicted product inhibition of immobilized cell growth.

pH feedback control of enzyme membranes

pH feedback on immobilized enzymes is theoretically examined with respect to substrate and pH levels, strength of acids produced by the reaction, buffering and asymmetry of the system. All the productions of proton by the different reactions are taken into account by using a 'symbolic species' H. The system of differential diffusion-reaction equations is then integrated using numerical methods. The local 'effective enzyme activity' modulated by an acidity factor enables us to predict and quantify evolutions of the systems: NonMichaelian behavior of an immobilized Michaelis-Menten-type enzyme is shown, even when pH back-actions are excluded; the analysis of intramembrane pH profiles shows that the shift of the optimal pH is a complex function of the substrate and pH levels, the intrinsic pH dependence of the enzyme, and the membrane characteristics. This study may easily be transposed to other types of effector such as divalent cations and used in examining self-regulations of multienzyme systems where pH-active reactions are involved.

Applications of purification reactions for minimizing reaction-generated enzyme poisoning

A mathematical model has been employed to examine the interplay of reaction and mass transfer in immobilized enzyme systems involving reaction-generated enzyme poisions. Deactivation rates can be significantly reduced in some cases by catalyzing a purification reaction in which the poison is transformed into an innocuous substance. This conclusion in illustrated experimentally for reaction-generated H2O2 in a continuous-flow stirred slurry reactor containing glucose oxidase immobilized on activated carbon.

Characteristics of unbuffered gel-immobilized urease particles. I. Internal pH

The overall rate of reaction of a gel-immobilized urease particle necessarily depends upon the hydrogen ion concentrations within the particle. When the particle is unbuffered, the internal hydrogen ion concentrations are a consequence of the local rates of reaction and the rate of egress of the products of hydrolysis. A simple apparatus has been devised which allows a fairly rapid determination of the hydrogen ion concentration in the center of a particle for any given size, enzyme concentration, substrate concentration, and external pH. The products of urea hydrolysis are self-buffering in the region of pH 8.83 and for an external pH less than the self-buffering pH, the pH within the particle is increased because of the reaction. When the external pH is greater than the self-buffering pH, the converse occurs. The pH at the center of the particle approaches the self-buffering pH with an increase in particle size and enzyme concentration. The external increase in the external substrate concentration has a limited effect, simply rendering the local rates of reaction to be of zero order. The center-line pH and therefore all internal hydrogen ion concentrations depend upon the parameter L square root pe and the external pH. Differences between the external and center-line pH values of the order of units are unexceptional. The implications of the internal pH profiles on the local and overall rates of reaction are explored.