Enzyme molecules in solitary confinement

A microreactor sealing method using adhesive tape for digital bioassays

Digital assays using microreactors fabricated on solid substrates are useful for carrying out sensitive assays of infectious diseases and other biological tests. However, sealing of the microchambers using fluid oil is difficult for non-experts, and thus hinders the widespread use of digital microreactor assays. Here, we propose the physical isolation of tiny reactors with adhesive tape (PITAT) using simple, commercially available pressure-sensitive adhesive (PSA) tape as a separator of the microreactors. We confirmed that PSA tape can effectively seal the microreactors and prevent molecules from diffusing out. By testing several types of adhesive tape, we found that rubber-based adhesives are the most suitable for this purpose. In addition, we demonstrated that single-molecule enzyme assays can be successfully performed inside microreactors sealed with PSA tape. The results obtained using PITAT are quantitatively comparable to conventional oil sealing, although it is quick and cost-effective. Finally, we demonstrated that single-particle virus counting of the influenza virus can be achieved using PITAT. Collectively, our results suggest that PITAT may be suitable for use in the design of sensitive tests for infectious diseases at the point of care, where no sophisticated equipment or machines are available.

Dynamic monitoring of a bi-enzymatic reaction at a single biomimetic giant vesicle

Giant unilamellar vesicles were used as individual biomimetic micro-reactors wherein a model bi-enzymatic reaction involving a glucose oxidase (GOx) and horseradish peroxidase (HRP) was monitored by confocal microscopy.

Self-Rolling Refillable Tubular Enzyme Containers Made of Recombinant Spider Silk and Chitosan

Encapsulation of enzymes is often necessary to stabilize them against environmental conditions or to protect them from other harmful enzymes such as proteases. Here, a refillable spatial confinement system was produced using a fully degradable self-rolling biopolymer bilayer. The enzyme containers comprise spider silk and chitosan and enable one-pot reactions in the micro- to milliliter regime by trapping the enzyme inside the semipermeable tube and allow the substrate and/or product either to diffuse freely or to be entrapped. The tubes are stable toward several organic and aqueous solvents. A one-tube system with esterase-2 was used to establish the system. Further, a two-tube system was applied to mimic enzymatic cascades, where the enzymes have to be separated, because they, for example, inhibit each other. The entrapment mode was also tested in the two-tube system, which is beneficial for toxic products or for obtaining high concentrations of the desired product.

Ultrasensitive Single-Molecule Enzyme Detection and Analysis Using a Polymer Microarray

This report describes a novel method for isolating and detecting individual enzyme molecules using polymer arrays of picoliter microwells. A fluidic flow-cell device containing an array of microwells is fabricated in cyclic olefin polymer (COP). The use of COP microwell arrays simplifies experiments by eliminating extensive device preparation and surface functionalization that are common in other microwell array formats. Using a simple and robust loading method to introduce the reaction solution, individual enzyme molecules are trapped in picoliter microwells and subsequently isolated and sealed by fluorinated oil. The sealing is stable for hours in the COP device. The picoliter microwell device can measure enzyme concentrations in the low-femtomolar range by counting the number of active wells using a digital read-out. These picoliter microwell arrays can also easily be regenerated and reused.

Monitoring the Activity of Immobilized Lipase with Quinizarin Diester Fluoro-Chromogenic Probe

Quinizarin diester is used as a fluoro-chromogenic substrate of the activity of lipase supported in poly(methylmetacrylate) beads (CALB, Novozym^® 435) dispersed in organic solvents. The monoester and diester of quinizarin are both non-fluorescent species contrasting with the enzymatic product quinizarin that shows optical absorption in the visible region and strong fluorescence signal. The enzymatic conversion is accomplished by spectroscopic measurements and it follows a sigmoid curve from which the mean reaction time of the enzymatic process can be determined. This parameter indicates the enzyme activity of the immobilized lipase. Its dependency with the amount of lipase allowed the determination of the ratio of the catalytic rate and the Michaelis constant (k_c/K_m) and the experimental value found was (1.0 ± 0.1) × 10⁻² mg⁻¹/min in the case of quinizarin diacetate.

Exploiting Femtoliter Microwells for the Sensitive Measurement of Protein Adsorption

A method is described for the sensitive measurement of adsorbed proteins using femtoliter microwells. Quantitative measurement of adsorbed protein is demonstrated at surface densities from 10 fg/cm² to 3 pg/cm². Determination of the efficacy of barrier coatings is also demonstrated using femtoliter microwells. Adsorption at low surface densities is measured, indicating the highest affinity sites on the surface and therefore the initial stages of adsorption. The femtoliter microwell method is shown to be useful in detecting differences between effective protective coatings.

Enzyme Kinetics in Femtoliter Arrays

Over the last decade, femtoliter arrays have been used as a simple and robust way to encapsulate and monitor the kinetics of single enzyme molecules. Encapsulating individual enzyme molecules within a femtoliter-sized reaction chamber does not require immobilization of the enzyme molecules or fluorescent tagging of the enzyme molecules, which offers the unique advantage of observing unmodified single enzyme molecules free in solution. Several fascinating details about enzyme kinetics have been revealed using these femtoliter arrays, which were unattainable from traditional ensemble experiments. Here, we discuss various considerations to take into account when developing single-molecule enzyme assays in femtoliter arrays and the advantages and disadvantages of various protocols.

Enzymatic reactions in confined environments

Within each biological cell, surface- and volume-confined enzymes control a highly complex network of chemical reactions. These reactions are efficient, timely, and spatially defined. Efforts to transfer such appealing features to in vitro systems have led to several successful examples of chemical reactions catalysed by isolated and immobilized enzymes. In most cases, these enzymes are either bound or adsorbed to an insoluble support, physically trapped in a macromolecular network, or encapsulated within compartments. Advanced applications of enzymatic cascade reactions with immobilized enzymes include enzymatic fuel cells and enzymatic nanoreactors, both for in vitro and possible in vivo applications. In this Review, we discuss some of the general principles of enzymatic reactions confined on surfaces, at interfaces, and inside small volumes. We also highlight the similarities and differences between the in vivo and in vitro cases and attempt to critically evaluate some of the necessary future steps to improve our fundamental understanding of these systems.

Special issue: single molecule techniques

Technological advances in the detection and manipulation of single molecules have enabled new insights into the function, structure and interactions of biomolecules. This Special Issue was launched to account for the rapid progress in the field of “Single Molecule Techniques”. Four original research articles and seven review articles provide an introduction, as well as an in-depth discussion, of technical developments that are indispensable for the characterization of individual biomolecules. Fluorescence microscopy takes center stage in this Special Issue because it is one of the most sensitive and flexible techniques, which has been adapted in many variations to the specific demands of single molecule analysis. Two additional articles are dedicated to single molecule detection based on atomic force microscopy.

A microdevice for producing monodispersed droplets under a jetting flow

A new capillary extended step microchannel for generating monodispersed droplets with a much wider narrowing jetting flow regime was specially designed.