Kinetic analysis for solid-supported enzymes

Engineered catalytic biofilms: Site-specific enzyme immobilization onto E. coli curli nanofibers

Biocatalytic transformations generally rely on purified enzymes or whole cells to perform complex transformations that are used on industrial scale for chemical, drug, and biofuel synthesis, pesticide decontamination, and water purification. However, both of these systems have inherent disadvantages related to the costs associated with enzyme purification, the long-term stability of immobilized enzymes, catalyst recovery, and compatibility with harsh reaction conditions. We developed a novel strategy for producing rationally designed biocatalytic surfaces based on Biofilm Integrated Nanofiber Display (BIND), which exploits the curli system of E. coli to create a functional nanofiber network capable of covalent immobilization of enzymes. This approach is attractive because it is scalable, represents a modular strategy for site-specific enzyme immobilization, and has the potential to stabilize enzymes under denaturing environmental conditions. We site-specifically immobilized a recombinant α-amylase, fused to the SpyCatcher attachment domain, onto E. coli curli fibers displaying complementary SpyTag capture domains. We characterized the effectiveness of this immobilization technique on the biofilms and tested the stability of immobilized α-amylase in unfavorable conditions. This enzyme-modified biofilm maintained its activity when exposed to a wide range of pH and organic solvent conditions. In contrast to other biofilm-based catalysts, which rely on high cellular metabolism, the modified curli-based biofilm remained active even after cell death due to organic solvent exposure. This work lays the foundation for a new and versatile method of using the extracellular polymeric matrix of E. coli for creating novel biocatalytic surfaces.

Determination of the intrinsic kinetic constants of immobilized glucose oxidase and catalase

Models of membrane systems containing immobilized glucose oxidase and catalase operating together or independently have been developed. A rotated disk electrode apparatus was employed with novel electrochemical operating conditions to experimentally determine mass transfer and intrinsic kinetic parameters of enzyme-containing membranes. The value of a mass transfer parameter that describes internal and external diffusion was first determined under conditions that do not permit the enzyme reactions. In a subsequent experiment with the reaction allowed, kinetic parameters corresponding to the intrinsic maximal velocity and Michaelis constants of the immobilized enzymes were estimated by regression analysis of data based on an appropriate two- or three- parameter model. It was found that immobilization reduced the maximal intrinsic velocity but had no detectable effect on the Michaelis constants. In all but one case- these methods for membrane characterization are nondestructive and can be used repeatedly on a given membrane. These techniques provide the means for quantitative comparisons of immobilization methods and make possible temporal studies of immobilized enzyme inactivation.

Specifically Immobilised Aldo/Keto Reductase AKR1A1 Shows a Dramatic Increase in Activity Relative to the Randomly Immobilised Enzyme

The difference between site-specific and random immobilisation of the aldo/keto reductase AKR1A1 was explored. AKR1A1 was recombinantly expressed as a thioester by the intein strategy. The thioester was selectively modified with a biotin label by the expressed protein ligation method, and subsequent immobilisation on streptavidin templates was performed. Adsorption of wild-type AKR1A1 to streptavidin templates and of biotinylated AKR1A1 to uncoated templates was used to study randomly immobilised enzymes. Investigation of the kinetic parameters revealed remarkably improved activity for the site-specifically immobilised enzyme, which was comparable to that of the wild-type enzyme in solution and 60-300-fold greater than that of the randomly immobilized enzymes. Furthermore, the enzyme was surprisingly stable. No loss of activity was observed for over a week, and even after 50 days more than 35% of activity was maintained.

Structure-function relationships in immobilized chymotrypsin catalysis

Specific activities and the amounts of active immobilized enzyme were determined for several different preparations of alpha-chymotrypsin immobilized on CNBr-activated Sepharose 4B. Electron paramagnetic resonance (EPR) spectroscopy of free and immobilized enzyme with a spin label coupled to the active site was used to probe the effects of different immobilization conditions on the immobilized enzyme active site configuration. Specific activity of active enzyme decreased and rotational correlation time of the spin label increased with increasing immobilized enzyme loading. Enzyme immobilized using an intermediate six-carbon spacer arm exhibited greater specific activity and spin label mobility than directly coupled enzyme. The observed activity changes due to immobilization were completely consistent with corresponding active site structure alterations revealed by EPR spectroscopy.

Immobilization of amyloglucosidase onto granular chicken bone

Amyloglucosidase was immobilized onto granular chicken bone (BIOBONE) by noncovalent interactions. The amount of activity bound relative to an equal amount of free enzyme was 13.6 +/- 0.4%. The estimated specific activity for amyloglucosidase decreased from 75.3 +/- 0.8 to 43.5 +/- 9.6 U/mg protein upon immobilization. The Km value of the bone-immobilized enzyme using glycogen as substrate increased from 3.04 +/- 0.38 mg/mL (free) to 9.04 +/- 1.51 mg/mL (immobilized), but Km showed no change upon immobilization when starches were used as substrates. A decrease in Vmax values occurred upon enzyme immobilization for all substrates, but this largely reflected the percentage of enzyme initially bound to the bone. Immobilization also improved enzyme stability in the presence of various additives (e.g., detergent, KCl, and ethanol) or under low or high pH reaction conditions. Bound amyloglucosidase maintained high activity (greater than 90%) following five cycles of continuous use at moderate (23 degrees C) and high (55 degrees C) temperatures. Data derived from Lineweaver-Burk and Arrhenius plots indicated that substrate and product diffusion limitation were minimal.

Flow microcalorimetric study of immobilized enzyme kinetics using the co-immobilized glucose oxidase-catalase system

The kinetics of the bi-substrate enzyme glucose oxidase (EC 1.1.3.4) (with catalase, EC 1.11.1.6) co-immobilized on glass was studied using flow microcalorimetry. Oxygen and glucose external-internal diffusion effects were resolved.

Diffusional falsification of kinetic constants on Lineweaver-Burk plots

The effect of mass transfer resistances on the Lineweaver-Burk plots in immobilized enzyme systems has been investigated numerically and with analytical approximate solutions. While Hamilton, Gardner & Colton (1974) studied the effect of internal diffusion resistances in planar geometry, our study was extended to the combined effect of internal and external diffusion in cylindrical and spherical geometries as well. The variation of Lineweaver-Burk plots with respect to the geometries was minimized by modifying the Thiele modulus and the Biot number with the shape factor. Especially for a small Biot number all the three Lineweaver-Burk plots fell on a single line. As was discussed by Hamilton et al. (1974), the curvature of the line for large external diffusion resistances was small enough to be assumed linear, which was confirmed from the two approximate solutions for large and small substrate concentrations. Two methods for obtaining intrinsic kinetic constants were proposed: First, we obtained both maximum reaction rate and Michaelis constant by fitting experimental data to a straight line where external diffusion resistance was relatively large, and second, we obtained Michaelis constant from apparent Michaelis constant from the figure in case we knew maximum reaction rate a priori.

Temperature and pH effects with immobilized electric eel acetylcholinesterase

Kinetic studies were made with 2 forms of immobilized acetylcholinesterase: enzyme trapped in polyacrylamide gel which was cut into slices; and enzyme attached to the inner surface of nylon tubing. Rates were measured at substrate concentrations which were low and high with reference to the Michaelis constant, and over the temperature range 16-40 degrees C. Low activation energies (1.7-2.7 kcal mol-1) were obtained at low substrate concentrations, indicating diffusion control. At high substrate concentrations the Arrhenius plots were non-linear and the activation energies substantially higher, and there is less diffusion control. With enzyme-polyacrylamide slices, there was a continuous increase in rate with increasing pH, in contrast to the bell-shaped behavior with free enzyme. A theoretical treatment suggests that this is due to the lowering of local pH as a result of the acid released in the hydrolysis.

Effects of carrier morphology and buffer diffusion on the expression of enzymatic activity

A very stable esterase (EC 3.1.1.-), which hydrolyses ethyl acetate, cephalosporin C and other acetyl esters with a maximum turnover number of 3-10(2) s-1, was isolated from Bacillus subtilis ATCC 6633 and immobilized on two supports: controlled-pore glass and powdered brick, a representative of carriers having a wide pore-size distribution. Carrier morphology determines diffusion rates and the expression of activity. Rate-limiting mass transfer of buffer leads to apparent losses of activity, gross distortions of molecular pH vs. activity profiles and to apparent deviations from Michaelis-Menten kinetics.

Temperature dependence of a diffusion-limited immobilized enzyme reaction

The apparent activation energy of N-alpha-benzoyl-L-arginine-ethyl ester (BAEE) hydrolysis by immobilized trypsin varies with the bulk substrate concentration from its maximum value, comparable to that of the free enzyme, to considerably lower values. Thus, with a concentration change from 3 x 10(-2) to 10(-4) M the apparent activation energy diminishes from 9.5 to 4.5 kcal/mol. This experimental finding is interpreted to be due to Michaelis-type kinetics in a heterogeneous system, in one case reflecting the temperature dependence of the maximal enzyme reaction rate, in another case illustrating the diffusion limited overall reaction at low substrate concentrations. As a consequence it may not be feasible to operate a reaction at elevated temperatures in a high conversion range, since diffusion limitation may restrict the enhancement of the overall reaction rate. Some further data are given concerning the buffer effect on the reaction rate, which should occur due to its limitation by proton transfer in the buffer-free system.

Immobilized electric eel acetylcholinesterase. I. Kinetics of acetylcholinesterase trapped in polyacrylamide membranes

Techniques are described for the trapping of electric eel acetylcholinesterase in polyacrylamide gel. The activity of the trapped enzyme was substantially reduced, the effect being due to inhibition by acrylamide, but the emzyme immobilized in polyacrylamide was considerable more stable than that in free solutionma kinetic study was made of the hydrolysis of acetylthiocholine, covering a range of membrane thicknesses, enzyme concentrations, substrate concentrations and temperatures. The results were interpreted with reference to the theoretical treatment of Sundaram, Tweedale and Laidler, and of Kobayaski and Laidler, and provided support for those treatments; Clear evidence was obtained for diffusion control with the thicker membranes. An activation energy was obtained for the diffusion of the substrate within the membrane, by combining the temperature results for thick and thin membranes at low substrate concentrations. The results lead to the conclusion that the in vivo kinetics of acetylcholinesterase are largely diffusion-free in muscle filaments, but are substantially diffusion-controlled in fibrils and fibers.