Pore diffusion model for a two-substrate enzymatic reaction: application to galactose oxidase immobilized on porous glass particles

Glucose oxidase immobilization on a novel cellulose acetate–polymethylmethacrylate membrane

Glucose oxidase (GOD) was immobilized on cellulose acetate-polymethylmethacrylate (CA-PMMA) membrane. The immobilized GOD showed better performance as compared to the free enzyme in terms of thermal stability retaining 46% of the original activity at 70 degrees C where the original activity corresponded to that obtained at 20 degrees C. FT-IR and SEM were employed to study the membrane morphology and structure after treatment at 70 degrees C. The pH profile of the immobilized and the free enzyme was found to be similar. A 2.4-fold increase in Km value was observed after immobilization whereas Vmax value was lower for the immobilized GOD. Immobilized glucose oxidase showed improved operational stability by maintaining 33% of the initial activity after 35 cycles of repeated use and was found to retain 94% of activity after 1 month storage period. Improved resistance against urea denaturation was achieved and the immobilized glucose oxidase retained 50% of the activity without urea in the presence of 5M urea whereas free enzyme retained only 8% activity.

Enzyme immobilization in novel alginate–chitosan core-shell microcapsules

Alginate-chitosan core-shell microcapsules were prepared in order to develop a biocompatible matrix for enzyme immobilization, where the protein is retained either in a liquid or solid core and the shell allows permeability control over substrates and products. The permeability coefficients of different molecular weight compounds (vitamin B2, vitamin B12, and myoglobin) were determined through sodium tripolyphosphate (Na-TPP)-crosslinked chitosan membrane. The microcapsule core was formed by crosslinking sodium alginate with either calcium or barium ions. The crosslinked alginate core was uniformly coated with a chitosan layer and crosslinked with Na-TPP. In the case of calcium alginate, the phosphate ions of Na-TPP were able to extract the calcium ions from alginate and liquefy the core. A model enzyme, beta-galactosidase, was immobilized in the alginate core and the catalytic activity was measured with o-nitrophenyl-beta-D-galactopyranoside (ONPG). Change in the activity of free and immobilized enzyme was determined at three different temperatures. Na-TPP crosslinked chitosan membranes were found to be permeable to solutes of up to 17,000Da molecular weight. The enzyme loading efficiency was higher in the barium alginate core (100%) as compared to the calcium alginate core (60%). The rate of ONPG conversion to o-nitrophenol was faster in the case of calcium alginate-chitosan microcapsules as compared to barium alginate-chitosan microcapsules. Barium alginate-chitosan microcapsules, however, did improve the stability of the enzyme at 37 degrees C relative to calcium alginate-chitosan microcapsules or free enzyme. This study illustrates a new method of enzyme immobilization for biotechnology applications using liquid or solid core and shell microcapsule technology.

Comparison of the coupling recoveries of immobilized aspartate aminotransferase. Specific activity, enzyme-bound coenzyme, and transaminationable active centers

Aspartate aminotransferase (AspAT, EC 2.6.1.1) was bound on CNBr-activated Sepharose and the effects of immobilization on the maximum velocity, biologically active pyridoxal-5'-phosphate (PLP), and transaminationable active centers were studied. By comparing these parameters of soluble and immobilized enzyme the factors decreasing the observed reaction rate upon immobilization were evaluated. Ninety percent of the soluble protein in the coupling mixture was bound to the support. The amount of enzyme-bound PLP of immobilized preparation was 83% of that of the soluble one. The coupling recovery of specific activity was 46%, which was 10%-units lower than that of the transaminationable active centers. This difference depends on the fact that a part of the active centers of immobilized enzyme had lower catalytic rate, due to the enzyme-matrix interactions or internal mass transfer limitations, than the others. The immobilized catalytically active AspAT had 80% of the turnover efficiency of the soluble enzyme. The affinity of the enzyme to its substrates did not significantly change upon immobilization, neither did the pH profile.

Factors affecting the activity of immobilized enzymes, I. Diffusional limitation

The measured (apparent) specific activities of immobilized lactate dehydrogenase, malate dehydrogenase, glucose dehydrogenase, alkaline phosphatase and chymotrypsin decrease with increasing activity loading and increase with decreasing particle diameter and with increasing substrate concentration. The observed inactivation is therefore concluded to be due to diffusional limitation. Real specific activities are not greatly affected by the immobilization, as has been demonstrated after enzymic digestion of the matrix. In the case of lactate dehydrogenase and of malate dehydrogenase real specific activities are not altered by a variation of the number of bonds to the carrier.

Analysis of the coupled transport, reaction, and deactivation phenomena in the immobilized glucose oxidase and catalase system

In a previous paper, the overall or macrokinetics of the immobilized glucose oxidase--catalase system has been presented. In this paper a detailed analysis of the interaction of diffusion and reaction in this system will be presented. The mathematical treatment includes two consecutive reactions with two-substrate kinetics. Furthermore, the deactivation of both enzymes due to the intermediate product peroxide is taken into account. The predicted results suggest that the efficiency of the glucose oxidase reaction depends on the concentration ranges of the two substrates. Furthermore, the external mass-transfer rate may cause a shift from glucose limitation to oxygen limitation. The efficiency of the coupled system is always higher than that predicted for the uncoupled reaction path. The calculations show that the economics of the coupled system depend a great deal on the deactivation of the enzymes.

A fast evaluation of diffusion effects on bound enzyme activity

As the kinetic behavior of bound enzymes is frequently affected by substrate diffusion between the bulk solution and the catalytic sites, a fast and simple method is proposed to detect and, subsequently, to remove diffusion effects on measured enzymic activities. The procedure makes use of the effectiveness factor concept and essentially involves the direct determination on two diagrams of the magnitude of both external and internal diffusion limitations. It requires a prior estimation of the volume and external surface area of the matrix, of the substrate external transport coefficient and internal diffusivity, and of the intrinsic Michaelis constant of the bound enzyme. However, it does not necessitate the knowledge of the quantity of bound enzyme. The two basic graphs have been calculated for Michaelis-Menten kinetics. They can also be used to evaluate diffusional effects on two-substrate reactions, as illustrated with previously published data.

Galactose oxidase: applications of the covalently immobilized enzyme in a packed bed configuration

Galactose oxidase (E.C. 1.1.3.9) was covalently immobilized to chemically modified porous silica particles by reaction of the native enzyme with pendant benzoyl azide groups on the carrier. The enzyme loading on the carrier was 100-150 units per milliliter. The immobilized enzyme was incorporated into a hardware assembly suitable for the determination of galactose or lactose concentrations in complex biological fluids. The prototype instrument as described is suitable for continuous, on-line monitoring or discrete sample analysis. Reaction conditions can be readily provided which maintain global first order kinetics within the reactor and strict linearity of the procedure over a wide range of sample concentrations. Auto-inactivation of the immobilized enzyme can be prevented by K3Fe(CN)6 and long-term reactor stability can be achieved by the periodic application of the reagent to the enzyme reactor in situ.