Operational effectiveness factors of immobilized enzyme systems

Immobilization of E. coli expressing Bacillus pumilus CynD in three organic polymer matrices

Cyanide is toxic to most living organisms. The toxicity of cyanide derives from its ability to inhibit the enzyme cytochrome C oxidase of the electronic transport chain. Despite its high toxicity, several industrial processes rely on the use of cyanide, and considerable amounts of industrial waste must be adequately treated before discharge. Biological treatments for the decontamination of cyanide waste include the use of microorganisms and enzymes. Regarding the use of enzymes, cyanide dihydratase (CynD), which catalyzes the conversion of cyanide into ammonia and formate, is an attractive candidate. Nevertheless, the main impediment to the effective use of this enzyme for the biodegradation of cyanide is the marked intolerance to the alkaline pH at which cyanide waste is kept. In this work, we explore the operational capabilities of whole E. coli cells overexpressing Bacillus pumilus CynD immobilized in three organic polymer matrices: chitosan, polyacrylamide, and agar. Remarkably, the immobilized cells on agar and polyacrylamide retained more than 80% activity even at pH 10 and displayed high reusability. Conversely, the cells immobilized on chitosan were not active. Finally, the suitability of the active complexes for the degradation of free cyanide from a solution derived from the gold processing industry was demonstrated.

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.