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Insolubilized enzymes. Kinetic behaviour of glucose oxidase bound to porous glass particles.

ABSTRACT: 1. The spectrophotometric and steady-state kinetic properties of glucose oxidase (EC 1.1.3.4, from Aspergillus niger) that is covalently linked to porous glass beads have been examined. These properties have been compared with those of soluble glucose oxidase, for which the kinetic mechanism at pH5.5 and 25 degrees C has been established previously by a combination of conventional and rapid-reaction techniques to be the following: [Formula: see text] where E(o) and E(r) represent oxidized and reduced forms of the enzyme, respectively. 2. The ratio k(+4)/k(+2) is unchanged after insolubilization, and evidence is presented which suggests that the absolute magnitudes of k(+4) and k(+2) are unchanged. 3. The kinetic efficiency of the insolubilized enzyme is greatly enhanced because of a 14-fold increase in the apparent affinity of glucose for E(o). This effect is attributed either to the binding of glucose to the glass surface or to a change in enzyme structure imposed by the insolubilization process. 4. Only 6% of the insolubilized enzyme which can be reduced by glucose is catalytically active. It is shown by calculation and direct experimental evidence that this fraction of catalytically active enzyme is bound to the exterior bead surface. The remaining 94% of the enzyme is bound within the pore network and may be subject to severe substrate diffusion control.

SUBMITTER: Weibel MK

PROVIDER: S-EPMC1177258 | biostudies-other | 1971 Oct

REPOSITORIES: biostudies-other

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Project description:Thermally induced transitions of the 13-subunit integral membrane protein bovine cytochrome c oxidase (CcO) have been studied by differential scanning calorimetry (DSC) and circular dichroism (CD). Thermal denaturation of dodecyl maltoside solubilized CcO proceeds in two consecutive, irreversible, kinetically driven steps with the apparent transition temperatures at ∼ 51°C and ∼ 61°C (5μM CcO at scan rate of 1.5 K/min). The thermal denaturation data were analyzed according to the Lyubarev and Kurganov model of two consecutive irreversible steps. However, because of the limitation of the model to describe the complex mechanism of the thermal denaturation of CcO, the obtained results were utilized only for comparison purposes of kinetic stabilities of CcO under specific protein concentration (5μM) and scan rate (1.5 K/min). This enabled us to show that both the amphiphilic environment and the self-association state of CcO affect its kinetic stability. Kinetic stabilities of both steps are significantly decreased when all of the phospholipids are removed from CcO by phospholipase A2 (the half-life decreases at 37°C). Conversely, dimerization of CcO induced by sodium cholate significantly increases its kinetic stability of only the first step (the half-life increases at 37°C). Protein concentration-dependent nonspecific oligomerization also indicate mild stabilization of CcO. Both, reversed-phase high-performance liquid chromatography (HPLC) and SDS-PAGE subunit analysis reveal that the first step of thermal denaturation involves dissociation of subunits III, VIa, VIb, and VIIa, whereas the second step is less well defined and most likely involves global unfold and aggregation of the remaining subunits. Electron transport activity of CcO decreases in a sigmoidal manner during the first transition and this dependence is very well described by kinetic parameters for the first step of the thermal transition. Therefore, dissociation of subunit III and/or VIIa is responsible for temperature-induced inactivation of CcO because VIa and VIb can be removed from CcO without affecting the enzyme activity. These results demonstrate an important role of tightly bound phospholipids and oligomeric state (particularly the dimeric form) of CcO for kinetic stability of the protein.

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Insolubilized enzymes. Kinetic behaviour of glucose oxidase bound to porous glass particles.

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