Catalytic Activity of Hexokinase in Reversed Micelles

Abstract

Reversed micelles can control the size of water pools and the physical property of water by changing W0(=[water]/[surfactant]). Hexokinase (HK) activity seems to be easily affected by the microenvironment in the neighborhood of the enzyme because it is assumed that HK binds to the outer mitochondrial membrane by insertion of its hydrophobic NH2 tail. The catalytic activity of HK was examined in reversed micelles in order to study the effect of the microenvironment in the neighborhood of HK on the activity. Sodium bis(2-ethylhexyl)sulfosuccinate (AOT), hexadecyltrimethyl ammonium chloride (HTAC), and octaoxyethylene dodecyl ether (C12E8) were used as anionic, cationic, and nonionic surfactants, respectively. HK activity was obtained by measuring ATP and ADP amounts with HPLC. The high electrostatic inner surfaces of AOT and HTAC reversed micelles were not favorable for HK to exhibit the catalytic activity, but the activity in HTAC reversed micelles was 2–3 times higher than that in AOT reversed micelles and the activities in both reversed micelles revealed an optimum at W0=10. The phenomenon was discussed in connection with the location of HK, nonuniform distribution of substrates, and the size and physical properties of the water pools. On the other hand, HK activity was much higher in C12E8 reversed micelles than in AOT and HTAC reversed micelles and increased with the concentration of C12E8. This suggests that HK activity is easily revealed in hydrated ethylene oxide chains. In conclusion, it was demonstrated that HK activity depends on the microenvironment such as the electrostatic field, the physical properties of water, and the hydrophobicity.