Reactivity of trypsin in reverse micelles: neglected role of aggregate size compared to water-pool components

Abstract

The catalytic efficiency of trypsin was estimated in cationic reverse micelles as a function of the concentration of water-pool components and aggregate size to determine their independent influence on enzyme activity. The variation in the aggregate size/water-pool size was achieved by changing both the W₀ (mole ratio of water to surfactant) and the headgroup area of surfactant through introduction of hydroxyethyl groups at the polar head. The local molar concentrations of water present inside the water-pool ([H₂O]_wp) of different cationic reverse micelles across varying W₀ was estimated using a modified phenyl cation-trapping protocol. The [H₂O]_wp in cationic reverse micelles (surfactant/isooctane/n-hexanol/water) increases with W₀ and attains the molarity of normal water beyond W₀ = 40 irrespective of the nature of headgroup. Concurrently, the catalytic activity of trypsin compartmentalized within the water-pool increases with the increase in [H₂O]_wp upto an optimal W₀ = 40 in organized solutions of any surfactant. The aggregate size (determined by static light scattering) also increases expectedly with W₀ and noticeably with the area of the surfactant headgroup at similar W₀. Since the enzyme activity rises both with the increase in water-pool size and [H₂O]_wp, trypsin's efficiency was compared with these two parameters across reverse micelles of varying surfactant headgroup size at similar W₀ to determine their probable independent influence in regulating the enzyme activity. Noticeably, the efficiency of trypsin rises two to ninefold in spite of the [H₂O]_wp being distinctly lower in case of hydroxyethyl group substituted surfactants compared to cetyltrimethylammonium bromide w/o microemulsions at similar W₀. Thus, the influence of the aggregate size possibly plays an important role alongwith the [H₂O]_wp in modulating the enzyme activity.