Figure 3.

Principle of the surface dilution model. A, The principle of the surface dilution model is presented in Equation 1 (Carman et al., 1995). According to this model, the action of the enzyme E consists of two consecutive steps. First, the enzyme interacts noncatalytically with the surface of detergent/lipid mixed micelles A. Subsequently, the enzyme mixed-micelle complex EA binds to the individual lipid substrate B presented on the surface, which leads to the conversion of substrate to product Q. The first association depends on the bulk concentration of both E and A, whereas the second step depends on the surface concentrations of EA and B. A is defined as the sum of the molar concentrations of detergent plus lipid substrate, and B is the mole fraction of lipid substrate in the mixed micelle. Equation 2 is the rate expression for the surface dilution kinetic model (Carman et al., 1995). Three kinetic parameters, V_max, K_s^A, and K_m^B, can be determined from this equation. V_max is the true V_max when both the bulk concentration and the surface concentration of the lipid substrate approach infinity. K_s^A, which equals k₋₁/k₁ and is expressed in bulk concentration terms, is the dissociation constant describing the interaction of the enzyme with the mixed micelles in the first binding step. K_m^B, equal to (k₋₂ + k₃)/k₂, defines the interfacial K_m for the second binding step and is expressed in surface concentration units, such as mole fraction. B, SphK activity in leaf lysates measured with increasing Triton X-100 molar concentrations in the mixed micelles. The molar concentration of Sph was held constant at 50 μm. Data are means ± se of five independent experiments.