Effect of arsenate contamination on free, immobilized and soil alkaline phosphatases: activity, kinetics and thermodynamics

Soil alkaline phosphatase plays a vital role in phosphorus cycling in the soil ecosystem. Arsenic, an ecotoxic element, affects the normal function of phosphatase in the soil environment. To understand the effects of arsenic on phosphatase, the activity, kinetic and thermodynamic characteristics were investigated systematically following the addition of arsenate to free soluble, clay-immobilized alkaline phosphatase (ALP) and soil ALP. The results showed that the ALP activity was strongly inhibited by arsenate. The Michaelis constant K_m increased linearly with increasing arsenate concentration, indicating a decrease in affinity of the enzyme for the substrate. The inhibition constant K_i increased after the ALP was immobilized on montmorillonite and goethite. Thermodynamic properties such as activation energy, activation enthalpy, Gibbs activation free energy and activation entropy increased when arsenate was added, which suggests that more energy is required to form the activated molecule and the enzyme–substrate complex. The reaction was less spontaneous and the enzyme–substrate complex was less ordered. Immobilization of the ALP on minerals reduced the variation in kinetic and thermodynamic properties in the presence of arsenate, indicating that immobilization provided protection from arsenate stress. Arsenate toxicity to soil ALP was affected markedly by soil texture. The results suggest that the type of arsenate inhibition on free soluble ALP was competitive inhibition, whereas for immobilized and soil ALPs, the inhibition was competitive or mixed inhibition depending on the mineral or soil property