Ca2+-surfactant interactions affect enzyme stability in detergent solutions

Detergent proteases and amylases generally bind Ca2+ ions. These bound ions enhance enzyme stability, reducing the rates of degradative reactions such as unfolding and proteolysis. Thus, surfactant aggregates, such as micelles, affect protease and amylase stability indirectly, by competing with the enzymes for Ca2+ ions. Dissociation constants for Ca2+ interactions with anionic surfactant micelles are in the 10(-3) to 10(-2) M range. These interactions are weak relative to enzyme-Ca2+ interactions (K-d of order 10(-6) M). However, surfactant is typically present at much higher concentration than enzyme, and it is the Ca2+-micelle equilibrium that largely determines the amount of free Ca2+ available for binding to enzymes. The problem of surfactant-mediated Ca2+ removal from enzymes can be avoided by adding calcium to a detergent formulation in an amount such that the concentration of free Ca2+ is around 10(-5)M.