Membrane topology modulates β-galactosidase activity against soluble substrates Original Research Article

The effect of bio-surfaces of contrasting curvature, on the kinetic parameters of ortho-nitrophenyl-β-d-galactopiranoside hydrolysis catalyzed by E. coli β-galactosidase, was investigated. The self-aggregating state and structure of the amphiphiles (Phosphatidylcholine, Lubrol-PX, Triton X-100, DocNa, SDS and CTAB) were inferred from their c.m.c. values and light-scattering measurements. Low curvature phosphatidylcholine or mixed phosphatidylcholine-detergent vesicles increased Vmax without affecting KM. High curvature micellar structures containing ionic detergents modulated negatively the enzyme activity (decreased or abolished Vmax and increased KM). Neither micelles containing non-ionic detergents nor the amphiphiles in a monomeric form, affected enzyme activity. CTAB at a concentration bellow its c.m.c but incorporated into a bilayer, became an activator (KM decreased respect to the control). Non-enzymatic interfacial hydrolysis of the substrate was discarded. Enzyme–membrane interaction and membrane elasticity, were evaluated using monomolecular layers at the air–water interface. Beyond particular molecular structures, topology affected the direction of the modulatory effects exerted by these amphiphiles on β-galactosidase activity.