| Abstract: | Previously it has been shown that the binding of porcine pancreatic phospholipase A2 to lipid-water interfaces is governed by the pK of the alpha-NH3+ group of the N-terminal alanine. Chemically modified phospholipases A2 in which the N-terminal Ala has been replaced by D-Ala or in which the polypeptide chain has been elongated with DL-Ala no longer display activity toward micellar substrate. The activity of DL-Ala-1-, [D-Ala1]-, and [Gly1]phospholipases A2 on substrate monolayers, which allow a continuous change in the packing density of the lipid molecule, was investigated. At pH 6 [Gly1]phospholipase A2 behaves like the native enzyme on lecithin monolayers. DL-Ala1- and [D-Ala1]phospholipases A2, although they are active in this system, showed a weaker lipid penetration capacity at this pH. Studies on the pH and Ca2+ ion dependency of the pre-steady-state kinetics and of the activity of these radiolabeled proteins showed that [D-Ala1]phospholipase A2 does not possess a second low-affinity site for Ca2+ ions in contrast to the native phospholipase A2. This second low-affinity Ca2+ binding site, which is also absent in [Gly1]phospholipase A2, is induced in the latter enzyme by the presence of lipid-water interfaces. |
