[Role of calcium ions in the development of the catalytic activity of phospholipase D]

Abstract

The peculiarities of the reaction of mycellar lecithine cleavage by phospholipase D and the role of Ca2+ in the catalytic activity of the enzyme were studied. It was shown that Ca2+ participates in the formation of a catalytically active intermediary enzyme-substrate complex with a strict stoichiometric ratio of substrate, phospholipase activator (Na-DS) and Ca2+. The kinetics of inhibition for this reaction by lantane and fluoride ions and EDTA were studied. The inhibition of the reaction in a lecithine-Na-Ca2+-phospholipase D system by La3+ is due to substitution of bivalent Ca2+ by trivalent La3+ within the intermediary complex. The inhibiting effect of F- is due to penetration of the electron-negative ion into the coordination sphere of the intermediary complex and to disturbances in the hydrophobic binding between the ligands. The inhibitory action of EDTA is revealed during protein binding to the substrate and is not due to the chelating capacity of EDTA with respect to Ca2+. It was demonstrated that the role of Ca2+ is not restricted by its binding to phospholipase C. Ca2+ are also necessary for neutralization of the extra charge on the surface of substrate mycelles, production of an appropriate ionic strength and modification of the substrate phase surface at the interface. The catalytic function of Ca2+ consists in a formation of a "recognition" site in the active center of phospholipase D.