ROLE OF CA2-TYPE ANALOG( IN THE BINDING OF PHOSPHOLIPASE A(2) WITH A MONOMERIC SUBSTRATE AND WITH ITS AMIDE)

Effects of Ca2+ on the kinetic parameters for the hydrolysis of monodi spersed 1,2-dihexanoyl-sn-glycero-3-phosphorylcholine (diC(6)PC), cata lyzed by Group I phospholipases A(2) (PLA(2)s) from Pseudechis austral is, Naja naja atra, and bovine pancreas and by Group II enzymes from V ipera russelli russelli, Agkistrodon, halys blomhoffii, and Trimeresur us flavoviridis, were studied by the pH-stat assay method at 25 degree s C, pH 7.5-8.2, and an ionic strength of 0.1 or 0.2 in the absence or presence of an amide-type substrate analog, 2-dodecanoyl-amino-1-hexa nol-phosphoglycol The binding of genuine substrate to the Group II enz ymes and that of its analog to the Groups I and II enzymes were marked ly facilitated by the binding of Ca2+ to the enzymes. On the other han d, the binding of genuine substrate to the Group I enzymes was found t o be independent of the Ca2+ binding. The former result suggests that the structures of the Group II enzyme-genuine substrate complexes and both types of enzyme-analog complexes are generally stabilized by the Ca2+ binding, whereas the latter indicates that the structures of the Group I enzyme-genuine substrate complexes are already similar to thos e of their Ca2+ complexes and that, therefore, these enzyme-substrate interactions are independent of the Ca2+ binding.