Zinc and barium inhibit the phospholipase A2 from Naja naja atra by different mechanisms

Abstract

The mode of inhibition of the phospholipase A2 (PLA2) enzyme from the Chinese cobra (Naja naja atra) by Zn2+ is qualitatively different from inhibition by Ba2+. Inhibition by Ba2+ shows the kinetic characteristics of a conventional competitive inhibitor acting to displace Ca2+ from a single essential site, but Zn2+ has the paradoxical property of being more inhibitory at high than at low Ca2+ concentration. Kinetic analysis of the Ca2+-dependence of enzymic activity shows a bimodal response, indicating the presence of two Ca2+-binding sites with affinities of 2.7 μM and 125 μM respectively, and we propose that these can be identified with the two Ca2+-binding sites revealed by crystallographic analysis. The results are consistent with the model that the enzyme is activated by two Ca2+ ions, one that is essential and can be displaced by Ba2+, and one that modulates the activity by a further 5-10-fold and which can be displaced by Zn2+. An alternative model is also presented in which the modulating Zn2+-binding site is a phenomenon of the lipid/water interface.