Spectral properties of Bitis gabonica venom phospholipase A2 in the presence of divalent metal ion, substrate and hydrolysis products

Abstract

Bitis gabonica phospholipase A2 binds 2 moles of Ca2+ per mole of dimer, giving rise to spectral perturbations of tryptophan chromophores in the enzyme. The spectral changes are taken to originate from both solvent and charge effects. A pH dependent perturbation is noted both in the presence and absence of Ca2+; however, the character of the perturbation in absence of metal ions is different from the perturbation observed in the presence of Ca2+. The data suggest that Ca2+ induces a conformational change in the enzyme, allowing the productive binding of substrate. A change in tryptophan absorption is also noted in the presence of dipalmitoyllecithin, lysolecithin and palmitic acid. However, perturbation by palmitic acid causes a blue shift of the spectrum as opposed to a red shift caused by lysolecithin and dipalmitoyllecithin. Fine structure in the 270–300 nm range due to tryptophan perturbation, is abolished when lysolecithin binds to the enzyme in the presence of Ca2+. This is interpreted to mean that the perturbable tryptophan is involved in substrate binding.