An essential tryptophan in the active site of phospholipase A2 from the venom of Bitis gabonica

The role of tryptophan in phospholipase A2 (EC 3.1.1.4) from the venom of the gaboon viper, Bitis gabonica, has been investigated. Modification of the enzyme with N-bromosuccinimide and 2-nitrophenylsulfenylchloride showed that the two tryptophan residues in the enzyme, viz. Trp-28 and Trp-59, differ in reactivity towards the reagents. Only Trp-28 reacted with N-bromosuccinimide while a preferential reaction occurred between Trp-59 and 2-nitrophenyl-sulfenylchloride. In each case it was found that loss of enzyme activity was specifically correlated with modification of TRP-28. CD spectra indicated that neither the local nor the gross conformation of the enzyme was altered by modification of Trp-28 and it was therefore concluded that Trp-28 is crucial for enzyme activity. The active enzyme was protected against N-bromosuccinimide inactivation by micellar concentrations of substrate or substrate analogue, suggesting that Trp-28 is involved in substrate binding.