STUDY OF STRUCTURE-ACTIVITY RELATIONSHIP OF FASCICULIN BY ACETYLATIONOF AMINO-GROUPS

Dendroaspis angusticeps (green mamba) has two toxins, fasciculins, tha t are non-competitive inhibitors of acetylcholinesterase. Amino groups of fasciculin 2 were acetylated with acetic anhydride. The monoacetyl derivatives of the E-amino groups (Lys 25, 32, 51 and 58) retained be tween 28 and 33% of the initial activity and that of the cu-amino grou p 72%. Acetylation of Lys 25 that has the most reactive amino group de creased the activity by 65% apparently without producing structural pe rturbations. since the circular dichroism spectrum was not affected. T he three-dimensional structure shows a cationic cluster formed by Lys 32, 51, Arg 24 and 28. A comparison of 175 sequences of homologous tox ins shows that Lys 32 is unique for fasciculin. Acetylation of lysine residues in the cluster had a large effect and reduced the activity by 72% (Lys 32) and 57% (Lys 51). This suggests an important role for th e cationic cluster. Lys 25 together with Lys 32 and 51 were, therefore , assumed to be in the active site.