MOLECULAR-STRUCTURE, CONFORMATIONAL-ANALYSIS, AND STRUCTURE-ACTIVITY STUDIES OF DENDROTOXIN AND ITS HOMOLOGS USING MOLECULAR MECHANICS AND MOLECULAR-DYNAMICS TECHNIQUES

Three-dimensional structures of Dendrotoxin (DtX), Toxin-I (DpI), and Toxin-K (DpK) were determined using molecular mechanics and molecular dynamics techniques. The overall molecular conformation and protein fo lding of the three dendrotoxins are very similar to the published crys tal structures of bovine pancreatic trypsin inhibitor (BPTI) and alpha -DtX. Major secondary structural regions of the dendrotoxins are stabl e without much fluctuation during the dynamics simulation; the regions corresponding to the turns and bends (rich in lysines and arginines) exhibit more fluctuations. The conformational angles and the C-alpha.. . C-alpha' distances of the three disulfides (in each of the dendrotox ins) are different from each other. Comparative model building studies , involving the dendrotoxins and the proteinases, reveal that the key interactions (observed in BPTI-trypsin complex) needed for anti-protea se activity are absent due to structural differences between the dendr otoxins and BPTI at the anti-protease loop; this explains the inabilit y of the dendrotoxins to inhibit proteinases. The model also suggests that the solvent-exposed p-turn region, rich in lysines (residues 26-2 8), might bind directly to the extracellular anionic sites of the rece ptors (K+ channels) by ionic interactions. The strikingly homologous c ysteine distribution (Cys-x-x-x-Cys) in DtX, DpI, and DpK, at the C-te rminus, induces the occurrence of a characteristic conformational moti f, consisting of an a-helix (in an amphiphilic environment) stabilized by two disulfides, one involving a cysteine at the beta-strand, and t he other at the N-terminus. This amphiphilic secondary structural elem ent seems to provide the rigid frame work needed for exposing the prop osed active site region of the dendrotoxins to the anionic sites of th e K+ channel receptors.