The structural events associated with the binding of divalent cations to beta-bungarotoxin

Toxicon. 2005 Feb;45(2):139-45. doi: 10.1016/j.toxicon.2004.08.012.

Abstract

In order to address the mechanism why the Ca2+ was crucial for the manifestation of the phospholipase A2 (PLA2) activity of beta-bungarotoxin (beta-BuTx), four divalent cations were used to assess their influences on the catalytic activity and the fine structures of beta-BuTx. Substitution Mg2+ or Sr2+ for Ca2+ in the substrate solution was found to cause a decrease in the PLA2 activity to approximately 15 or 6% of that in the presence of Ca2+. However, only marginally detectable PLA2 activity was observed with the addition of Ba2+. The nonpolarity of 8-anilinonaphthalene-1-sulfonate (ANS)-binding site of beta-BuTx markedly increased with the binding of cations to beta-BuTx. The negative ellipticity noted with the CD spectra of beta-BuTx increased upon the binding of cations too. With the exception of Ba2+, the order of the ability of cations to enhance the intensity of ANS fluorescence or increase the increment of negative ellipticity was Sr2+ > Ca2+ > Mg2+, which was the same order as the increase in their atomic radii. However, the energy transfer from Trp fluorescence emission to ANS was most effective upon the addition of Ca2+. Moreover, the extent of glutaraldehyde crosslinking between A chain and B chain decreased in the presence of cations. Nevertheless, the binding affinities of beta-BuTx for the four cations were similar. These results, together with the findings that the ANS molecule binds at the active site of the A chain in beta-BuTx, suggest that the binding of Ca2+ to beta-BuTx induces subtly conformational changes occurred at the active site for exerting the activity of beta-BuTx. Moreover, the change in the gross conformation induced by the binding of Ca2+ may affect the interaction between A chain and B chain, and consequently the activity of beta-BuTx as well.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Barium / metabolism
  • Bungarotoxins / chemistry*
  • Bungarotoxins / metabolism
  • Calcium / metabolism
  • Cations, Divalent / metabolism*
  • Magnesium / metabolism
  • Phosphatidylcholines / metabolism
  • Phospholipases A / chemistry
  • Phospholipases A / metabolism
  • Phospholipases A2
  • Protein Binding
  • Spectrometry, Fluorescence
  • Strontium / metabolism

Substances

  • Bungarotoxins
  • Cations, Divalent
  • Phosphatidylcholines
  • Barium
  • Phospholipases A
  • Phospholipases A2
  • Magnesium
  • Calcium
  • Strontium