Structurally homologous toxins isolated from the Taiwan cobra (Naja naja atra) differ significantly in their structural stability

Cardiotoxin and neurotoxin analogues isolated from snag, venom sources are highly homologous proteins (>50% homology) with similar three-dimensional s tructures but exhibit drastically different biological properties. In the p resent study, we compare the conformational stability of cardiotoxin analog ue III (CTX III) and cobrotoxin (CBTX), a neurotoxin analogue, from the Tai wan cobra (Naja naja atra), using circular dichroism spectroscopy and hydro gen-deuterium (H/D) exchange techniques in conjunction with two-dimensional NMR methods. Contrary to expectations, it is found that CTX III and CBTX d iffer significantly in their structural stabilities. The three-dimensional structure of CBTX is less stable than that of CTX III. The amide protons of residues at the N- and C-terminal ends of the CTX III molecule are strongl y protected against H/D exchange, implying that the terminal ends of the mo lecule are bridged together by significant numbers of hydrogen bonds. Howev er, in CBTX, amide protons at the terminal ends of the molecule do not exhi bit an significant protection against H/D exchange. Comparison of the prote ction factors of the various amide protons in CTX III and CBTX reveals that the extraordinary stability of CTX III stems from the strong network of in teractions among the residues at the N- and C-terminal ends and also due to the tight and ordered packing of the nonpolar residues involved in the tri ple-stranded, anti-parallel, beta-sheet segment of the molecule. (C) 1999 A cademic Press.