Investigation of the structural stability of cardiotoxin analogue III fromthe Taiwan cobra by hydrogen-deuterium exchange kinetics

The conformational stability of a small (similar to 7 kDa), all beta-sheet protein, cardiotoxin analogue III (CTX III), from the Venom of the Taiwan c obra has been investigated by hydrogen-deuterium (HID) exchange using two-d imensional NMR spectroscopy. The H/D exchange kinetics of backbone amide pr otons in CTX III has been monitored at pD 3.6 and 6.6 (at 25 degrees C), fo r over 5000 h. Examination of H/D exchange kinetics in the protein showed t hat a number of slowly exchanging residues are in the hydrophobic core of t he protein. The average protection factor of the amide protons of residues belonging to the triple-stranded beta-sheet domain is about 20 times greate r than that of those in the double-stranded beta-sheet segment. The residue s in the C-terminal tail of the molecule, though structureless, have been f ound to exhibit significant protection against H/D exchange. Comparison of the quenched-flow H/D exchange data on CTX III with those obtained in the p resent study reveals that the most slowly exchanging portion constitutes th e folding core of the protein.