Cobrotoxin chemical modification

Cobrotoxin is a basic protein having four free amino groups and six arginine residues at positions 28, 30, 33, 36, 39 and 59 in sequence (Fig. 2). The important roles of cationic groups in cobrotoxin in relation to the lethal toxicity and antigenic specificity were investigated by selective and stepwise chemical modification of arginine and lysine residues. 

Selective and stepwise chemical modifications of cobrotoxin indicate that at least two cationic groups, e-amino group of Lys-47 and guanidino group of Arg-33, both of which are common to all known postsynaptic snake neurotoxins, held at a certain critical distance in the molecule are functionally important for its neuromuscular blocking activity. 

In our laboratory we first isolated the major lethal protein (termed Cobrotoxin) of non-enzymatic nature from the venom of Taiwan cobra Naja naja atra) in 1964 and subsequently purified and crystallized the protein. The primary structure and the disulfide linkages with various efforts by chemical modification and immunological methods in elucidation of the structure-function relationship of this important venom neurotoxin have since been accomplished. Structure-activity correlations have been drawn from chemical modification carried out on both pre- and post-synaptic neurotoxins. With recent advances in DNA recombination and protein engineering, we feel that the time is now ripe to apply these techniques to the isolation and characterization of the genes encoding these toxins. Detailed structural and site-specific mutational studies on the cDNA clones of neurotoxins of both types may complement our previous chemical modifications of the functional role of some amino acid residues in neurotoxins and lead to insight into the modes of action for these biologically active molecules. 

Fig. 9. Backbone folding of short neurotoxin molecule as derived from Naja naja neurotoxin II study. Additional contacts from the studies of other neurotoxin are indicated as follows Mos - neurotoxin III Naja mossambica mossambica (36), Erb - erabutoxin b Laticauda semifasciata(40) and Ct - cobrotoxin Naja Naja atra(39). Abbreviation pK -deprotonation of ionogenic group influence on chemical shifts, + - charge effect on pK values, NOE - nuclear Overhauser effect, CS - chemical shift changes upon selective modification, SL - spin label broadening effects.

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