Identification of 'molten globule'-like state in all beta-sheet protein.

The cardiotoxin analogue III (CTX III), isolated from the Taiwan Cobra venom (Naja naja atra), is a sixty amino acid, all beta-sheet protein. The 2,2,2-trifluoro ethanol (TFE) induced unfolding of CTX III is studied under acidic conditions (pH 2.5). Using circular dichroism, 1-anilino-8-napthalene sulphonic acid binding and NMR experiments, it is shown that stable, partially structured state(s) ['molten globule'-like state] is formed between 50 and 80% TFE concentrations. The protein was found to exist in an unfolded state in 80% TFE containing 2M urea. The TFE induced unfolding process is shown to be completely reversible. In the 'molten globule' state of CTX III in 80% TFE, though portion(s) of the backbone of the protein assume helical conformation, most of the original beta-sheet secondary structural elements in the protein are intact. In our opinion, this is the first report of the identification of a 'molten globule'-like state in the unfolding pathway of an all beta-sheet monomeric protein.