Characterization of a partially structured state in an all-beta-sheet protein

Cardiotoxin analogue III (CTX III) is a low-molecular-mass all-α-sheet protein isolated from the Taiwan cobra (Naja naja atra) venom. A stable partially structured state similar to the ‘molten globule’ state has been identified for CTX III in a 3% (w/v) solution of 2,2,2-trichloroacetic acid at 298 K. This stable state has been structurally characterized using a variety of techniques such as CD, 1-anilinonaphthalene-8-sulphonate fluorescence binding, Fourier transform IR and two-dimensional NMR spectroscopy techniques. Direct assignment of the homonuclear two-dimensional NMR spectra of the protein in 3% trichloroacetic acid showed that drastic structural perturbation had not taken place in the protein and that the ‘intermediate’ state retained a significant portion of the native secondary-structural interactions. It is found that about 65% of the native α-sheet structural contacts are maintained in the partially structured state of CTX III in 3% trichloroacetic acid.