The Quenching-Resolved Fluorescence Spectrum and Its Application to Studies of the Folding/Unfolding of Trypsin Inhibitor from Seeds of the Bitter Gourd

With reference to the local conformation of a protein, it is interesting to differentiate the individual fluorescence properties of included tryptophan residues without modification. The fluorescence spectrum of bitter gourd trypsin inhibitor (BGTI) was separated into two emission bands by the quenching-resolved fluorescence method. One emission band was given as a fraction with the Stern-Volmer quenching constant, 44.9 x 10(-3) M(-1), against the fluorescence quenching by KI, and it showed an emission maximum intensity at 341 nm. The fluorescence quenching constant of the other band was 1.58 x 10(-3) M(-1), and the maximum wavelength was found at 337 nm. These separated emissions were due to the fluorescence of Trp54 and Trp9 of BGTI. The quenching resolved-fluorescence spectrum was effectively applied to the precise description of the polar circumstances surrounding the Trp residues in the unfolding intermediate state of BGTI. The results suggested that the molten globule-like state of BGTI adopted such a peculiar conformation that the helix domain including Trp9 was packed more densely while the other loop domain partially unfolded.