Photophysics of the single tryptophan residue in Fusarium solani Cutinase: evidence for the occurrence of conformational substates with unusual fluorescence behaviour

The single tryptophan residue, at position 69 in the amino acid sequence, was used as an intrinsic probe to obtain structural and dynamical information on the lipolytic enzyme Fusarium solani cutinase. In the enzyme's native state the tryptophan fluorescence is highly quenched. Time-resolved experiments reveal that the majority of the excited state species is characterized by an unusually fast decay time of approximately 40 ps, indicating the occurrence of a very efficient nonradiative relaxation process, possibly via the adjacent disulphide bond or via the peptide bonds of a nearby loop. A minority of the excited state species relaxes on a nanosecond time scale. Irradiation of the enzyme in the tryptophan absorption band causes an increase by an order of magnitude of the fluorescence quantum yield. This increase is ascribed to a photo-induced, subtle structural change of a minor subset of species whose fluorescence is not highly quenched. The structural change is accompanied by a tightening of the local environment of the tryptophan moiety, as indicated by results from time-resolved fluorescence anisotropy which reveal a complete disappearance of the segmental flexibility of the tryptophan moiety.