MULTITRYPTOPHAN-FLUORESCENCE-EMISSION DECAY OF BETA-GLYCOSIDASE FROM THE EXTREMELY THERMOPHILIC ARCHAEON SULFOLOBUS-SOLFATARICUS

The emission decay of intrinsic fluorescence of the extremely thermoph ilic beta-glycosidase from Sulfolobus solfataricus has been investigat ed as functions of temperature and of iodide-quencher concentration by frequency-domain fluorometry. This protein contains 68 tryptophans an d provides a matrix for correlation of the average spectroscopic behav iour with solvent exposure and local dynamics. At each temperature, th e emission is very heterogeneous and interpretable in terms of quasico ntinuous bimodal distribution of fluorescence lifetimes. We associate the component of the bimodal distribution to two distinct classes of t ryptophanyl residues that differ in microenvironmental characteristics . Temperature and quenching experiments show that the long-lived compo nent includes tryptophanyl residues located in buried regions with hig h rigidity; the short distributional component corresponds to tryptoph ans embedded in more flexible and exposed regions. This proposal has b een confirmed by examination of the crystallographic structure. The da ta suggest that, at least for this protein, there is a good correlatio n between residue exposure and lifetime distributional components. The conformational dynamics of the two classes of tryptophanyl residues i s affected differently by temperature, suggesting that the protein reg ions in which they are located give different contributions to enzyme properties, such as flexibility, stability and function.