Fluorescence and quenching comparative studies of halophilic and bovine glutamate dehydrogenase

Fluorescence techniques have been used to study the structural characteristics of many proteins. The halophilic enzyme NADP-glutamate dehydrogenase from Haloferax mediterranei is found to be a hexameric enzyme composed of identical subunits. Fluorescence spectra of native and denatured halophilic and bovine glutamate dehydrogenase (h-GDH and b-GDH) have been analysed. Native h-GDH presents the maximum emission at 338 nm, whereas for b-GDH the maximum appears at 332 nm. The denaturation process is accompanied by an exposure to the solvent of the tryptophan residues, as manifested by the red shift of the emission maximum in both cases. The unfolding of h-GDH is a gradual process, which is accompanied by a loss in enzyme activity. Fluorescence quenching by external quenchers, KI and acrylamide, has also been carried out. The tryptophan residues in the protein are more exposed to the solvent in h-GDH than in b-GDH. The total amount of tryptophan residues is nearly the same for both enzymes.