MOLECULAR-PROPERTIES OF GLUTAMATE-DEHYDROGENASE FROM THE EXTREME THERMOPHILIC ARCHAEBACTERIUM SULFOLOBUS-SOLFATARICUS

This study is concerned with the structural characterization in soluti on of the glutamate dehydrogenase from the Archaeon Sulfolobus solfata ricus. At neutral pH both alpha-helix and beta-sheet constitute the se condary structure of this enzyme, on the basis of circular dichroism, A complex, temperature dependent self-association equilibrium regulate s the formation of the enzyme quaternary structure, which seems to be accompanied by a reversible structural change. At 25 degrees C the enz yme is mostly represented by monomeric subunits at concentrations lowe r than 0.02 mg/ml, while oligomers are predominant at concentrations h igher than 0,12 mg/ml, The mid-point of the association curve shifts f rom 0.05 mg/ml at 25 degrees C to about 0.1 mg/ml at 45 degrees C. Onl y the oligomeric form appears to be temperature resistant. Monomeric a nd oligomeric enzyme show distinct behaviour on guanidine hydrochlorid e perturbation at neutral pH. The monomer denaturation, although compl ex, is reversible. Two fluorescent tryptophan classes are detectable i n the monomer, monitoring the independent unfolding of two regions thr ough a multistate transition. Instead, the oligomeric protein shows a complex denaturation pattern with the tendency to aggregate irreversib ly at high denaturant concentration.