Intermediates in guanidine-HC1 unfolding of glutamine synthetase from the extreme thermophile, Bacillus caldolyticus. 
Glutamine synthetase is expressed in Bacillus caldolyticus as two isoforms that differ in physico-chemical and regulatory properties.  Biphasic kinetics of thermal denaturation of E-I and E-II (Merkler, D.J., et al (1987) Biochemistry 26, 7805), suggested the formation of intermediates.  CD spectral changes of E-II induced by guanidine-HC1 clearly indicate a three-state pathway for unfolding (N----I----D).  Refolding of E-II from 6 M GuHCl led to only 15% recovery of activity, compared to greater than or equal to 90% with E-I.