Isolation and partial characterization of the N-terminal functional unit of subunit RtH1 from Rapana thomasiana grosse hemocyanin

Two different structural subunits were identified in Rapana thomasiana hemocyanin: RtH1 and RtH2. RtH1-a is the N-terminal functional unit in the subunit RtH1 and its stability toward temperature and chemical denaturation by guanidinium hydrochloride (Gdn.HCl) are studied and compared with the structural subunit RtH1 and the whole Rapana hemocyanin molecule. The conformational changes, induced by the various treatments, were monitored by CD and fluorescence spectroscopy. The critical temperatures (T-c) for RtH1-a, the structural subunits and the native He, determined by fluorescence spectroscopy, coincide closely with the melting temperatures (T-m), determined by CD spectroscopy. The free energy of stabilization in water, Delta G(D)(H2O), determined from (Gdn.HCl) denaturation studies, is about two times higher for the structural subunit KtH1 and the whole hemocyanin molecule as compared to the functional unit RtH1-a. The oligomerization between the structural subunits or the eight functional units, assembled in subunit RtH1, has a stabilizing effect on the whole molecule as well as the structural subunits. (C) 2000 Elsevier Science Ltd. All rights reserved.