Sequential unfolding of papain in molten globule state

Papain exhibits the characteristics of molten globule under acidic conditio ns as seen by circular dichroism, fluorescence and ANS binding. Between pH 2.0-2.5 the protein exhibits substantial secondary structure as indicated b y far-UV CD spectrum but loses the persistent tertiary interactions of the native state. Enhanced binding of ANS to the state at pH 2.0 in relation to the native and unfolded states at neutral pH indicates a considerable expo sure of aromatic side chains. Temperature and guanidine hydrochloride induc ed unfolding of papain in this state is noncooperative and the transition c urves are biphasic in nature. As papain molecule consists of two domains, t he results suggest that the domains unfold independently and sequentially. (C) 1998 Academic Press.