Unfolding of multimeric proteins in presence of denaturants. A case study of helianthinin from Helianthus annuus L.

Helianthinin (11S), a multisubunit protein from Sunflower Seeds (Helianthus annuus L.) dissociates to its monomer (2S) through a trimeric (7S) interme diate as a function of guanidine hydrochloride (GuHCl) and guanidine thiocy anate (GuHSCN) concentration. Measurements of viscosity, velocity sedimenta tion patterns and spectroscopic parameters of the protein in presence of th ese denaturants both at equilibrium and as a function of time clearly sugge st that the dissociation, unfolding and aggregation of this multimeric prot ein occur sequentially. The unfolding of the protein in guanidinium salts h as two transitions with the first transition occurring between native to in termediate and the second transition occurring between intermediate state a nd unfolded state. The midpoint concentration for the major transition is 1 .75 M for GuHCl and 0.8 M for GuHSCN. Evaluation of this data suggests that during the process of denaturation the simultaneous unfolding of acidic an d basic subunits of the protein takes place. At intermediate concentrations of denaturant namely 1.6 M of GuHCl or 1.1 M of GuHSCN the aggregation of the protein was found to be maximum. The results suggests the possibility o f a mechanism for the dissociation, denaturation and unfolding of multimeri c proteins in presence of the chemical denaturants.