INTERACTION OF SODIUM DODECYL-SULFATE WITH MULTISUBUNIT PROTEINS - A CASE-STUDY WITH CARMIN

Sodium dodecyl sulfate (SDS) dissociates the multimeric protein, carmi n, into its monomers (2 S) at low concentration. Dissociation begins a t and above 1.6 mm SDS concentration and reaches 50% at 5 mM SDS conce ntration. Denaturation occurs above 5 mM SDS concentration. The dissoc iation step involves binding of 540 +/- 50 mol of SDS/mol of protein w ith an association constant, K(a) of 6.90 +/- 0.35 x 10(2) M-1. The in teraction reflects a DELTAG0 = -4.0 +/- 0.1 kcal mol-1. In the denatur ation step, the K(a) has the same value, and the gamma value is nearly 2-fold higher. Dissociation of carmin thus begins only above a bindin g of 0.60 g of SDS/g of protein. Analysis of the binding data at 37-de grees-C indicates a maximum of 1030 +/- 90 mol of SDS bound/mol of pro tein, which is equivalent to 1.14 +/- 0.10 g of SDS/g of protein. Upon denaturation, the alpha-helix content of the protein increases from 4 to 15%. Kinetically, the denaturation process consists of a two-step process (a fast and a slow step). The first order rate constants for t hese steps are 89.6 +/- 8.1 and 15.8 +/- 1.5 min-1, respectively, at 6 .3 mM SDS concentration. The processes of dissociation and denaturatio n occur sequentially. Dissociation of the protein is reversible, where as the process of denaturation is only partially reversible as reflect ed by sedimentation velocity and conformational analysis. These data a re taken as a model for general understanding of the dissociation and/ or denaturation processes, which could be either sequential or simulta neous in multimeric proteins.