Interaction of sodium dodecyl sulfate with multi-subunit proteins. A case study with carmin.

Sodium dodecyl sulfate (SDS) dissociates the multimeric protein, carmin, into its monomers (2 S) at low concentration. Dissociation begins at and above 1.6 mM SDS concentration and reaches 50% at 5 mM SDS concentration. Denaturation occurs above 5 mM SDS concentration. The dissociation step involves binding of 540 +/- 50 mol of SDS/mol of protein with an association constant, K alpha of 6.90 +/- 0.35 x 10(2) M-1. The interaction reflects a delta G(0) = -4.0 +/- 0.1 kcal mol-1. In the denaturation step, the K alpha has the same value, and the gamma value is nearly 2-fold higher. Dissociation of carmin thus begins only above a binding of 0.60 g of SDS/g of protein. Analysis of the binding data at 37 degrees C indicates a maximum of 1030 +/- 90 mol of SDS bound/mol of protein, 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 these 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 denaturation occur sequentially. Dissociation of the protein is reversible, whereas the process of denaturation is only partially reversible as reflected by sedimentation velocity and conformational analysis. These data are taken as a model for general understanding of the dissociation and/or denaturation processes, which could be either sequential or simultaneous in multimeric proteins.