Interaction of Sodium Dodecyl Sulfate with Multi-subunit Proteins: A case study with CARMIN

Sodium dodecyl sulfate (SDS) dissociates the multi-
meric 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 640 f 50 mol of SDS/mol of protein with an associ-
ation constant, K, of 6.90 f 0.35 X 10’ M-‘. The inter-
action reflects a AG“ = -4.0 f 0.1 kcal mol”. In the
denaturation step, the K, has the same value, and the
y 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 “C indicates
a maximum of 1030 f 90 mol of SDS bound/mol of
protein, which is equivalent to 1.14 f 0.10 g of SDS/g
of protein. Upon denaturation, the a-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 2 8.1 and 16.8 f 1.5 min-’, re-
spectively, at 6.3 mM SDS concentration. The proc-
esses of dissociation and denaturation occur sequen-
tially. Dissociation of the protein is reversible, whereas
the process of denaturation is only partially reversible
as reflected by sedimentation velocity and conforma-
tional analysis. These data are taken as a model for
general understanding of the dissociation and/or de-
naturation processes, which could be either sequential
or simultaneous in multimeric proteins.