Susceptibility of Cu(I) transport ATPases to sodium dodecyl sulfate. Relevance of the composition of the micellar phase

Sodium dodecylsulfate (SDS) is a well-known protein denaturing agent. A less known property of this detergent is that it can activate or inactivate some enzymes at sub-denaturing concentrations. In this work we explore the effect of SDS at sub-denaturing concentrations on the ATPase activity of a hyper-thermophilic and a mesophilic Cu(I) ATPase reconstituted in mixed micelles of phospholipids and a non-denaturing detergent. We first develop an iterative procedure to evaluate the partition of SDS between the aqueous and the micellar phases. This procedure allows to determine the composition of micelles prepared with variable amphiphiles content. When incubating the enzymes with SDS in the presence of different amounts of phospholipids, it can be observed that higher SDS concentrations are required to obtain the same degree of inactivation when the initial concentration of phospholipids is increased. Notably, we found that, if represented as a function of the mole fraction of SDS in the micelle, the degree of inactivation obtained at different amounts of amphiphiles converges to a single inactivation curve. To interpret this result, we propose a simple model involving active and inactive enzyme molecules in equilibrium. This model allowed us to determine the Gibbs free energy change for the inactivation process and its derivative respect to the mole fraction of SDS in the micellar phase, this last being a measure of the susceptibility of the enzyme to SDS. Our results showed that the inactivation free energy changes are similar for both proteins, and indicate that the equilibrium is highly shifted towards the active form in both enzymes. Conversely, susceptibility to SDS is significantly lower for the hyperthermophilic ATPase, suggesting an inverse relation between thermophilicity and susceptibility to SDS.