ATP-dependent active transport simulations based on a phosphatase-channel-kinase membrane structure

Simulations of coupled interactions involving enzymatic reaction diffusion and electrostatic interactions were conducted under a fixed phosphatase-channel-kinase (PCK) topology oriented from the outside to the inside of a charged membrane structure. Depending on the phosphatase and kinase locations, we recently demonstrated that active transport of a phosphorylated substrate may occur via this PCK topology. The present analysis demonstrates that, if in addition to this topology, a phosphatase activity (P(1)) is also present on the inner side of the membrane, but outside the unstirred layer surrounding the inner membrane surface, then active transport of the corresponding unphosphorylated substrate may also occur. Therefore, this PCK membrane topology, which behaves as a specific ATP-dependent transporter, appears as a general topology permitting; first, on its own the active transport of a phosphorylated substrate; second, when associated with a phosphatase acting in the bulk of the receiver compartment, the active transport of the corresponding unphosphorylated substrate, that is, in most cases, the transport of an uncharged substrate. The general mathematical model given permits the active transport of a phosphorylated substrate to be analyzed (in the absence of P(1)), the active transport of an unphosphorylated substrate (in the presence of P(1)), whatever the charge distributions on both sides of the membrane surface and whatever the positions of the membrane-bound phosphatase and the membrane-bound kinase. This general model also takes into account the consumption of ATP occurring into the receiver compartment during the time course of these transport phenomena. A broad analysis of the role played by the main parameters taken into account in the model was conducted to precisely define the physicochemical conditions and the membrane topology needed for the highest active transports within the shortest time.