A kinetic model for the action of a resistance efflux pump

ArsA is the catalytic subunit of the arsenical pump, coupling ATP hydrolysi s to the efflux of arsenicals through the ArsB membrane protein. It is a pa radigm for understanding the structure-function of the nucleotide binding d omains (NBD) of medically important efflux pumps, such as P-glycoprotein, b ecause it has two sequence-related, interacting NBD, for which the structur e is known. On the basis of a rigorous analysis of the pre-steady-state kin etics of nucleotide binding and hydrolysis, we propose a model in which Ars A alternates between two mutually exclusive conformations as follows: the A rsA(1) conformation in which the A1 site is closed but the A2 site open; an d the ArsA(2) conformation, in which the A1 and A2 sites are open and close d, respectively. Antimonite elicits its effects by sequestering ArsA in the ArsA(1) conformation, which catalyzes rapid ATP hydrolysis at the A2 site to drive ArsA between conformations that have high (nucleotide-bound ArsA) and low affinity (nucleotide-free ArsA) for Sb(III). ArsA potentially utili zes this process to sequester Sb(III) from the medium and eject it into the channel of ArsB.