Tryptophan fluorescence reports nucleotide-induced conformational changes in a domain of the ArsA ATPase

The ATPase mechanism of ArsA, the catalytic subunit of the arsenite pump

The ArsA ATPase is the catalytic subunit of a novel arsenite pump, with two nucleotide-binding consensus sequences in the N- and C-terminal halves of the protein. The single tryptophan-containing Trp159 ArsA was used to elucidate the elementary steps of the ATPase mechanism by fluorescence stopped-flow experiments. The binding and hydrolysis of MgATP is a multistep process with a minimal kinetic mechanism (Mechanism 1). A notable feature of the reaction is that MgATP binding induces a slow transient increase in fluorescence of ArsA, which is independent of the ATP concentration, indicative of the build-up of a pre-steady state intermediate. This finding, coupled with a phosphate burst, implies that the steady-state intermediate builds up subsequent to product release. We propose that the rate-limiting step is an isomerization between different conformational forms of ArsA. kcat is faster than the phosphate burst, indicating that both nucleotide binding sites of ArsA are catalytic. Consistent with this interpretation, approximately 2 mol of phosphate are released per mole of ArsA during the phosphate burst.

Asp45 is a Mg2+ ligand in the ArsA ATPase

The ATPase activity of ArsA, the catalytic subunit of the plasmid-encoded, ATP-dependent extrusion pump for arsenicals and antimonials in Escherichia coli, is allosterically activated by arsenite or antimonite. Magnesium is essential for ATPase activity. To examine the role of Asp45, mutants were constructed in which Asp45 was changed to Glu, Asn, or Ala. Cells expressing these mutated arsA genes lost arsenite resistance to varying degrees. Purified D45A and D45N enzymes were inactive. The purified D45E enzyme exhibited approximately 5% of the wild type activity with about a 5-fold decrease in affinity for Mg2+. Intrinsic tryptophan fluorescence was used to probe Mg2+ binding. ArsA containing only Trp159 exhibited fluorescence enhancement upon the addition of MgATP, which was absent in D45N and D45A. As another measure of conformation, limited trypsin digestion was used to estimate the surface accessibility of residues in ArsA. ATP and Sb(III) synergistically protected wild type ArsA from trypsin digestion. Subsequent addition of Mg2+ increased trypsin sensitivity. D45N and D45A remained protected by ATP and Sb(III) but lost the Mg2+ effect. D45E exhibited an intermediate Mg2+ response. These results indicate that Asp45 is a Mg2+-responsive residue, consistent with its function as a Mg2+ ligand.