FIGURE 8.
Model of metalloid transport by Acr3 and ArsB. In cells of E. coli uptake of As(OH)3 or Sb(OH)3, uptake is facilitated by the GlpF channel. ArsB is an antiporter that exchanges As(OH)3 or Sb(OH)3 in exchange for a proton. Exchange of either neutral metalloid with positively charged H+ couples efflux to the protonmotive force. Acr3 is a uniporter that extrudes the As(OH)2O− anion driven by the membrane potential, positive outside. The arsenite anion is in equilibrium with the neutral acid form according to its pKa of 9.2. With a pKa of 11.8, the concentration of the antimonite anion in cytosol of E. coli is too low for significant amounts of Sb(III) to be transported by Acr3. Thus the difference in pKa of the substrates explains the difference in selectivity between Acr3 and ArsB.