To illustrate the emerging class of anion exchange proteins in bacteria, this article discusses the biochemical and physiological properties of phosphate (Pi)-linked antiporters that accept glucose 6-phosphate (G6P) as their primary substrate. These systems have a bifunctional active site that binds a pair of negative charges, whether presented as a single divalent anion or a pair of monovalent substrates. Exchange stoichiometry therefore .moves between the limits of 2:1 and 2:2 according to the ratio of mono- and divalent substrates at either membrane surface. This predicts an interesting reaction sequence in vivo because internal pH is more alkaline than external pH; one expects an asymmetric exchange as a pair of monovalent G6P anions moves against a single divalent G6P, and in this way an otherwise futile selfexchange of G6P can result in a net inward flux driven (indirectly) by the pH gradient. Despite their biochemical complexity, at a molecular level the Pi-linked antiporters resemble other secondary carriers. Indeed, the current listing of nearly two dozen such proteins suggests a structural theme in which the minimal functional unit has two sets of six transmembrane alpha helices separated by a central hydrophilic loop. Presently described examples show that this topology can derive from either a single protein or from pairs of identical subunits. The finding of this common structure makes it possible to begin building more detailed structural models that have more general implications.
Liquid phase exfoliation of carbonate-intercalated layered double hydroxides
