Resolution and reconstitution of anion exchange reactions.
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 self-exchange 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.
Maloney
P C
PC
Department of Physiology, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
D000838:Anions; D001419:Bacteria / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D008961:Models, Structural; D011487:Protein Conformation
D000838:Anions; D002352:Carrier Proteins