Anion exchange reactions in bacteria.

Bacterial anion exchange now includes both "carboxylate-linked" reactions in which there is an antiport of mono- and dicarboxylic acids, and "Pi-linked" reactions that build on phosphate (Pi) and organic phosphates. To illustrate the general features of this expanding class, this article discussed the biochemistry, physiology, and molecular biology of Pi-linked antiporters that accept glucose 6-phosphate (G6P) as their primary substrate. Kinetic and biochemical analysis suggests that Pi-linked exchangers have a bifunctional active site that accepts a pair of negative charges. For this reason, exchange stoichiometry moves between the limits of 2:1 and 2:2 to reflect the ratio of mono- and divalent substrates at either membrane surface. This results in a particularly interesting reaction sequence in vivo, where, because cytosolic pH is relatively alkaline, one can expect the asymmetric exchange of two monovalent G6P anions against a single divalent G6P. In this way, an otherwise futile self-exchange of G6P gives a net flux driven (indirectly) by the pH gradient. Despite this biochemical and physiological complexity, Pi-linked carriers resemble all other secondary carriers at a molecular level. Indeed, sequence analysis leads one to infer a common (albeit low resolution) structural theme in which each functional unit has two sets of six trans-membrane alpha helices separated by a central hydrophilic loop. Present examples show that this topology can derive from either a single protein, as is typical in bacteria, or from pairs of identical subunits, as found in mitochondria and chloroplasts. The finding of this common structure should make it possible to build detailed structural models that have implications for all membrane carrier proteins.