Substrate-dependent reversal of anion transport site orientation in the human red blood cell anion-exchange protein, AE1.
The tightly coupled, one-for-one exchange of anions mediated by the human red blood cell AE1 anion-exchange protein involves a ping-pong mechanism, in which AE1 alternates between a state with the anion-binding site facing inward toward the cytoplasm (Ei) and a state with the site facing outward toward the external medium (Eo). The conformational shift (Ei <--> Eo) is only permitted when a suitable substrate such as Cl(-) or HCO(3)(-) (B(-)) is bound. With no anions bound, or with Cl(-) bound, far more AE1 molecules are in the inward-facing than the outward-facing forms (Ei Eo, ECli EClo). We have constructed a model for CI(-)-B(-) exchange based on Cl(-)-Cl(-) and B(-)-B(-) exchange data, and have used it to predict the heteroexchange flux under extremely asymmetric conditions, with either all Cl(-) inside and all B(-) outside (Cli-Bo) or vice versa (Bi-Clo). The experimental values of the ratio of the exchange rate for Bi-Clo to that for Cli-Bo are only compatible with the model if the asymmetry of bicarbonate-loaded sites (A(B) = EBo/EBi) > 10, the opposite of the asymmetry for unloaded or Cl-loaded sites. Furthermore, the Eo form has a higher affinity for HCO(3)(-) than for Cl(-), whereas the Ei form has a higher affinity for Cl(-). The fact that this "passive" system exhibits changes in substrate selectivity with site orientation ("sidedness"), a characteristic usually associated with energy-coupled "active" pumps, suggests that changes in affinity with changes in sidedness are a more general property of transport proteins than previously thought.
Knauf
Philip A
PA
Department of Biochemistry and Biophysics, Box 712, University of Rochester Medical Center, 601 Elmwood Avenue, Rochester, NY 14642, USA. philip_knauf@rochester.edu
Law
Foon-Yee
FY
Leung
Tze-Wah Vivian
TW
Gehret
Austin U
AU
Perez
Martha L
ML
D001457:Anion Exchange Protein 1, Erythrocyte / Q000378:metabolism*; D000838:Anions; D001639:Bicarbonates / Q000378:metabolism; D001692:Biological Transport; D002712:Chlorides / Q000378:metabolism; D004912:Erythrocytes / Q000378:metabolism*; D006801:Humans
D001457:Anion Exchange Protein 1, Erythrocyte; D000838:Anions; D001639:Bicarbonates; D002712:Chlorides