Substrate-dependent reversal of anion transport site orientation in the human red blood cell anion-exchange protein, AE1

AUTOR(ES)
FONTE

National Academy of Sciences

RESUMO

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\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}_{3}^{-}\end{equation*}\end{document} (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 (AB = EBo/EBi) > 10, the opposite of the asymmetry for unloaded or Cl-loaded sites. Furthermore, the Eo form has a higher affinity for HCO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}_{3}^{-}\end{equation*}\end{document} 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.

ACESSO AO ARTIGO

Documentos Relacionados