The Journal of General Physiology, Vol 100, 301-339, Copyright © 1992 by The Rockefeller University Press
The human erythrocyte anion transport protein, band 3. Characterization of exofacial alkaline titratable groups involved in anion binding/translocation
PJ Bjerrum
Department of Clinical Chemistry, Herlev Hospital, University of Copenhagen, Denmark.
Chloride self-exchange across the human erythrocyte membrane at alkaline
extracellular pH (pHO) and constant neutral intracellular pH (pH(i)) can be
described by an exofacial deprotonatable reciprocating anion binding site
model. The conversion of the transport system from the neutral to the
alkaline state is related to deprotonation of a positively charged ionic
strength- and substrate-sensitive group. In the absence of substrate ions
([ClO] = 0) the group has a pK of approximately 9.4 at constant high ionic
strength (equivalent to approximately 150 mM KCl) and a pK of approximately
8.7 at approximately zero ionic strength. The alkaline ping-pong system
(examined at constant high ionic strength) demonstrates outward recruitment
of the binding sites with an asymmetry factor of approximately 0.2, as
compared with the inward recruitment of the transport system at neutral pHO
with an asymmetry factor of approximately 10. The intrinsic half-saturation
constant for chloride binding, with [Cli] = [Clo], increased from
approximately 30 mM at neutral to approximately 110 mM at alkaline pHO. The
maximal transport rate was a factor of approximately 1.7 higher at alkaline
pHO. This increase explains the stimulation of anion transport, the
"modifier hump," observed at alkaline pHO. The translocation of anions at
alkaline pHO was inhibited by deprotonation of another substrate- sensitive
group with an intrinsic pK of approximately 11.3. This group together with
the group with a pK of approximately 9.4 appear to form the essential part
of the exofacial anion binding site. The effect of extracellular iodide
inhibition on chloride transport as a function of pHO could, moreover, be
simulated if three extracellular iodide binding constants were included in
the model: namely, a competitive intrinsic iodide binding constant of
approximately 1 mM in the neutral state, a self-inhibitor binding constant
of approximately 120 mM in the neutral state, and a competitive intrinsic
binding constant of approximately 38 mM in the alkaline state.
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