The carboxyl side chain of glutamate 681 interacts with a chloride binding modifier site that allosterically modulates the dimeric conformational state of band 3 (AE1). Implications for the mechanism of anion/proton cotransport.

Abstract

Glutamate 681 is thought to be located within the transport channel of band 3 (AE1, the chloride/bicarbonate exchanger), where it acts as a proton donor for the anion/proton cotransport function. Here we show that neutralization of the negative charge on glutamate 681 by chemically modifying band 3 with Woodward's reagent K plus sodium borohydride (i.e., the modification process) exposes a cryptic, conformationally active chloride-binding site which functions to modulate allosterically the conformational state of the band 3 dimer. Chloride binding was determined by measuring the effect of increasing chloride concentration on the rate of DBDS (4,4'-dibenzamido-2,2'-stilbenedisulfonate) release from band 3 using a stopped-flow fluorescence kinetic inhibitor replacement assay with DIDS (4,4'-diisothiocyanato-2,2'-stilbenedisulfonate) as the replacing inhibitor. The time course for DBDS release from unmodified, control band 3 was monophasic and exponential. Chloride binding to the transport site accelerated the rate of DBDS release, with the observed rate constant showing a hyperbolic dependence on chloride concentration, while the total change in reaction fluorescence remained constant. After modification of glutamate 681, DBDS release was monophasic in the absence of chloride, but the rapid addition of chloride at constant ionic strength induced a doubling in the fluorescence quantum yield for the bound DBDS molecules. This was associated with the development of 50:50 biphasic kinetics for DBDS release. Such changes were independent of the degree of modification of the band 3 subunit population between the 66% and 91% levels. Titration of the increase in total reaction fluorescence gave an apparent chloride binding K(d) of between 7 and 10 mM, which is 25-40-fold higher in affinity than chloride binding to the transport site. The dependence of the kinetic constants for both phases of the DBDS release reaction on chloride concentration was nonhyperbolic, which contrasts with unmodified band 3, and is indicative of the presence of two classes of chloride-binding sites on the modified transporter. We have also found that the fraction of subunits capable of binding DBDS reversibly, or DIDS covalently, decreased nonlinearly in the absence of chloride as the level of modification of the band 3 subunit population increased. In contrast, the same DBDS binding correlation plot showed a maximum in the presence of saturating chloride. The observation of such nonlinear correlation plots is consistent with a noncooperative dimer model for the modification process, where each dimeric species must possess different properties with respect to stilbenedisulfonate binding capacity and with respect to the spectral-kinetic response of bound stilbenedisulfonate molecules to the addition of chloride. Within the context of this model, the fractions of the three molecular dimeric species (i.e., the unmodified dimer, the dimer with one subunit modified, and the fully modified band 3 dimer) are calculated as a function of the level of modification of the band 3 subunit population. Nonlinear correlation plots are generated by then assigning the following specific properties to each dimeric species. The unmodified dimer binds DBDS but does not change its fluorescence quantum yield upon addition of chloride. The half-modified dimer binds DBDS on both modified and unmodified subunits, and both of those DBDS molecules increase their fluorescence quantum yield by 2-fold when chloride is added, and the system develops 50:50 biphasic DBDS release kinetics. Finally, the model requires that the fully modified dimer does not bind DBDS or DIDS. This model generates theoretical correlation plots that can represent the data presented in this study. We propose that neutralization of glutamate 681 on the half-modified band 3 dimer exposes an allosteric, chloride-binding modifier site which functions to facilitate the anion/proton cotransport process (a) by blocking the "redocking" of the carboxyl side chain of glutamate (thus raising its pK) and (b) by inducing amate (thus raising its pK) and (b) by inducing a conformational change in the band 3 dimer from a symmetrical to an asymmetrical state.