Stilbenedisulfonate Binding Kinetics to Band 3 (AE 1): Relationship between Transport and Stilbenedisulfonate Binding Sites and Role of Subunit Interactions in Transport

Abstract

ABSTRACTStilbenedisulfonates are competitive inhibitors of band 3 (AE1) anion exchange. It has been assumed that competitive binding implies that the stilbenedisulfonates bind to the transport site. In this paper, I summarize briefly an extensive series of stopped-flow fluorescence kinetic studies which indicate that stilbenedisulfonates do not compete with substrate anions directly, but rather that they behave as allosteric competitive inhibitors of monovalent anion binding to band 3. Monovalent anions lower stilbenedisulfonate affinity by accelerating the rate of their release from band 3, without changing the value of the initial second-order “on” kinetic constant. In addition, partial covalent labeling of the band 3 population with stilbenedisulfonates revealed subunit interaction effects: (a) in steady-state and pre-steady-state transport kinetic studies, (b) in studies on the kinetics of reversible stilbenedisulfonate binding to the unlabeled portion of the band 3 population, and (c) in microcalorimetric studies of the thermal unfolding of the membrane domain of band 3. Studies on the kinetics of reversible stilbenedisulfonate binding to erythrocyte membranes from an individual with Southeast Asian ovalocytosis also revealed subunit interaction effects. The demonstration of allosteric competition between stilbenedisulfonates and substrate anions and the observation of numerous examples of subunit interaction effects suggest that allosteric effects may play a significant role in band 3 function.