Falke, Joseph J. and Kanes, Katherine J. and Chan, Sunney I. (1985) The kinetic equation for the chloride transport cycle of band 3. A 35Cl and 37Cl NMR study. Journal of Biological Chemistry, 260 (17). pp. 9545-9551. ISSN 0021-9258. http://resolver.caltech.edu/CaltechAUTHORS:FALjbc85a
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The nature of a transmembrane transport process depends largely on the identity of the reaction that is rate-limiting in the transport cycle. The one-for-one exchange of two chloride ions across the red cell membrane by band 3 can be decomposed into two component reactions: 1) the binding and dissociation of chloride at the transport site, and 2) the translocation of bound chloride across the membrane. The present work utilizes 35 Cl NMR and 37 Cl NMR to set lower limits on the rates of chloride binding and dissociation at the saturated inward- and outward-facing band 3 transport sites (≥10^(5) events site^-1 s^-1 in all cases). At both 0-3 and 37 degrees C, the NMR data specify that chloride binding and dissociation a the saturated transport sites are not rate-limiting, indicating that translocation of bound chloride across the membrane is the slowest step in the overall transport cycle. Using these results, it is now possible to describe many features of the kinetic equation for the ping-pong transport cycle of band 3. This transport cycle can be decomposed into two half-reactions associated with the transport of two chloride ions in opposite directions across the membrane, where each half-reaction is composed of sequential binding, translocation, and dissociation events. One half-reaction contains the rate-limiting translocation event that controls the turnover of the transport cycle; in this half-reaction, translocation must be slower than binding and dissociation. The other half-reaction contains the non-rate-limiting translocation event that in principle could be faster than binding or dissociation. However, when the following sufficient (but not necessary) condition is satisfied, both translocation events are slower than binding and dissociation: if the non-rate-limiting translocation rate is within a factor of 10^(2) (0-3 degrees C) or 2 (37 degrees C) of the overall turnover rate, then translocation is rate-limiting in each saturated half-reaction. Thus, even though chloride appears to migrate through a channel that leads from the transport site to solution, the results support a picture in which the binding, dissociation, and channel migration events are rapid compared to the translocation of bound chloride across the membrane. In this case, chloride binding to the transport site can be described by a simple dissociation constant (KD = kappa OFF/kappa ON) rather than by a Michaelis-Menten constant (KM = (kappa OFF + kappa TRANSLOCATION)/KAPPA ON).
|Additional Information:||Copyright © 1985 by the American Society for Biochemistry and Molecular Biology. (Received for publication, August 30, 1984) This work was supported by National Institute of General Medical Sciences Grant GM-22432 (to S.I.C.) and by a National Science Foundation predoctoral fellowship (to J.J.F.). Contribution 7081 from the Arthur Amos Noyes Laboratory. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.|
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