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Tryptophan Contributions to the Empirical Free-Energy Profile in Gramicidin A/M Heterodimer Channels

Deptartment of Physiology and Developmental Biology, Brigham Young University, Provo, Utah

Correspondence: Address reprint requests to David D. Busath, Dept. of Physiology and Developmental Biology, Brigham Young University, Provo, UT 84602. Tel.: 801-422-8753; Fax: 801-422-0700; E-mail: david_busath{at}byu.edu.

Gramicidin A/gramicidin M heterodimer conductances were measured in planar lipid bilayers and found to form two distinguishable populations about halfway between the gramicidin A and gramicidin M homodimer conductances. This implies that the principle difference in the gramicidin A and gramicidin M transport free-energy profiles occurs at the channel center, where it would produce similar effects on the rate-limiting barrier for the two heterodimers. Kinetic analysis based on this and nearly all previously published homodimer conductance data for both gramicidin A and gramicidin M channels confirms this conclusion, indicating that the translocation step is 100-fold slower in gramicidin M homodimers than in gramicidin A homodimers and that first- and second-ion exit-rate constants are higher by factors of 24 and 10, respectively. Assuming that the ratios of rate constants are related to the free-energy difference between gramicidin A and gramicidin M, we construct an effective ion-Trp free-energy interaction profile that has a minimum at the channel center.