Tryptophan contributions to the empirical free-energy profile in gramicidin A/M heterodimer channels.
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 approximately 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.
Durrant
Jacob Devin
JD
Deptartment of Physiology and Developmental Biology, Brigham Young University, Provo, Utah 84602, USA.
Caywood
Devin
D
Busath
David D
DD
D003198:Computer Simulation; D019281:Dimerization; D004735:Energy Transfer; D006096:Gramicidin / Q000031:analogs & derivatives* / Q000737:chemistry*; D015640:Ion Channel Gating; D008051:Lipid Bilayers / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D014364:Tryptophan / Q000737:chemistry*
D008051:Lipid Bilayers; D006096:Gramicidin; C036306:gramicidin A, 15-des-Trp-Phe-; D014364:Tryptophan