Gramicidin D conformation, dynamics and membrane ion transport

The linear pentadecapeptide antibiotic, gramicidin D, a heterogeneous mixtu re of six six components, is a naturally occurring product of Bacillus brev is known to form ion channels in synthetic and natural membranes,. The conf ormation of gramicidin A in the solid state, in organic solvents, and in pl anar lipid bilayers and the relationship between the composition and the co nformation of gramicidin and its selective transport of ions across membran es has been the subject of intense investigation for over 50 years. The x-r ay crystal structure and nmr solution spectroscopy agree fully with one ano ther and reveal that entirely different conformations of gramicidin are pre sent in uncomplexed and ion complexed farms. Precise refinements of the thr ee-dimensional structures of naturally occurring gramicidin D in crystals o btained from methanol, ethanol, and n-propanol demonstrate the unexpected p resence of stable left-handed antiparallel double-helical heterodimers that vary with the crystallization solvent. The side chains of Trp residues in the three structures exhibit sequence-specific patterns conformational pref erence. Tyr substitution for Trp at position II appears to favor beta ribbo n formation and stabilization of the antiparallel double helix. This confor mation acts as a template for gramicidin folding and nucleation of the diff erent crystal forms. The fact that a minor component in a heterogeneous mix ture influences aggregation and crystal nucleation has potential applicatio ns to other systems in which anomalous behavior is exhibited by aggregation of apparently homogeneous materials, such as the enigmatic behavior of pri on proteins. The crystallographically determined structures of cesium, pota ssium, rubidium, and hydronium ion complexes of gramicidin A are in excelle nt agreement with the nmr structure determination of the cesium ion gramici din complex in a methanol chloroform mixture (50 : 50). The right-handed an tiparallel double stranded double helical structures (DSDHR) also exhibit g eometric features compatible with the solid-state N-15 and H-2 nmr data rec orded for gramicidin in pla,lar lipid bilayers and attributed to the active form of gramicidin A. The DSDHR, crystal structures reveal an ion channel with a single partially solvated cation distributed over three ion binding sites. The channel lumen is relatively smooth and electrostatically negativ e as required for cation passage, while, the exterior is electrostatically neutral, a requirement for membrane insertion. The "coordination" of the Cs ' ion is achieved by interaction with the pi orbitals of the carbonyls whic h do not point toward the ions. The Kt binding sites, which are similar in position to Cs' binding sites, are shifted off center slightly toward the w all of the channel. (C) 1999 John Wiley & Sons, Inc.