1.D.1 The Gramicidin A (Gramicidin A) Channel Family
Gramicidin A, a pentadecapeptide antibiotic, is made by Bacillus brevis and forms channels in synthetic and natural bilayers that are selective for monovalent cations such as H+, Tl+, NH4+ and the alkali metals. X-ray crystal structures, 15N-NMR and CD data reveal alternate structures that Gramicidin can assume. The functional channel in lipid bilayers is probably a transmembrane helical dimer. Two monomeric β-helices meet at their N-termini in the center of the membrane. Transport of ions may occur by single file transfer through the gramicidin channel. Gramicidin also forms double helical structures which consists of two hydrogen bonded β-strands that are rolled up to form double β-helicies which can span the thickness of the bilayer. Only under limited conditions do double helical forms conduct ions.
Gramicidin is not synthesized by a ribosomal-dependent mechanism, and it contains six D amino acids, all leucine and valine residues. The sequence of gramicidin A is: HCO-L-Val1-Gly2-L-Ala3-D-Leu4-L-Ala5-D-Val6-L-Val7-D-Val8-L-Trp9-D-Leu10-L-Trp11-D-Leu12-L-Trp13-D-Leu14-L-Trp15-NHCH2-CH2OH. Because it is not encoded by a gene, gramicidin is not included in the databases, and no accession number is available. In contrast to valinomycin which complexes with K+ and shuttles across the membrane, in a 'carrier'-like process, gramicidin forms a static channel and serves as the prototype for protein-mediated channel formation across biological membranes.
The generalized reaction catalyzed by gramicidin is:
Monovalent cation (in) → Monovalent cation (out).