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. Some aspects of its structure and mechanism are debatable (Andersen et al. 2005; Kelkar and Chattopadhyay 2007), and one report suggests that gramicidin may not form pores (Ashrafuzzaman et al. 2008). Gramicidin A can catalyzed phospholipid flipping from one monolayer to the other (Anglin et al. 2007). Gramicidin can passively translocate across membranes (McKay et al. 2018). An increase in the conductance and lifetime of gramicidin A channels induced by the alkaloids, benzylamines, is related to alteration in the membrane dipole potential not to decrease in membrane stiffness (Efimova et al. 2020). Membrane-mediated lateral interactions regulate the lifetime of gramicidin channels (Kondrashov et al. 2020).
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. Gramicidin has been shown to block tumor growth and angiogenesis (David et al. 2014). Applications of pore-forming gramicidin include small- and macromolecule-sensing, targeted cancer therapy and drug delivery (Gurnev and Nestorovich 2014). Ten analogues, sharing a similar ion channel function, have different cytotoxic, hemolytic, and antibacterial activities (Takada et al. 2020).
In addition to its role as a K+ channel, gramicidin increases lipid flip-flop (lipid scrambling between the two monolayers of the bilayer) in both symmetric and asymmetric lipid vesicles (Doktorova et al. 2019). A series of glycoside-peptide conjugates were prepared by engineering at the N-terminus of gramicidin A. The conjugate containing a galactose moiety formed a unimolecular transmembrane channel and mediated ion transport to induce apoptosis of cancer cells. It exhibited liver cancer cell-targeting behavior due to galactose-asialoglycoprotein receptor recognition (Haoyang et al. 2021). Small changes to water-channel interactions alter the free energy barrier for ion permeation (Ngo et al. 2021). Gramicidin A accumulates in mitochondria, reduces ATP levels, induces mitophagy, and inhibits cancer cell growth (Xue et al. 2022). Two derivatives, gramicidin-ethylenediamine and gramicidin-histamine exhibit pH-dependent single-channel behaviour over the pH ranges 9-11 and 6.5-8.5, respectively (Kumar and Madhavan 2023).
The generalized reaction catalyzed by gramicidin is:
Monovalent cation (in) → Monovalent cation (out).