Measurements of Ion Binding to Lipid-Hosted Ionophores by Affinity Chromatography

Theoretical investigation of hydroxylated analogues of valinomycin as potassium transporter

Valinomycin is a potent ionophore known for its ability to transport potassium ions across biological membranes. The study focuses on the hydroxylated analogues of valinomycin (HyVLMs) and compares their energy profiles and capabilities for transporting potassium ions across phospholipid membranes. Using metadynamics, we investigated the energy profiles of wildtype valinomycin (VLM_1) and its three hydroxylated analogues (VLM_2, VLM_3, and VLM_4). We observed that all analogues exhibited energy maxima in the centre of the membrane and preferred positions below the phospholipid heads. Furthermore, the entry barriers for membrane penetration were similar among the analogues, suggesting that the hydroxyl group did not significantly affect their passage through the membrane. Transition state calculations provided insights into the ability of valinomycin analogues to capture potassium ions, with VLM_4 showing the lowest activation energy and VLM_2 displaying the highest. Our findings contribute to understanding the mechanisms of potassium transport by valinomycin analogues and highlight their potential as ionophores. The presence of the hydroxyl group is of particular importance because it paves the way for subsequent chemical modifications and the synthesis of new antiviral agents with reduced intrinsic toxicity.

Single-Molecule Electrical Currents Associated with Valinomycin Transport of K. ACS NANO 2023;17:8829-8836. [PMID: 37068060 DOI: 10.1021/acsnano.3c02825]

A quantitative description of ionophore-mediated ion transport is important in understanding ionophore activity in biological systems and developing ionophore applications. Herein, we describe the direct measurement of the electrical current resulting from K⁺ transport mediated by individual valinomycin (val) ionophores. Step fluctuations in current measured across a 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) bilayer suspended over a ∼400 nm radius glass nanopore result from dynamic partitioning of val between the bilayer and torus region, effectively increasing or decreasing the total number of val present in the membrane. In our studies, approximately 30 val are present in the membrane on average with a val entering or leaving the bilayer approximately every 50 s, allowing measurement of changes in electrical current associated with individual val. The single-molecule val(K⁺) transport current at 0.1 V applied potential is (1.3 ± 0.6) × 10^-15 A, consistent with estimates of the transport kinetics based on large val ensembles. This methodology for analyzing single ionophore transport is general and can be applied to other carrier-type ionophores.

Ion-Pairing Mechanism for the Valinomycin-Mediated Transport of Potassium Ions across Phospholipid Bilayers

The role of the anion on the ionophore properties of valinomycin was studied in a model floating bilayer lipid membrane (fBLM) using supporting electrolytes containing K⁺ with four different counter anion species (ClO₄^-, H₂PO₄^-, Cl^-, and F^-). The electrochemical impedance spectra indicate that the membrane resistance of the bilayer decreases with the decrease of Gibbs free energy of anion solvation. The IR spectra demonstrate that valinomycin does not readily bind to K⁺ in the KH₂PO₄, KCl, and KF electrolyte solutions, but in the presence of KClO₄, valinomycin readily binds to K⁺, forming a valinomycin-K⁺ complex. The results in the present paper reveal the role of the counter anion on the transport of cations by valinomycin across the lipid bilayer. The valinomycin-cation complex creates an ion pair with the anion, and this ion pair can enter the hydrophobic region of the bilayer transporting the cation across the membrane. Anions with low solvation energies facilitate the formation of the ion pair improving the ion conductivity of valinomycin-incorporated bilayers. This paper sheds new light on the transport mechanism of valinomycin ionophores and provides new information about the bioactivity of this molecule.

Rethinking Ion Transport by Ionophores: Experimental and Computational Investigation of Single Water Hydration in Valinomycin-K+ Complexes

Valinomycin is a macrocyclic ionophore that transports K⁺ across hydrophobic membranes. Its function depends on selectivity, capture, transport, and release of the ion. While thermodynamics clearly indicate that valinomycin binds K⁺ preferentially over all other alkali ions, characterizing the capture/transport/release of K⁺ by valinomycin at the molecular level remains a challenge. The bracelet-like structure of valinomycin-K⁺ (K⁺VM) has the ion completely enveloped, facilitating transport through the cell membrane. We report that hydration by a single water molecule, (K⁺VM)(H₂O), produces three different conformers, identified by infrared spectroscopy and supporting computational studies. For two minor conformers, the water prevents the ionophore from closing, a conformation that would inhibit diffusion through the membrane. However, the dominant conformer encloses both the ion and the water, replicating the bracelet-like K⁺VM and arguably enhancing diffusion through the membrane. This potential for active participation of water in transport through the hydrophobic cellular membrane has never been previously considered.

Antibacterial mechanism of brevilaterin B: an amphiphilic lipopeptide targeting the membrane of Listeria monocytogenes

Antimicrobial peptides (AMPs) are recognized as promising safe alternatives to antibiotics for its low drug-resistance. Brevilaterin B, a newly discovered antimicrobial lipopeptide produced by Brevibacillus laterosporus S62-9, exhibits efficient antibacterial activity on Listeria monocytogenes with a minimum inhibitory concentration of 1 μg mL^-1. The present research aimed to investigate the antibacterial mechanism of brevilaterin B against Listeria monocytogenes. Brevilaterin B caused membrane depolarization and the breakup of the cytomembrane as measured by 3,3-dipropylthiadicarbocyanine iodide and transmission electron microscopy, respectively. Using 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (7:3) as a model membrane, results proved that brevilaterin B could bind to liposomes, integrate into the lipid bilayer, and consequently increase the permeability of liposomes to calcein. The secondary structure of brevilaterin B also changed from an unstructured coil to a mainly β-sheet conformation as measured by circular dichroism. Brevilaterin B exhibits antibacterial activity by a membrane interaction mechanism, which provides a theoretical basis for using brevilaterin B as a promising natural and effective antimicrobial agent against pathogenic bacteria. KEY POINTS: • Brevilaterin B exhibited antibacterial activity against Listeria monocytogenes. • Brevilaterin B exhibited membrane interaction mechanism. • Brevilaterin B showed conformational change when interacted with liposome.

How Valinomycin Ionophores Enter and Transport K+ across Model Lipid Bilayer Membranes

Valinomycin, a cyclic peptide, was incorporated into a biomimetic lipid membrane tethered to the surface of a gold (111) electrode. Electrochemical impedance spectroscopy was used to study the ionophore properties of the peptide, and polarization modulation infrared reflection absorption spectroscopy was employed to determine the conformation and orientation of valinomycin in the membrane. The combination of these two techniques provided unique information about the ionophore mechanism where valinomycin transports ions across the membrane by creating a complex with potassium ions and forming an ion pair with a counter anion. The ion pair resides within the hydrophobic fragment of the membrane and adopts a small angle of ∼22° with respect to the surface normal. This novel study provides new insights explaining the valinomycin ion transport mechanism in model biological membranes.