Ionophores and channels

Abstract

The present state in the chemistry of peptides and proteins capable of transporting ions across biological and model membranes have been reviewed. The mechanisms of action of peptide ionophores valinomycin and enniatin and also the channel-forming peptide gramicidin-A have been discussed on the basis of spatial structure and dynamic conformational properties of the compounds. Recent progress in the studies of bacteriorhodopsin, cholinergic receptor from postsynaptic membrane, and sodium channel from the excitable nerve membrane, the integral membrane proteins acting as proton or metal ion channel formers is also discussed.Among the basic approaches to the structural and dynamic characteristics of membranes, a prominent place is occupied by ion transport. This is quite understandable for ion transport plays one of the key roles in the functioning of bioenergetic and receptor systems, in the transmission of nerve impulses and in other processes in the organism.During the last two decades we, at the Shemyakin Institute, have been engaged in the systematic study of peptides and proteins participating in the transmembrane ion transport. The present paper summarizes the results obtained with the peptide antibiotics, valinomycin, enniatins, and gramicidin-A, performing the transport of alkali metal ions; recent data on bacteriorhodopsin, the light driven proton pump, as well as membrane proteins involved in nerve transmission, are also described.Valinomycin is a classical representative of low molecular bioregulators called ionophores, i.e., compounds which are able to bind metal ions in solution and to carry the bound ion along in one or the other stage of the transmembrane ion transporting process.Valinomycin consists of three identical D-Val-L-Lac-L-Val-D-Hyi fragments (Fig. 1). Its 36-membered ring contains six amide and six ester bonds; the side chains are hydrophobic aikyl radicals. Valinomycin is capable of binding numerous cations including those of the alkali, alkaline-earth, and transition metals [ 11. An outstanding property of this ionophore is its remarkably high K/Na complexing selectivity, the stability of the K+ complex being three to four orders of magnitude higher than that of the Na+ complex. Neither in animate nor inanimate nature are any substances known that could at least approach valinomycin in this respect. Valinomycin is quite universal in its action on membranes, inducing K+ conductivity in any kind of membrane system with almost no exception. Its action is manifested already at low concentrations ( lod8 and lower).

Analysis of light-induced transmembrane ion gradients and membrane potential in Photosystem I proteoliposomes

“…As it has been previously shown, that approximation is better for narrow distributions, when the vesicle population approaches the permeability kinetics of a homogeneous population [48]. Regarding the final values for the membrane permeability to ions (table I) It has been often been assumed that valinomycin only affects the membrane permeability to potassium, since the very high affinity of potassium ions to this ionophore [53] causes a significant increase of the potassium flux through lipid membranes [54][55][56]. Micromolar concentrations of valinomycin can increase 2 to 4 orders of magnitude the membrane conductance to potassium [55;57;58].…”