Single channels of 9, 11, 13, 15-destryptophyl-phenylalanyl-gramicidin A

Energy profiles in the gramicidin A channel

Gramicidin A (GA) is a linear pentadecapeptide made of alternating D and L residues, in which the N-and C-terminals are blocked by a formyl group (head) and an ethanolamine end (tail), respectively (Sarges & Witkop, 1964):

Synthesis and characterization of Tyr(Bzl)9,11,13,15 and Tyr9,11,13,15 gramicidin A

Tyr(Bzl) and Tyr gramicidin A were prepared by the solid phase method using a 4-(oxymethyl)-Pam resin and Bpoc as alpha-amino-protecting group. The benzylated analog [Gr.T(Bzl)] was purified by chromatography on silica gel and then on LH60 Sephadex. Removal of benzyl groups was carried out by hydrogenolysis and the debenzylated derivative (Gr.T) was purified in the same way. Both gramicidins were checked and characterized by t.l.c., HPLC, circular dichroism, 1H n.m.r. and single channel measurements. CD spectra were found to be different for Gr.T(Bzl) and Gr.T and strongly dependent upon the solvent and the concentration. Single channel conductance of Gr. T is slightly lower than that of Gr.A (A Gr.T approximately equal to 0.7 A Gr.T).

Linear gramicidins at the air-water interface

The behavior of four linear gramicidins, which differ by the nature of their 9, 11, 13, and 15 aromatic residues, together with a covalent "head to tail" retro GA-DAla-GA dimer, has been examined at the air-water interface. It is shown that all four "monomers" have almost the same molecular area, which is compatible with either a single-stranded or a double-stranded helical model, whereas it is suggested that retro GA-DAla-GA could adopt another conformation. The surface potential measurements agree with those of different groups of molecules characterized by their single-channel behaviors.

Effects of ionizing radiation on artificial (planar) lipid membranes. I. Radiation inactivation of the ion channel gramicidin A

The ion channel formed by the pentadecapeptide gramicidin A in planar lipid membranes is extremely sensitive to ionizing radiation. The membrane conductance may drop by several orders of magnitude under appropriate experimental conditions (low pH and presence of oxygen). The radiation sensitivity is strongly reduced for gramicidin M-. This analogue has the four tryptophan residues replaced by phenylalanines. Experiments performed in the presence of various radical scavengers suggest that the inactivation of the channel is due to a combined action of OH and of HO2 radicals at the tryptophan residues. The shape of the inactivation curves following continuous radiolysis or pulse radiolysis were found to be in fair agreement with a simple model which assumes that the damage of a single tryptophan residue is sufficient for channel inactivation. The conductance of inactivated channels could not be resolved within the experimental accuracy. This is contrary to photolysis of gramicidin channels found by Busath and Waldbilling (1983), where a broad distribution of low conductance states was observed. The inactivation by radiolysis seems to represent an 'all-or-none-process' of the channel conductance.

How pore mouth charge distributions alter the permeability of transmembrane ionic channels

This paper investigates the effects that surface dipole layers and surface charge layers along the pore mouth-water interface can have on the electrical properties of a transmembrane channel. Three specific molecular sources are considered: dipole layers formed by membrane phospholipids, dipole layers lining the mouth of a channel-forming protein, and charged groups in the mouth of a channel-forming protein. We find, consistent with previous work, that changing the lipid-water potential difference only influences channel conduction if the rate-limiting step takes place well inside the channel constriction. We find that either mouth dipoles or mouth charges can act as powerful ion attractors increasing either cation or anion concentration near the channel entrance to many times its bulk value, especially at low ionic strengths. The effects are sufficient to reconcile the apparently contradictory properties of high selectivity and high conductivity, observed for a number of K+ channel systems. We find that localizing the electrical sources closer to the constriction entrance substantially increases their effectiveness as ion attractors; this phenomenon is especially marked for dipolar distributions. An approximate treatment of electrolyte shielding is used to discriminate between the various mechanisms for increasing ionic concentration near the constriction entrance. Dipolar potentials are far less sensitive to ionic strength variation than potentials due to fixed charges. We suggest that the K+ channel from sarcoplasmic reticulum does not have a fixed negative charge near the constriction entrance; we suggest further that the Ca+2-activated K+ channel from transverse tubule does have such a charge.

Analysis of the ion transfer through the channel of 9,11,13,15-phenylalanylgramicidin A

The behavior of an analogue of gramicidin A in which all four tryptophanyl residues are substituted by phenylalanyl and which shows a strong voltage effect on the single channel conductance is analyzed on the basis of a 'three-barrier--two-site' model. It is shown that in the gramicidin family the side chains of some amino acids, in spite of their location, which point outside the channel can play a major role in the binding of ions in the channel and thus can significantly modify the energy profile of the channel.

Conformations of gramicidin A and its 9,11,13,15-phenylalanyl analog in dimethyl sulfoxide and chloroform

In order to understand the difference in single channel behavior of gramicidin A as compared to that of gramicidin M- which is the mirror image of gramicidin M (all four tryptophanyl residues substituted by phenylalanine), conformational investigations were made under several experimental conditions. It is shown that, when examined under identical conditions, both molecules adopt the same conformations which could be identified in dimethyl sulfoxide (DMSO) and chloroform. In DMSO the conformation is based on a succession of beta-turns while in chloroform gramicidin A and M- can adopt a dimeric hybrid structure: a double helix terminated by two single-stranded helices involving the N- and C-terminal parts, respectively. It is therefore concluded that the difference in the energy profile between both gramicidins which was deduced from the ion transfer data has its origin in the nature of the aromatic side chains.

Single-channel studies on linear gramicidins with altered amino acid side chains. Effects of altering the polarity of the side chain at position 1 in gramicidin A

The modulation of gramicidin A single-channel characteristics by the amino acid side chains was investigated using gramicidin A analogues in which the NH2 terminal valine was chemically replaced by other amino acids. The replacements were chosen such that pairs of analogues would have essentially isosteric side chains of different polarities at position 1 (valine vs. trifluorovaline or hexafluorovaline; norvaline vs. S-methyl-cysteine; and norleucine vs. methionine). Even though the side chains are not in direct contact with the permeating ions, the single-channel conductances for Na+ and Cs+ are markedly affected by the changes in the physico-chemical characteristics of the side chains. The maximum single-channel conductance for Na+ is decreased by as much as 10-fold in channels formed by analogues with polar side chains at position 1 compared with their counterparts with nonpolar side chains, while the Na+ affinity is fairly insensitive to these changes. The relative conductance changes seen with Cs+ were less than those seen with Na+; the ion selectivity of the channels with polar side chains at position 1 was increased. Hybrid channels could form between compounds with a polar side chain at position 1 and either valine gramicidin A or their counterparts with a nonpolar side chain at position 1. The structure of channels formed by the modified gramicidins is thus essentially identical to the structure of channels formed by valine gramicidin A. The polarity of the side chain at position 1 is an important determinant of the permeability characteristics of the gramicidin A channel. We discuss the importance of having structural information when interpreting the functional consequences of site-directed amino acid modifications.

How shortening a channel may lower its conductance. The case of des-Val7-DVal8-gramicidin A

A shortened analog of gramicidin A has been shown by Urry et al. (Biochim. Biophys. Acta 775, 115-119) to have lower conductance than native gramicidin A. They argue this suggests that the major current carrier is the doubly occupied channel. A different perspective is presented here. Channel formation does not alter bilayer width. In a shortened channel an ion approaching the binding site moves further toward the center of the lipid-pore system. The electrostatic contribution to the energy barrier near the constriction mouth is greater for the shorter channel. As long as entry to the channel is rate limiting singly occupied short channels should exhibit lower conductance. The data are not inconsistent with singly occupied channels being the major current carriers. Experiments on other gramicidin analogs are suggested to more clearly distinguish between singly and doubly occupied channels as the dominant conducting species.

The effect of the amino-acid side chains on the energy profiles for ion transport in the gramicidin A channel

Computations on the energy profiles for Na+ in the gramicidin A (GA) channel have been extended by introducing the effect, previously neglected, of the amino acid side chains of GA, fixed in their most stable conformations. The calculations have been performed in two approximations: 1) with the ethanolamine tail fixed in its most stable conformation, 2) with the tail allowed to optimize its conformation upon the progression of the ion. In both approximations the overall shape of the energy profile is very similar to that obtained in the absence of the side chains. One observes, however, a general lowering of the profile upon the adjunction of the side chains. The analysis of the factors responsible for this energy lowering indicates that it is due essentially to the electrostatic and polarisation components of the interaction which interplay differently, however, in the different parts of the channel. A particular role is attributed in this respect to the tryptophan residues of GA. The role of the 4 tryptophans present, Trp 15, 13, 11 and 9, is individualized by stripping of one of them at a time. The strongest effect on the energy deepening is due to Trp 13 and is particularly prominent in the entrance zone at 14.5A from the center of the channel. The result indicates the possibility of investigating theoretically the effect on the energy profiles of the substitution of the "natural" side chain by others.

Single-channel studies on linear gramicidins with altered amino acid sequences. A comparison of phenylalanine, tryptophane, and tyrosine substitutions at positions 1 and 11

The relation between chemical structure and permeability characteristics of transmembrane channels has been investigated with the linear gramicidins (A, B, and C), where the amino acid at position 1 was chemically replaced by phenylalanine, tryptophane or tyrosine. The purity of most of the compounds was estimated to be greater than 99.99%. The modifications resulted in a wide range of conductance changes in NaCl solutions: sixfold from tryptophane gramicidin A to tyrosine gramicidin B. The conductance changes induced by a given amino acid substitution at position 1 are not the same as at position 11. The only important change in the Na+ affinity was observed when the first amino acid was tyrosine. No major conformational changes of the polypeptide backbone structure could be detected on the basis of experiments with mixtures of different analogues and valine gramicidin A (except possibly with tyrosine at position 1), as all the compounds investigated could form hybrid channels with valine gramicidin A. The side chains are not in direct contact with the permeating ions. The results were therefore interpreted in terms of modifications of the energy profile for ion movement through the channel, possibly due to an electrostatic interaction between the dipoles of the side chains and ions in the channel.

Ca2+-gramicidin A interactions and blocking effects on the ionic channel

From spectroscopic data (infrared, CD and 13C-NMR) it is shown that Ca2+ interacts with gramicidin A and that a head-to-head gramicidin A dimer can have two Ca2+-binding sites located near the COOH termini. On the basis of this result, we can propose an explanation for the blocking effect of Ca2+ on the transport of alkali metal ions (Cs+ and K+) through gramicidin channels. The binding of Ca2+ is competitive with the alkali metal ion binding; Ca2+ cannot cross the gramicidin channel and its binding in the channel is voltage dependent. From the proposed model, it is possible to account for the influence of the addition of Ca2+ on the single-channel limiting conductance and on the variation of the single-channel current as a function of the voltage in the presence of Cs+ or K+.