Salting Out the Ionic Selectivity of a Wide Channel: The Asymmetry of OmpF

Alcaraz, Antonio; Nestorovich, Ekaterina M.; Aguilella‐Arzo, Marcel; Aguilella, Vicente M.; Bezrukov, Sergey M.

doi:10.1529/biophysj.104/043414

Cited by 157 publications

(273 citation statements)

References 57 publications

(107 reference statements)

Supporting

Mentioning

247

Contrasting

Unclassified

Order By: Relevance

“…It will be higher in one direction and lower in the other, and we will analyze this effect in more detail below. We note that such a directional behavior has been observed most recently in the channel protein OmpF (26).…”

Section: [22]supporting

confidence: 69%

Molecular transport through channels and pores: Effects of in-channel interactions and blocking

Bauer¹,

Nadler²

2006

Proc. Natl. Acad. Sci. U.S.A.

View full text Add to dashboard Cite

Facilitated translocation of molecules through channels and pores is of fundamental importance for transmembrane transport in biological systems. Several such systems have specific binding sites inside the channel, but a clear understanding of how the interaction between channel and molecules affects the flow is still missing. We present a generic analytical treatment of the problem that relates molecular flow to the first passage time across and the number of particles inside the channel. Both quantities depend in different ways on the channel properties. For the idealized case of noninteracting molecules, we find an increased flow whenever there is a binding site in the channel, despite an increased first passage time. In the more realistic case that molecules may block the channel, we find an increase of flow only up to a certain threshold value of the binding strength and a dependence on the sign of the concentration gradient, i.e., asymmetric transport. The optimal binding strength in that case is analyzed. In all cases the reason for transport facilitation is an increased occupation probability of a particle inside the channel that overcomes any increase in the first passage time because of binding.binding site ͉ first passage time ͉ membrane D iffusion of molecules through channels and pores of an otherwise impermeable membrane is an important issue in biological transport at the cellular level (1, 2). Early considerations of channel transport have been concerned mainly with the effect of barriers inside channels (3). However, various means of facilitated transport across a membrane have also been considered, e.g., shuttle mechanisms (4). In recent years it has been noted that there are several cases where the molecules transported interact strongly with regions inside the channel (5-13), apparently leading to an increase in transmembrane transport. However, a fundamental understanding and quantitative description of such an increased transport due to in-channel binding sites is still missing.From an intuitive point of view, it is not clear at all why a strong interaction with the channel should facilitate transport. Conversely, one could argue that a strong binding is associated with a longer residence time inside the channel, which reduces flow. Furthermore, molecules bound temporarily inside the channel may hamper transport of other molecules, especially when they are large (13,14), and block the channel. So, why do traps and͞or reaction sites within the channel facilitate molecular flow? These questions have to be addressed in the generic biological setting of a macroscopic concentration gradient across the membrane (see Fig. 1). An appropriate quantitative description should give the flow for a given concentration difference depending on the potential and other parameters describing the molecule-channel interaction. Physical insight can be gained if the flow can be related to other global properties of the system in question.Past efforts (15,16) to analyze this situation used the concepts of conditio...

show abstract

Section: [22]supporting

confidence: 69%

Molecular transport through channels and pores: Effects of in-channel interactions and blocking

Bauer¹,

Nadler²

2006

Proc. Natl. Acad. Sci. U.S.A.

View full text Add to dashboard Cite

show abstract

“…A detailed description of the reconstitution procedure can be found elsewhere. 17,26 The electric potential V is positive when it is higher at the trans side of the membrane cell. An Axopatch 200B amplifier (Molecular Devices, Sunnyvale, CA) in the voltage-clamp mode is used for measuring both V and the electric current (I) through the channel.…”

mentioning

confidence: 99%

“…being the solution pH the most significant one. 26 At low pH, the channel is positively charged and selective to anions, whereas at high pH, the channel is negatively charged and selective to cations. 26,28 For pH values close to neutrality, the channel exhibits a slight selectivity to cations.…”

mentioning

confidence: 99%

“…26 At low pH, the channel is positively charged and selective to anions, whereas at high pH, the channel is negatively charged and selective to cations. 26,28 For pH values close to neutrality, the channel exhibits a slight selectivity to cations. We have shown previously that not only the ion selectivity but also the I-V characteristics of OmpF depend strongly on the pH of the external solutions.…”

mentioning

confidence: 99%

“…4 However, the OmpF channel is only partially selective to cations, 26,28 as it is the case of most ion channel and nanopores. The fact that both cations and anions contribute to the overall current in a proportion which could change considerably with the applied voltage has not been addressed previously.…”

mentioning

confidence: 99%

See 2 more Smart Citations

Electrical pumping of potassium ions against an external concentration gradient in a biological ion channel

Queralt-Martín

García-Giménez

Aguilella

et al. 2013

Applied Physics Letters

Self Cite

View full text Add to dashboard Cite

We show experimentally and theoretically that significant currents can be obtained with a biological ion channel, the OmpF porin of Escherichia coli, using zero-average potentials as driving forces. The channel rectifying properties can be used to pump potassium ions against an external concentration gradient under asymmetric pH conditions. The results are discussed in terms of the ionic selectivity and rectification ratio of the channel. The physical concepts involved may be applied to separation processes with synthetic nanopores and to bioelectrical phenomena. Ratchet systems have the ability of rectifying unbiased fluctuations into directed transport. 1-3 Both synthetic and biological nanochannels may show non-zero electrical currents driven by zero-average time dependent forces. [4][5][6][7] The electrical rectification is on the basis of the observed phenomena 4,5,8,9 and the potential applications concern separation processes and energy conversion. 10-12 Electrical rectification and ratchet phenomena can also be of relevance for signal averaging of weak electric fields and cell plasticity because ion channels, together with ion pumps and gap junction complexes, play a crucial role in cell communication and control. [13][14][15][16] In this Letter, we present measurements and model calculations of the rectifying properties of a biological ion channel (the outer membrane protein F, OmpF, a porin expressed in Escherichia coli bacteria 17 ). We consider the pH-controlled net current induced by zero-average oscillating potentials and the electrical pumping of potassium ions against an external concentration gradient, a phenomenon previously reported in synthetic nanopores. 4,5 We show that the complex interplay between the current rectification and the channel ionic selectivity is crucial for understanding the uphill transport of ions. The results are of relevance for the mechanisms regulating the transport through ion channels, [17][18][19] for electric signal transduction in cell cycle and embryogenesis, 14,16 and for building ionic circuits in the emerging field of nanofluidics. 6,[20][21][22][23][24][25] The set-up used in the experiments is shown schematically in Fig. 1. A single OmpF ion channel is reconstituted on a planar lipid bilayer and communicates the two halves of a conductivity cell filled with KCl solutions of concentrations c cis (side of protein addition) and c trans . pH cis and pH trans denote the corresponding pH values. A detailed description of the reconstitution procedure can be found elsewhere. 17,26 The electric potential V is positive when it is higher at the trans side of the membrane cell. An Axopatch 200B amplifier (Molecular Devices, Sunnyvale, CA) in the voltage-clamp mode is used for measuring both V and the electric current (I) through the channel. I is positive when it flows from solution trans to solution cis in Fig. 1.The OmpF porin contains both basic (positive) and acidic (negative) residues, 27 and thus, the net charge of the channel depends on the ionization state of th...

show abstract

Ion-Current Rectification in Nanopores and Nanotubes with Broken Symmetry

2006

View full text Add to dashboard Cite

This article focuses on ion transport through nanoporous systems with special emphasis on rectification phenomena. The effect of ion‐current rectification is observed as asymmetric current–voltage (I–V) curves, with the current recorded for one voltage polarity higher than the current recorded for the same absolute value of voltage of opposite polarity. This diode‐like I–V curve indicates that there is a preferential direction for ion flow. Experimental evidence that ion‐current rectification is inherent to asymmetric, e.g., tapered, nanoporous systems with excess surface charge is provided and discussed. The fabrication and operation of asymmetric polymer nanopores, gold nanotubes, glass nanocapillaries, and silicon nanopores are presented. The possibility of tuning the direction and extent of rectification is discussed in detail. Theoretical models that have been developed to explain the ion‐current rectification effect are also presented.

show abstract

Salting Out the Ionic Selectivity of a Wide Channel: The Asymmetry of OmpF

Cited by 157 publications

References 57 publications

Molecular transport through channels and pores: Effects of in-channel interactions and blocking

Molecular transport through channels and pores: Effects of in-channel interactions and blocking

Electrical pumping of potassium ions against an external concentration gradient in a biological ion channel

Ion-Current Rectification in Nanopores and Nanotubes with Broken Symmetry

Contact Info

Product

Resources

About