Optimizing diffusive transport through a synthetic membrane channel.

Abstract

Position x P1 P2 P3 Transport of ions, [ 1 , 2 ] proteins, [ 3 ] antibiotics [ 4 , 5 ] and other macromolecular solutes through channels and pores is ubiquitous in nature. In particular channel-facilitated diffusion relies on optimized binding sites for the transported particles inside the channel. [ 6 ] Well characterized examples include membrane channels such as maltoporins [ 7 , 8 ] or aquaglyceroporin [ 9 ] found in abundance in bacterial membranes. This has been confi rmed by (i) ex situ crystallographic structure studies, [ 10 , 11 ] (ii) indirect ionic current measurements through protein channels reconstituted into planar lipid bilayers [ 12–14 ] and (iii) molecular dynamics simulations. [ 9 ] These results suggest that organisms can maximize nutrient uptake driven by diffusion by favoring intimate interaction between the protein channel and the translocating species. This is counterintuitive since a strong binding site implies a long residence time in the channel. However, a few theoretical studies have independently rationalized such fi ndings by considering the transport of particles through a channel using a continuum diffusion model based on the Smoluchowski equation, [ 15 ] discrete stochastic models [ 16 , 17 ] or a generalized macroscopic Fick’s diffusion law, [ 18 ] all demonstrating that an attractive potential in the channel may enhance the particle fl ux. Remarkably, one intriguing approach predicts a maximum in the diffusive current with respect to the binding potential. [ 15 ]