Transport Properties of Nanoporous, Chemically Forced Biological Lattices.

Abstract

Permselective nanochannels are ubiquitous in biological systems, controlling ion transport and maintaining a potential difference across a cell surface. Surface layers (S-layers) are proteinaceous, generally charged lattices punctuated with nano-scale pores that form the outermost cell envelope component of virtually all archaea and many bacteria. Ammonia Oxidizing Archaea (AOA) obtain their energy exclusively from oxidizing ammonia directly below the S-layer lattice, but how the charged surfaces and nanochannels affects availability of NH4+ at the reaction site is unknown. Here, we examine the electrochemical properties of negatively charged S-layers for asymmetrically forced ion transport governed by Michaelis-Menten kinetics at ultra-low concentrations. Our 3-dimensional electro-diffusion reaction simulations revealed that a negatively charged S-layer can invert the potential across the nanochannel to favor chemically forcedNH4+ transport, analogous to polarity switching in nanofluidic field effect transistors. Polarity switching was not observed when only the interior of the nanochannels was charged. We found that S-layer charge, nanochannel geometry and enzymatic turnover rate are finely tuned to elevate NH4+ concentration at the active site, potentially enabling AOA to occupy nutrient-poor ecological niches. Strikingly, and in contrast to voltage-biased systems, magnitudes of the co- and counter-ion currents in the charged nanochannels were nearly equal and amplified disproportionally to the NH4+ current. Our simulations suggest that engineered arrays of crystalline proteinaceous membranes could find unique applications in industrial energy conversion or separation processes.