Highly permeable polymeric membranes based on the incorporation of the functional water channel protein Aquaporin Z

Abstract

The permeability and solute transport characteristics of amphiphilic triblock-polymer vesicles containing the bacterial water-channel protein Aquaporin Z (AqpZ) were investigated. The vesicles were made of a block copolymer with symmetric poly-(2-methyloxazoline)-poly-(dimethylsiloxane)-poly-(2-methyloxazoline) (PMOXA 15-PDMS110-PMOXA 15) repeat units. Light-scattering measurements on pure polymer vesicles subject to an outwardly directed salt gradient in a stopped-flow apparatus indicated that the polymer vesicles were highly impermeable. However, a large enhancement in water productivity (permeability per unit driving force) of up to Ϸ800 times that of pure polymer was observed when AqpZ was incorporated. The activation energy (E a) of water transport for the protein-polymer vesicles (3.4 kcal/mol) corresponded to that reported for water-channel-mediated water transport in lipid membranes. The solute reflection coefficients of glucose, glycerol, salt, and urea were also calculated, and indicated that these solutes are completely rejected. The productivity of AqpZ-incorporated polymer membranes was at least an order of magnitude larger than values for existing salt-rejecting polymeric membranes. The approach followed here may lead to more productive and sustainable water treatment membranes, whereas the variable levels of permeability obtained with different concentrations of AqpZ may provide a key property for drug delivery applications.permeability ͉ triblock copolymer ͉ water treatment B iological membranes have excellent water transport characteristics, with certain membranes able to regulate permeability over a wide range. The permeability of membranes such as those present in the proximal tubules of the human kidney (1) can be increased by insertion of specific water-channel membrane proteins known as Aquaporins (AQPs). Other biological membranes, such as those in mammalian optic lenses (2), erythrocytes (3), and many other cell membranes (4) are constitutively AQP-rich. The permeabilities of AQP-rich membranes are orders of magnitude higher than those observed for unmodified phospholipid membranes (5). Additionally, some members of the AQP family have excellent solute retention capabilities for small solutes such as urea, glycerol, and glucose, even at high water transport rates (5, 6). These properties result from the unique structure of the water-selective AQPs. These AQPs have six membrane-spanning domains and a unique hourglass structure (7) with conserved charged residues that form a pore that allows both selective water transport and solute rejection. The AQP used in this study was a bacterial aquaporin from Escherichia coli, Aquaporin Z (AqpZ). AqpZ was selected because it can enhance the permeability of lipid vesicles by an order of magnitude while retaining small uncharged solutes (5). Additionally, AqpZ can be expressed in relatively large quantities in E. coli and has been reported to be quite stable under different reducing conditions and at temperatures of 4°C (5)-properties that make it att...

Bio-Decorated Polymer Membranes: A New Approach in Diagnostics and Therapeutics

“…Different behavior was seen in the case of AqpZ insertion in polymeric membranes. A ratio of 1:50, protein to block copolymer, has been found to yield the highest permeability to the polymeric vesicles [36]. The results using the PMOXA-PDMS-PMOXA block copolymer, but with different membrane proteins (OmpF, Aqpz respectively) reinforce the general conclusion that quantitative insertion of membrane proteins in membranes depends on the nature of the protein.…”

“…The PMOXA-PDMS-PMOXA membrane is impermeable to small molecules, including water [36], and its permeability was increased by reconstituting channel proteins, allowing the transport of various molecules through the polymeric membrane (Figure 7). Similarly, other membrane proteins have been successfully reconstituted in PMOXA-PDMS-PMOXA membranes: LamB, FhuA, Tsx, AqpZ, Complex I [34,36,37,59], as shown by the movement of different molecules (water, NADH, nucleotides, DNA, enzyme substrates) through the polymeric membrane. F0F1-ATP synthase and bacteriorhodopsin reconstituted in poly(2-ethyl-2-oxazoline)-bpoly(dimethylsiloxane)-b-poly(2-ethyl-2-oxazoline) (PEtOz-PDMS-PEtOz) polymeric vesicles are further examples of transmembrane proteins that retained their activity after reconstitution in polymer membranes [38].…”

Optimized reconstitution of membrane proteins into synthetic membranes

“…The vesicles were formed using a variation of the film rehydration method 28,54 Tables 1-3). The final volume was 500 µL and the membrane concentration 4 mg/mL.…”

The energy–water nexus: Water use trends in sustainable energy and opportunities for materials research and development

“…Laboratory studies suggest that these materials have the potential to increase permeability by up to two or three orders of magnitude over those of conventional polymer materials ( Figure 4b ). [34][35][36] In both cases, the underlying idea is to incorporate the carbon nanotubes or protein into a polymer matrix to provide …”