A novel method of AquaporinZ incorporation via binary-lipid Langmuir monolayers

Optimization of detergents in solubilization and reconstitution of Aquaporin Z: A structural approach

BACKGROUND

The exceptional capacities of aquaporins in terms of water permeation and selectivity have made them an interesting system for membrane applications. Despite the multiple attempts for immobilizing the aquaporins over a porous substrate, there is a lack of studies related to the purification and reconstitution steps, principally associated with the use of detergents in solubilization and destabilization steps. This study analyzed the effect of detergents in Aquaporin Z solubilization, considering the purity and structural homogeneity of the protein.

METHODS

The extraction process was optimized by the addition of detergent at the sonication step, which enabled the omission of the ultracentrifugation and resuspension steps. Two detergents, Triton X-100, and octyl-glucoside were also evaluated. Destabilization mediated by detergents was used as reconstitution method. Saturation and solubilization points were defined by detergent concentration and both, liposomes and proteoliposomes, were analyzed by size distribution and permeability assays. Detergent removal with Bio-beads was also analyzed.

RESULTS

Octyl glucoside ensures structural stability and homogeneity of Aquaporin Z. However, high concentrations of detergents induce the presence of defects in proteoliposomes. While saturated liposomes create homogeneous and functional structures, solubilized liposomes get affected by a reassembly process, creating vesicle defects with anomalous permeability profiles.

CONCLUSIONS

Detergent concentration affects the structural conformation of proteoliposomes in the reconstitution process.

GENERAL SIGNIFICANCE

Since the destabilization process is dependent on vesicle, detergent, and buffer composition, optimization of this process should be mandatory for further studies. All these considerations will allow achieving the potential of Aquaporins and any other integral membrane protein in their applications for industrial purposes.

Increasing Salt Rejection of Polybenzimidazole Nanofiltration Membranes via the Addition of Immobilized and Aligned Aquaporins

Aquaporins are water channel proteins in cell membrane, highly specific for water molecules while restricting the passage of contaminants and small molecules, such as urea and boric acid. Cysteine functional groups were installed on aquaporin Z for covalent attachment to the polymer membrane matrix so that the proteins could be immobilized to the membranes and aligned in the direction of the flow. Depth profiling using x-ray photoelectron spectrometer (XPS) analysis showed the presence of functional groups corresponding to aquaporin Z modified with cysteine (Aqp-SH). Aqp-SH modified membranes showed a higher salt rejection as compared to unmodified membranes. For 2 M NaCl and CaCl₂ solutions, the rejection obtained from Aqp-SH membranes was 49.3 ± 7.5% and 59.1 ± 5.1%. On the other hand, the rejections obtained for 2 M NaCl and CaCl₂ solutions from unmodified membranes were 0.8 ± 0.4% and 1.3 ± 0.2% respectively. Furthermore, Aqp-SH membranes did not show a significant decrease in salt rejection with increasing feed concentrations, as was observed with other membranes. Through simulation studies, it was determined that there was approximately 24% capping of membrane pores by dispersed aquaporins.

Hollow fiber nanofiltration membranes with integrated aquaporin Z

AqpZ reconstituted proteoliposomes integrated into polyamide matrix using different ultrafiltration support membranes successfully with a performance increase of 12 times.

Bioinspired materials for water supply and management: water collection, water purification and separation of water from oil

Access to a safe supply of water is a human right. However, with growing populations, global warming and contamination due to human activity, it is one that is increasingly under threat. It is hoped that nature can inspire the creation of materials to aid in the supply and management of water, from water collection and purification to water source clean-up and rehabilitation from oil contamination. Many species thrive in even the driest places, with some surviving on water harvested from fog. By studying these species, new materials can be developed to provide a source of fresh water from fog for communities across the globe. The vast majority of water on the Earth is in the oceans. However, current desalination processes are energy-intensive. Systems in our own bodies have evolved to transport water efficiently while blocking other molecules and ions. Inspiration can be taken from such to improve the efficiency of desalination and help purify water containing other contaminants. Finally, oil contamination of water from spills or the fracking technique can be a devastating environmental disaster. By studying how natural surfaces interact with liquids, new techniques can be developed to clean up oil spills and further protect our most precious resource.This article is part of the themed issue 'Bioinspired hierarchically structured surfaces for green science'.

Can Stabilization and Inhibition of Aquaporins Contribute to Future Development of Biomimetic Membranes?

In recent years, the use of biomimetic membranes that incorporate membrane proteins, i.e., biomimetic-hybrid membranes, has increased almost exponentially. Key membrane proteins in these systems have been aquaporins, which selectively permeabilize cellular membranes to water. Aquaporins may be incorporated into synthetic lipid bilayers or to more stable structures made of block copolymers or solid-state nanopores. However, translocation of aquaporins to these alien environments has adverse consequences in terms of performance and stability. Aquaporins incorporated in biomimetic membranes for use in water purification and desalination should also withstand the harsh environment that may prevail in these conditions, such as high pressure, and presence of salt or other chemicals. In this respect, modified aquaporins that can be adapted to these new environments should be developed. Another challenge is that biomimetic membranes that incorporate high densities of aquaporin should be defect-free, and this can only be efficiently ascertained with the availability of completely inactive mutants that behave otherwise like the wild type aquaporin, or with effective non-toxic water channel inhibitors that are so far inexistent. In this review, we describe approaches that can potentially be used to overcome these challenges.

Aquaporin-Based Biomimetic Polymeric Membranes: Approaches and Challenges

In recent years, aquaporin biomimetic membranes (ABMs) for water separation have gained considerable interest. Although the first ABMs are commercially available, there are still many challenges associated with further ABM development. Here, we discuss the interplay of the main components of ABMs: aquaporin proteins (AQPs), block copolymers for AQP reconstitution, and polymer-based supporting structures. First, we briefly cover challenges and review recent developments in understanding the interplay between AQP and block copolymers. Second, we review some experimental characterization methods for investigating AQP incorporation including freeze-fracture transmission electron microscopy, fluorescence correlation spectroscopy, stopped-flow light scattering, and small-angle X-ray scattering. Third, we focus on recent efforts in embedding reconstituted AQPs in membrane designs that are based on conventional thin film interfacial polymerization techniques. Finally, we describe some new developments in interfacial polymerization using polyhedral oligomeric silsesquioxane cages for increasing the physical and chemical durability of thin film composite membranes.

Multiscale simulation of surfactant–aquaporin complex formation and water permeability

Molecular dynamics simulation reveals distinctive roles of electrostatic and hydrophobic interactions in surfactant (SDS)–protein (AqpZ) complex formation and functionality.

Stabilization and immobilization of aquaporin reconstituted lipid vesicles for water purification

Aquaporins are water channel proteins in biological membranes that have extraordinary water permeability and selectivity. In this work, we have demonstrated that one of their family members, AquaporinZ (AqpZ), can be possibly applied in a pressure-driven water purification process. A nanofiltration membrane was designed and fabricated by immobilization of AqpZ-reconstituted liposomes on a polydopamine (PDA) coated microporous membrane. Amine-functionalized proteoliposomes were first deposited via gentle vacuum suction and subsequently conjugated on the PDA layer via an amine-catechol adduct formation. Due to the existence of a polymer network within the lipid bilayers, the membrane could sustain hydraulic pressure of 5 bar as well as the strong surface agitation in nanofiltration tests, indicating a relatively stable membrane structure. In comparison with membrane without AqpZ incorporation, the membrane with AqpZ-to-lipid weight ratio of 1:100 increased the water flux by 65% with enhanced NaCl and MgCl(2) rejections of 66.2% and 88.1%, respectively. With AqpZ incorporation, the vesicle immobilized membrane exhibits a promising strategy for high productivity water purification.