Towards biomimetic water treatment - from a molecular dynamics perspective

Abstract

Water is the media of life, and access to renewable freshwater resources is a vital necessity for humans. Sadly, a substantial amount of the world's population lives in water stressed areas where clean water is unavailable. This is caused by droughts or floodings combined with poor sanitation. It is estimated that 50 % of the world's population will be living in water stressed areas by 2025. Wastewater from municipalities and industries contains significant amounts of toxic compounds, but also valuable and scarce resources. Water treatment is thus essential for securing the future clean water supply, by removing pollutants and reclaiming valuable resources. Through evolution, nature has optimized biological processes in which proteins have been specialized to perform specific tasks. Such processes include the transport of solvent and solutes across biological membranes, which occurs with extreme selectivity due to atomic scale interactions. The biomimetics engineering field is inspired by the designs of nature and aims to implement these solutions in technologies. This includes protein based water treatment by biomimetic membranes and bioengineering. Understanding the molecular interactions is therefore imperative for the biomimetic development for environmental applications. This can be achieved by employing molecular modelling to study, optimize and utilize proteins for wastewater treatment. In this thesis, two classes of proteins have been investigated by means of molecular modelling and molecular dynamics simulations. The first part studies a newly crystallized human membrane protein, called aquaporin 10, which transports water and glycerol in human tissues. The study focuses on elucidating the complex transport through this protein, and how to obtain quantitative information. A novel simulation protocol is employed and the results suggests how the protein changes substrate selectivity by a change in pH. The results indicate that glycerol flux nearly doubles at low pH, while the water transport rate is halved. The difference in transport is attributed to the change in hydrophobic environment by a novel gating mechanism of the protein. The employed protocol can readily be transferred to other membrane channels and tested for embedment in biomimetic membranes. The second part of the thesis focuses on capturing specific compounds from wastewater solutions by employing specialized globular proteins, called substrate binding proteins. This class of proteins bind and release specific molecules from a given solution. Phosphate is in particular a scarce resource, and therefore focus is put on how phosphate binding proteins may be employed for recovery of this anion. A biomolecular concept is designed, and it is illustrated how the covalent attachment of a chromophore may give conformational control of the protein by illumination with light. It is hypothesized that by controlling the conformational equilibrium, the binding and release of phosphate can be controlled. This renders the concept useful for phosphate recovery in wastewater treatment. Performing molecular dynamics simulations on charged particles such as phosphate has certain limitations and issues. Due to the conformational similarities with phosphate binding protein, a principal study is performed on the maltodextrin binding protein instead. The binding and release mechanism of the maltodextrin binding protein is studied for two ligands, and a difference of the binding modes is found. Overall the study illustrates an approach for studying the class of substrate binding proteins, which can be used to evaluate the induced conformational photo-control.