Relevance of water in biological membranes

Abstract

Water stabilizes the structure of biological macromolecules and supramolecular lipid arrangements, controls biochemical activities, and regulates interfacial/intermolecular interactions. In consequence, due to their unique properties, ?water? is an essential commodity for all functions in biology and, therefore, to act as a link between membrane and metabolic phenomena. However, current models of biomembranes do not include water as a structural, dynamic and thermodynamic determinant component. In a recent proposal water is incorporated in the membrane structure as a dynamic component and its properties allow to explain the responsiveness of lipid assembles on thermodynamic grounds. In this view, chemical and mechanical stresses acting on the membrane produces water exchange with the surrounding media. Thus, in thermodynamic terms the membrane is an open system. In order to analyze the membrane under this approach, it is considered as composed by a bidimensional solution of hydrated polar head groups immersed in water with solvent properties different than the bulk. This interphase region extends partially into the hydrocarbon region and expands along two?three layers from the head groups. This last possibility is reasonable when considering the cell as a crowded system in which interphases of macromolecules and supramolecular lipid structures can overlap. This would account for propagation phenomena of events between metabolism and cell membrane. The main water property that emerges in this coupling is the cooperativity of hydrogen bonds and proton displacement along them. Water is organized at the interphase region around polar head groups and acyl chains. However, not all of water content in lipids is active in terms of contributing to fundamental functions at different temporal and spatial scales, such as response to different biologically active compounds (bioeffectors) in the environment. The interrelation between the hydration properties at molecular level and the thermodynamic response is a particularly important point to discuss.