Water at Membrane Interphases: The Hidden Motor of Life

Cells, as the unit of life, can be defined within the following three concepts: compartmentalization, crowding and complexity.

Compartmentalization

Cells consist of a specific environment separated from the aqueous surrounding under the premise that if all the cell components were extracted and expanded in the ocean no life would exist [3]. By this, it is meant that all cell components must condense in a particular state of aggregation. In this regard, two main quite extremely opposed views have been proposed: the membrane theory and the colloidal approach. The widely accepted membrane theory considers that cell components are enclosed by a selective membrane composed by lipids and proteins that maintain an asymmetry ionic distribution across the membrane giving place to an electric potential at expense of metabolic energy. Under this view, most of the biological and biophysical studies are designed under the paradigm of the mosaic fluid membrane model (MFMM) and their modifications [4]. Due to inconsistencies in the evaluation of energy to maintain the electric membrane potential, an opposed theory was formulated, which has had little impact in the scientific community except a revival in the last years. It considers that cells consist of a protein gel where water is embedded [5] in equilibrium with the environment. Contrary to the MFMM, no membrane structure is required to explain the energetics to maintain cell functions.

Crowding

The implicit message of MFMM is that in the cell interior proteins are dissolved in water that mostly conserves the properties of the free bulk [4,6]. However, this premise is actually under revision with the introduction of the concept of crowding [7]. In this view, cell interior is not a soup in which proteins and organelles are floating. Considering that the inner cell concentration of salt is 150mM K+ solution, the distance between ions is around 1.9 nm, that is, no more than three water molecules length. Thus, around 5-6 water layers adjacent to surfaces are influenced by the presence of the cell components (proteins and lipid membrane). Thus, the new feature merging in crowded systems is that water near cell components has properties different from bulk pure water. In cells due to specific arrangements defined as the interphase region. Water around molecular condensates are organized in cell cytoplasm either as membrane-bound or membrane- less organelles [8].

Complexity

Considering the two concepts previously defined, cell performance takes place on the base of the coordination and interconnection between different processes involving water in which, chemical, mechanical and electrical phenomena are coupled. In this context, the complexity of the crowded state implies synergism and cooperativity. In particular, the exchange of matter and energy makes the cell a complex system in regard that the reductionistic approach to search the overall cell behaviour in simple molecular components fails. Instead, complexity looks for a holistic explanation in which the whole system properties are not the sum of each of the components.

A main concept implicit in complexity is that cells are in permanent contact with the environment. Therefore, it is an open system maintained out of equilibrium, more precisely, in a steady state due to the exchange of mater and energy. This timeinvariant state is such that it is not in the minimum of free energy and therefore it has the propensity to react when chased with chemical, mechanical and electrical stimuli [9]. Taken together, the three main concepts of cells have been resumed by Lehninger [10] as: “The cell is an open, self-assembled, self-organized and self-regulated system that exchanges matter and energy with the surrounding environment under the principle of minimum energy cost, organizing its interior at the expense of the disorder of the environment.

This minireview aims to demonstrate or, at least, to put into discussion that the three components of cell definition described above can be linked considering the water interphase phenomena as a motor of cell performance. In this approach, water organized in restricted environments plays a key role through specific structural arrangements which derives in thermodynamic responses.

Compartmentalization is a consequence of self-aggregation; crowding emerges from self-organization and self-regulation is the consequence of the system complexity in thermodynamic terms. Thus, it is clearly observed that a complete definition of living cells requires thermodynamic and structural bases for a physically realistic view.

A new insight is achieved by introducing the relevant physical chemical properties merging from the interphases.

In words of Einstein [11], an air-water interface of a drop of water has its specific heat and entropy potential independent of the bulk [12].

The link between the three concepts mentioned above and, in addition, the convergence of both theories of cells, is achieved by redefining the role aqueous interphases [13].

Water is a unique structured liquid [14]. Due to the possibility that water molecules can form one, two, three or four H bonds as well as donor or as proton acceptor, water may form a multiplicity of structures, as shown by the seven different kinds of ice in the complete phase diagram [15]. The versatility in structures makes feasible that water may easily adapt to different types of surfaces (hydrophilic, hydrophobic, and in the case of cells lipids as well as proteins). These arrangements are essentially cooperative since the formation of one H bond enhances the formation of others and vice versa. Thus, as the perturbation in the interphase must be conserved, this leads to propagation [11].

The proton mobility along the H bond produces changes in polarity without molecular rotation and makes possible signal propagations [16].

These properties of water are implicit in the three concepts ascribed to the cell but not often included in the proposed models and mechanisms. The formation of closed compartments enclosing biomolecular condensates is a consequence of the entropicdriven hydrophobic interaction resulting in aggregates with a non-polar core and exposing a hydrophilic hydrated surface [17]. One example of this structure is the spontaneous stabilization of lipids in a bilayer, the main backbone in which the MFMM is based incorporating hydrophobic proteins to explain cell function [4]. However. the properties of the hydrated interphase resulting from the polarization of water in phosphate groups and carbonyls are not taken into account [18].

Moreover, water is also found in the hydrocarbon core due to the formation of pockets along the double bonds of the acyl chains. Both types of water, hydration water in the polar head groups and confined water in the acyl chains play a major role in the interaction and stabilization of peptides in the membrane and amino acids [18- 20]. The critical revision of the classical paradigms describing the membrane as a pure dielectric and a closed system has allowed to propose a new model of lipid membranes considering the role of membrane aqueous interphases [21,22].

This new description of biomembranes introduces new structural concepts and with thermodynamic consequences in terms of interphase properties and biological response. In this view, water molecules are tightly bound to phosphates and carbonyl groups and loose water is present in a second shell around those groups and the hydrocarbon chains [1,2]. Loose water can be exchanged by mechanical work (compression and expansion, creation of curvature, osmotic stress), electrical work by ionbinding and surface polarization and density changes (osmotic and hydric stress). Instead, displacement of the first hydration layer requires more drastic treatments and when this is achieved, cells are irreversibly damaged unless water is replaced by molecules mimicking the distribution of H bonds in water [23]. The complete functional description of a biomembrane fits into the approach of thermodynamic of irreversible processes [22], that is, processes that are not in thermodynamic equilibrium. The merit of this approach is that water is a component that can be exchanged with the media and thus, membrane is not a close inert material but it has the propensity to change with the surrounding conditions. In crowded systems, this exchange takes place in the region between surface of membranes and biomolecular condensates in a limit of 3-4 water layers [21].

The propensity of the cell to react to external stimuli is because it is not in the minimum of free energy. It is said that chemical equilibrium means the death of the cell. Thus, all the machinery is to maintain the cell in a controlled steady state to respond consequently. The response of cells takes place with a decrease in free energy (exergonic processes) that cell must recover to maintain its responsiveness at expense of reorganization and input of energy (endergonic processes) [24].

The biological antenna to trigger cell processes are the membrane surface by means of receptors, lipid configurations or water arrangements [25,26]. From there, signal must be immediately transmitted to cell machinery and this is also provided by water along the propagation of protons by the H bonds [27-29].

In this brief review the purpose is to emphasize that water properties are essential to biological functions and cannot be disregarded for a complete understanding of biological functions. Protein and membrane hydration carried out within a holistic view of cells as a complex functional unity [27] could open new strategies to understand physiological phenomena and pathological states as those caused by hydric stress, peroxidation, amyloid fibrils, lung surfactant functions among others [30-35].

A deeper knowledge of the membrane components in relation to water would help in the design of biomimetic systems employed in bio medicine and drug delivery systems as antimicrobial peptides and to the creation of artificial cells for biotechnological purposes [1].

This work was supported with funds from ANPCyT PICT- 2020-A- 01500. EAD and MAF are members of the research career of the National Research Council of Argentina (CONICET).

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