Role of interfacial water in biological function

Respiration rate of the yeast cells linearly decreases with water content upon dehydration and apparently stops at hydration level h 0.20 g/g [463] (Fig. 91). For lichens, two switching points in the hydration-induced metabolism were found [464]. Limited metabolism appears when water content is below 10% of the fully hydrated samples, and at hydrations above 20%, another class of enzymes becomes active. Seeds of plants may stay for years in dehydrated state but germinate promptly upon hydration. This makes the analysis of the evolution of physiological activities of seeds with increasing hydration possible. The rate of O2 consumption and the rate of CO2 evolution by dry seeds are very low, indicating an absence of mitochondrial metabolism. It 

At molecular level, the manifestations of the biological activity appear in specific biochemical reactions, conformational behavior, and dynamical properties of biomolecules. Experimental studies of various partially hydrated enzymatic proteins show that their activity accelerates rapidly at some critical hydration levels. Onset of the enzymatic activity of urease occurs at 0.15 g/g [469]. In the presence of chymotrypsin, the acylation reaction is undetectable at hydrations /i 0.12 g/g, but its rate grows sharply above this critical hydration level [470]. The rate of enzymatic activity of glucose-6-phosphate dehydrogenase, hexoki-nase, and fumarase becomes detectable and start to increase sharply at hK 0.20 g/g, whereas this critical hydration is about 0.15 g/g for phosphoglucose isomerase [471]. Enzymatic activity of lysozyme can be detected only when hydration level achieves h 0.20 g/g [472, 473] (see Fig. 92). 

Existence of the critical hydration level he for enzymatic activity may reflect the fact that hydration water can serve as a transport media for the substrates and/or for the products of the reactions only above he [471]. This possibility was explored by the experiments with gas-phase 

Bacteriorhodopsin is an intramembrane protein, which uses adsorbed light energy to transfer a proton through the membrane. The microscopic mechanism of the proton pumping is based on the set of isomerization processes initiated by the light adsorption. Upon dehydration, photoisomerization of bacteriorhodopsin reduces [480-484] and proton pumping stops below 60% relative humidity [483—486]. The above examples show 

B to A transition was also observed in concentrated solutions of some nonelectrolytes miscible in water [492,493]. 

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We have considered various manifestations of the importance of water in biological function. In most cases, there are clear indications on the crucial role of interfacial water in Bfe. Two mam aspects of the phase behavior of interfacial water can be distinguished a) condensation of a layer of hydration water at biosurfaces and b) effect of temperature and pressure on the state and properties of this hydration layer. These two aspects are considered in the Sections 8 and 9, respectively. 

Further studies of interfacial water are necessary to clarify its role in functioning of living systems. This is a long-standing problem, and many efforts have been already made to understand the physical mechanisms behind the crucial role of water in biological function. Some aspects of this problem were clarified, but the self-consistent picture is far from being completed. There is an understanding that hydration water forms an environment for biomolecules, which enables their conformational dynamics and serves as a media for the mass... 

The proportion of a-helix in native proteins is variable and sometimes not very great. It is a mistake to over emphasize its role in interfacial structures, but where it is present its radial distribution of side chains means that its orientation in the interface will be governed by its over-all hydrophobicity. The presence of hydrophobic groups directed into the water is then possible as well as others contributing to cohesion between adjacent molecules as in the monolayers considered here. Those directed into the water may function as sites for the binding of other molecules in the aqueous phase. This is also a possibility for the p conformation but not for extended chain conformations where under pressure the hydrophobic side chains are directed away from the surface and the hydrophilic ones into the water. While this latter model has been accepted by surface chemists 37), the conformation appears unlikely both from a biological and a stereochemical standpoint. Indeed except where there is a regular alternation of hydrophobic and hydrophilic side chains, the conformation is probably not one acceptable within the usual criteria for polypeptide structures (5). 

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