The Organization and Function of Water in Protein Crystals

Abstract

That living tissue contains around 70% water is by now a well-worn cliché. To what extent and in what manner water is necessary to the operation of a biological system is, however, still uncertain. We do know that at least part of the water is not an inert solvent; it is an essential element in many biological processes. ^{(200,307,330,527,849)} The folding, structural stability, and dynamics of globular proteins are thought to be extensively controlled by solvent interactions, stress being generally placed on the poorly understood so-called “hydrophobic” or “apolar” interaction.* Similar driving forces are invoked to explain the energetics of enzyme–substrate binding, the binding of a hormone to its receptor, and protein–protein interactions in general. The structural integrity of membranes—the major component being amphiphilic lipids—depends upon solvent interactions. Within a cell, diffusion proceeds in a largely aqueous cytoplasm, the state of which may be perturbed by the presence of ions, large molecules, or other interfaces. The solvent close to a protein surface—the so-called “hydration shell”—has properties that differ significantly from the bulk, though how far this perturbation extends from the surface is still a matter of debate. In particular, a lowered water mobility would allow more rapid proton transfer, a potentially significant effect for enzyme activity. Recent studies on the photocycle of bacteriorhodopsin in purple membrane stress the importance of the immediate hydration state in the operation of the system’s light-driven proton pump.⁽⁵¹⁰⁾ Similar effects may be of general importance to energy-transducing membranes.⁽⁵¹⁰⁾ The state of the solvent may be significant in electron-transfer interactions.⁽⁷⁴³⁾ The relative insensitivity to temperature of light-induced oxidation of cytochrome in chromatium suggests quantum tunneling of electrons in photosynthesis^{(241,242,610)}; the ease of any tunneling outside the protein will depend strongly on the state of the solvent region. The rate-limiting step of certain enzyme catalyses is the diffusion of a small molecule to the enzyme, e.g., in triose phosphate isomerase⁽⁵⁰²⁾; such diffusion rates will be dependent on the state of the solvent medium. The water molecule has been invoked as an active molecule in several enzyme catalyses.^{(620,726,900)}