The Dance of Water Molecules around Proteins

Abstract

A combination of experiments, simulations, and modeling has revealed the anomalous diffusion of water molecules along the surfaces of proteins. by Ralf Metzler * P roteins are indispensable molecules in living cells, charged with highly specific tasks ranging from in-formation transmission to metabolism. To perform their duties, proteins need to be in an aqueous so-lution, which allows them to fold, wrap, and wind. These conformational changes, which are often related to a pro-tein's functioning, are influenced by the water molecules that surround the solvated protein. But the influence goes both ways since water molecules can be restricted in their movement as they navigate around the complex surface of a protein. A new study of water dynamics around proteins has combined experiments, simulations, and modeling to paint a clear picture of the hopping behavior of surface water molecules [1]. These results might help researchers under-stand the aging of biopolymer-associated water molecules and the role molecular collisions play in rejuvenating them. Gone are the days when x-ray images of crystallized pro-teins stirred much excitement. Instead, ever-more-refined experimental and computational approaches have given us direct glimpses of proteins and other biopolymers in their natural habitat—the wet, crowded environments of living biological cells and membranes. This " wildlife mi-croscopy " has revealed the importance of conformational changes in proteins. For instance, DNA-binding proteins structurally switch between unbound, search, and recogni-tion modes. Similarly, enzymes—the workhorses of cellular metabolism—change shape upon binding and release of a substrate. Given this constant reshaping, one important question is, how does a single protein dynamically couple to its immediate environment, and in particular, to water molecules and ions forming a layer on its surface (Fig. 1)? Experiments that track the dynamics of this so-called hy-dration water would provide an indirect picture of this coupling. The study by Liang Hong from Shanghai Jiao Tong Uni-versity, China, and co-workers is a step in this direction