Water Is 'Designer Fluid' That Helps Proteins Change Shape

"Water, as we know it, does not exist within our bodies," said Martin Gruebele, a William H. and Janet Lycan Professor of Chemistry at the University of Illinois. "Water in our bodies has different physical properties from ordinary bulk water, because of the presence of proteins and other biomolecules. Proteins change the properties of water to perform particular tasks in different parts of our cells."

Consisting of two hydrogen atoms and one oxygen atom, water molecules are by far the body's largest component, constituting about 75 percent of body volume. When bound to proteins, water molecules participate in a carefully choreographed ballet that permits the proteins to fold into their functional, native states. This delicate dance is essential to life.

"While it is well known that water plays an important role in the folding process, we usually only look at the motion of the protein," said Gruebele, who also is the director of the U. of I.'s Center for Biophysics and Computational Biology, and a researcher at the Beckman Institute. "This is the first time we've been able to look at the motion of water molecules during the folding process."

Using a technique called terahertz absorption spectroscopy, Gruebele and his collaborator Martina Havenith at the Ruhr-University Bochum studied the motions of a protein on a picosecond time scale (a picosecond is 1 trillionth of a second).

The technique, which uses ultrashort laser pulses, also allowed the researchers to study the motions of nearby water molecules as the protein folded into its native state.

The researchers present their findings in a paper published July 23 in the online version of the chemistry journal Angewandte Chemie.

Terahertz spectroscopy provides a window on protein-water rearrangements during the folding process, such as breaking protein-water-hydrogen bonds and replacing them with protein-protein-hydrogen bonds, Gruebele said. The remaking of hydrogen bonds helps organize the structure of a protein.

In tests on ubiquitin, a common protein in cells, the researchers found that water molecules bound to the protein changed to a native-type arrangement much faster than the protein. The water motion helped establish the correct configuration, making it much easier for the protein to fold.

"Water can be viewed as a 'designer fluid' in living cells," Gruebele said. "Our experiments showed that the volume of active water was about the same size as that of the protein."

The diameter of a single water molecule is about 3 angstroms (an angstrom is about one hundred-millionth of a centimeter), while that of a typical protein is about 30 angstroms. Although the average protein has only 10 times the diameter of a water molecule, it has 1,000 times the volume. Larger proteins can have hundreds of thousands times the volume. A single protein can therefore affect, and be influenced by, thousands of water molecules.

"We previously thought proteins would affect only those water molecules directly stuck to them," Gruebele said. "Now we know proteins will affect a volume of water comparable to their own. That's pretty amazing."

With Gruebele and Havenith, co-authors of the paper are graduate student Seung Joong Kim at the U. of I., and graduate student Benjamin Born at the Ruhr-University Bochum.

Funding was provided by the Human Frontier Science Program and the National Science Foundation.