NEWS

By DAVID WILLIAMSON
UNC News Services

CHAPEL HILL -- Contrary to what scientists have thought, there is no correlation between a dissolved substance’s impact on water’s structure and that solute’s effect on protein stability, University of North Carolina at Chapel Hill chemists have discovered.

"These are important, fundamental findings because they increase our understanding of how nature designed proteins and how to manipulate their stability," said Dr. Gary J. Pielak, professor of chemistry at UNC. "One of our honors undergraduate students, Joseph Batchelor, did most of the work and did it very well."

Among possible benefits of altering protein stability are more effective or stable pharmaceuticals and more useful compounds for an untold number of industrial and domestic purposes, Pielak said.

For more than a century, researchers have been interested in determining how solutes such as urea, sugars and guanidinium salts and trimethylamine N-oxide affect the stability, solubility and solvation of globular proteins, he said. A key recurring hypothesis has held that solutes affect protein stability indirectly by making or breaking water structure.

"Our results indicate that efforts to explain solute effects should focus on other hypotheses," Pielak said.

A report on the experiments appears this month in the Journal of the American Chemical Society. Batchelor, working under Pielak’s supervision, conducted the experiments and wrote the paper.

"In liquid water, the molecules are arranged in two main ways, an ice-like, more ordered, less dense arrangement and a less ordered, denser arrangement," Batchelor said. "I used a new technique -- pressure perturbation calorimetry -- to measure whether water structure is made or broken by 17 solutes commonly employed to affect protein stability."

UNC has one of the few pressure perturbation calorimetry instruments in the world, he said, and he was lucky to have been given access to it in the university’s Macromolecular Interaction Facility.

"A protein's stability, basically, is how much heat it takes to unfold a folded protein," Batchelor said. "The more heat that is required, the stabler the protein."

Certain salts will increase or decrease a protein's stability. After measuring the effect on water structure of solutes commonly used to stabilize or destabilize proteins, the student looked for a correlation between each solute's effect on water structure and its effect on protein stability.

"Contrary to what we expected, we found no correlation between the two effects," he said.

"Water is very strange stuff," said Dr. George Rose, professor of biophysics and biology at Johns Hopkins University. "A being from another solar system might think of it the way we think of silly putty. Although a liquid at room temperature, water is nevertheless highly structured. We don't understand water. We don't understand proteins either."

As an example, Rose said that common ions dissolved in water will either stabilize or destabilize protein molecules and to varying degrees. Given that it is a general phenomenon and that water is so highly structured, a favorite idea has it that co-solvents affect proteins indirectly by decreasing or increasing the structure of water.

"Does this attractive idea account for the observed trends?" Rose said. "No one knows, because it is hard to disentangle the structure-making or breaking effect from other effects of adding co-solvents. Until now. Batchelor, Pielak and their colleagues have managed to do exactly that -- disentangle the data.

"All of this might seem like a dry -- no pun intended -- academic exercise, but just the opposite is true," he said. "Protein molecules are responsible for all life, and, under suitable solvent conditions, they assemble themselves spontaneously, reliably and autonomously into exquisitely fashioned, functionally relevant three-dimensional shapes."

How do proteins accomplish that vital feat?

"This is the ‘protein folding’ problem, one of the great, unsolved scientific problems of all time," he said. "The one thing we do know is that correct folding depends critically upon the interaction between proteins and water.

"In protein folding, as in all human cognition, what we see depends upon the perspective from which we look. This paper by Batchelor, Pielak and colleagues is an important milestone because it eliminates a large class of incorrect models and instead points us in the right direction."

The National Science Foundation and the Smallwood Foundation supported the study, said Pielak, who is affiliated with UNC’s College of Arts and Sciences and Lineberger Comprehensive Cancer Center at the UNC School of Medicine.

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Note: The authors can be reached at (919) 966-3671 or gary_pielak@unc.edu.  The paper is at http://www.unc.edu/~jdbatche/older%20stuff/paper/Batchelor%20et%20a1_3.doc

News Services Contact: David Williamson or Mike McFarland, (919) 962-2091.