Experiments by biochemists at the University of California, Davis show for the first time that a law of physics, the ergodic theorem, can be demonstrated by a collection of individual protein molecules -- specifically, a protein that unwinds DNA. The work will be published online by the journal Nature on July 14.
Using technology invented at UC Davis for watching single enzymes at work, Bian Liu, a graduate student in the Biophysics Graduate Group and professor Steve Kowalczykowski, Department of Microbiology and Molecular Genetics and UC Davis Cancer Center, found that when they paused and restarted a single molecule of the DNA-unwinding enzyme RecBCD, it could restart at any speed achieved by the whole population of enzymes.
"It's pretty impressive," said Daniel Cox, a physics professor at UC Davis who was not involved in the work. "The laws of physics should apply to biological systems, and it turns out they do."
The results also have implications for understanding how proteins fold into their correct shape, for exploring interactions between drugs and their targets, and for engineering enzymes for new functions.
The ergodic theorem, proposed by mathematician George Birkhoff in 1931, holds that if you follow an individual particle over an infinite amount of time, it will go through all the states that are seen in an infinite population at an instant in time. It's a fundamental assumption in statistical mechanics -- but difficult to prove in an experiment.
Liu and Kowalczykowski weren't attempting to test laws of physics when they began the work. They wanted to know why RecBCD, an enzyme that unwinds DNA in E. coli bacteria, showed so much variability in its rate of action.
RecBCD attaches to and moves along DNA, unwinding the double helix into two separate strands. It has two jobs in the cell: to allow damaged DNA to be repaired, and to break down invading "foreign" DNA from viruses.
|Contact: Andy Fell|
University of California - Davis