Shaped a bit like a tennis racket over a basket, this tiny arm of the protein moves, through thermal fluctuations, to open or close the binding site near the myoglobin's iron-filled center. "As the atoms move in one direction it becomes easier to bind oxygen," says DeWitt, "and as they move in the reverse direction it becomes less easy."
And how this distal histidine moves should vary between the whale and the cheetah. "I would imagine," says Kelvin Chu, associate professor of physics and DeWitt's co-author, "that there has been evolutionary pressure on every species to adapt this motion in the myoglobin for their particular oxygen-binding needs."
"That's a testable hypothesis," Chu says. "What we would expect to see across species is that the tennis rackets are in different places or move different amounts."
DeWitt and Chu's work extends far beyond myoglobin. The two physicists see broad application of their new method in creating custom-crafted proteins.
"Once you know what these motions are and what the important atoms are," says Chu, "you can make mutants of proteins that have different binding attributes." And these different attributes have promise in developing new biotechnologies "ranging from blood substitutes to organic solar cells," he says.
Proteins are a cell's heavy laborers: hauling water, taking out the trash, carrying in the groceries -- and trillions of other tasks that make life. But how the shape of a protein determines its function remains one of the most vexing and important questions in the physics of biology.
Proteins are not the static, Lego-like objects you might see in an x-ray photograph in a biochemistry textbook. Instead, made from long chains of amino acids scrunched into various blobs and globs, a protein is always jumping between slightly different structural arrangements due to thermal motion of its atoms. Even a modest-sized protein like myoglobin has more possible
|Contact: Joshua Brown|
University of Vermont