Proteins play a fundamental role in living cells, acting as catalysts for all chemical reactions. Proteins also act as a kind of "nervous system" for a cell, transmitting signals from the outside environment. They assist in transporting nutrients into the cell, and also help convert that food into energy.
The function of a particular protein is determined by its shape. A protein consists of a rigid backbone of carbon, oxygen and nitrogen atoms. Loosely attached to this backbone are chains of amino acids (the link between each amino acid is called a peptide bond). This chainlike molecule, which may contain from 50 to several hundred amino acids, is called a polypeptide. Some proteins only consist of one polypeptide chain, while others consist of several chains held together by weak molecular bonds. These chains of amino acids determine the shape of the protein.
"Starting with information about the amino acid sequence, it should be possible, using computational methods, to discover the structure of a protein from the sequence alone," says Andrew Pohorille, a scientist at the Ames Research Center.
But scientists have found it very difficult to determine the structure of a protein even when they know the sequence of its amino acids, because each amino acid can have five possible different orientations. Because proteins contain at least 50 amino acids, even the shortest proteins can have amino acids arranged in thousands of different possible combinations.
Pohorille says there are a variety of computational methods that try to address the protein-folding problem. The most direct method tries to determine the series of small steps that led to the protein being folded into a certain shape.
This method is extremely time-consuming, however, because there are approximately a thousand trillion steps. A computer capable of calculating one million of these steps per second would have to work round-the-clock for 32 years to complete this task. The largest computer simulations performed so far have extended to only a trillion steps.
"We hope that by a combination of new, powerful computers, efficient parallel programming and novel algorithms, we will be able to achieve our goal," says Pohorille. "The techniques of 3D visualization and structure manipulation that are being developed by Chris Henze and his group should greatly aid our efforts."
"What used to take a year to calculate on a single processor might be done in less than a day on a 512-processor machine," says Henze, another scientist at Ames who is working on the simulations of protein formation with Pohorille. "Nevertheless, with current supercomputer power it'll take months or years of calculations to simulate how even a small protein molecule folds into a certain shape."
"One of the goals of astrobiology is to understand the origin of life on Earth and elsewhere in the Universe," says Pohorille. "Undoubtedly, proteins played an important role in this process. So far, attempts to construct simple, but still functional proteins have been largely unsuccessful. We want to design such proteins."
Pohorille and his team are collaborating with the Harvard Medical School on a new experimental technique to create proteins. Pohorille says the Harvard researchers have developed small proteins that perform certain desired functions. The research results will be published in an upcoming issue of Nature.
"However, the structure of this new protein remains unknown," says Pohorille. "We want to be able to fold this protein and other, similar proteins that will be evolved soon."
Pohorille is also working on a project that involves modeling a small protein. This protein is capable of inserting itself into the cell membrane and forming channels that transport protons into the cell. Such proton transport is an essential step in converting energy to drive the cell's chemistry.
"We already gained some understanding how this protein works - it was a subject of a short article in New Scientist last year," says Pohorille. "However, to make it useful for protocells, the protein needs to be somewhat redesigned. The new computer should make it possible."