A team of Howard Hughes Medical Institute researchers based at MGH and Harvard Medical School created a completely artificial enzyme in vitro. Their methodology? The evolution!
HHMI investigator Jack W. Szostak and Burckhard Seelig, a postdoctoral associate in his Massachusetts General Hospital and Harvard Medical School laboratory, show in a paper published in the August 16, 2007, issue of the journal Nature the steps they took to create the artificial enzyme, an RNA ligase that catalyzes a reaction joining two types of RNA chains…
So far, most scientists experimenting with artificial enzymes have started with naturally occurring enzymes, and used genetic mutation and screening to evolve variants that work under different conditions or catalyze a slightly different reaction…
Szostak’s approach relies instead on evolution. The technique enabled the researchers to generate a new RNA ligase without any pre-existing model of how it would work. According to Szostak, “There is no known biological enzyme that carries out this reaction.”
To create one, the researchers assembled a library of 4 trillion small protein molecules — each with slight variations on an initial protein sequence — then subjected those molecules to evolutionary selection in the laboratory. “Here,” Szostak says, “the hard work is in designing a good starting library, and an effective selection process. Since we do not impose a bias on how the enzyme does its job, whatever mechanism is easiest to evolve is what will emerge.”
The enzyme that emerged from the group’s experiments is what Szostak characterizes as “small and not very stable, and not very active compared to most biological enzymes.” Nevertheless, Szostak’s group is optimistic about their ability to select for versions of the enzyme that are more stable and more active.
Szostak’s and Seelig’s successful creation of an enzyme in the laboratory was the culmination of a long series of developments, beginning 10 years ago with the development of messenger RNA-display, which binds proteins to the messenger RNA molecules that encode them. Szostak could then select proteins with certain characteristics, such as binding to a specific target molecule, and produce more of that protein. Six years ago Szostak’s team described the first isolation of a completely artificially evolved non-biological protein, an ATP-binding protein. Three years ago his group published the first of several papers on the optimization of that new protein. “We have been working on the enzyme evolution project for the past five years,” says Szostak, “and it has been a long hard struggle.”