Science
Researchers have proven that nature can adapt to incorporate silicon into carbon-based molecules, the building blocks of life. Scientists "bred" a bacterial protein to have the ability to make the man-made bonds, a finding that has applications in several industries and could extend our search for extraterrestrial life.
New research is the first to show that living organisms can be made to create silicon-carbon bonds—something only chemists had done before. The scientists involved "bred" a bacterial protein to have the ability to make the man-made bonds, a finding that has applications in several industries.
The top image is an artist rendering of organosilicon-based life. Organosilicon compounds contain carbon-silicon bonds. The research from the laboratory of Frances Arnold shows, for the first time, that bacteria can create organosilicon compounds. This does not prove that silicon- or organosilicon-based life is possible, but shows that life could be persuaded to incorporate silicon into its basic components.
"This study shows how quickly nature can adapt to new challenges."
Molecules with silicon-carbon, or organosilicon, compounds are found in pharmaceuticals as well as in many other products, including agricultural chemicals, paints, semiconductors, and computer and TV screens. These products are currently made synthetically, since the silicon-carbon bonds are not found in nature.Carbon and silicon are chemically very similar. They both can form bonds to four atoms simultaneously, making them well suited to form the long chains of molecules found in life, such as proteins and DNA.
The new research, which recently won Caltech's Dow Sustainability Innovation Student Challenge Award (SISCA) grand prize, demonstrates that biology can instead be used to manufacture these bonds in ways that are more environmentally friendly and potentially much less expensive.
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| Image Source - Caltech |
"We decided to get nature to do what only chemists could do—only better," says Frances Arnold, Caltech's Dick and Barbara Dickinson Professor of Chemical Engineering, Bioengineering and Biochemistry, and principal investigator of the new research, published in the journal Science.
The study is also the first to show that nature can adapt to incorporate silicon into carbon-based molecules, the building blocks of life.
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"No living organism is known to put silicon-carbon bonds together, even though silicon is so abundant, all around us, in rocks and all over the beach," says Jennifer Kan, a postdoctoral scholar in Arnold's lab and lead author of the new study. Silicon is the second most abundant element in Earth's crust.The researchers used a method called directed evolution, pioneered by Arnold in the early 1990s, in which new and better enzymes are created in labs by artificial selection, similar to the way that breeders modify crops, livestock or domestic pets. Directed evolution has been used for many years to make enzymes for household products, like detergents; and for "green" sustainable routes to making pharmaceuticals, agricultural chemicals, and fuels.
Enzymes are a class of proteins that catalyze, or facilitate, chemical reactions. The directed evolution process begins with an enzyme that scientists want to enhance. The DNA coding for the enzyme is mutated in more-or-less random ways, and the resulting enzymes are tested for a desired trait. The top-performing enzyme is then mutated again, and the process is repeated until an enzyme that performs much better than the original is created.
The new study's, goal was not just to improve an enzyme's biological function but to actually persuade it to do something that it had not done before. The researchers' first step was to find a suitable candidate, an enzyme showing potential for making the silicon-carbon bonds.
"It's like breeding a racehorse," says Arnold, who is also the director of the Donna and Benjamin M. Rosen Bioengineering Center at Caltech. "A good breeder recognizes the inherent ability of a horse to become a racer and has to bring that out in successive generations. We just do it with proteins."
The ideal candidate turned out to be a protein from a bacterium that grows in hot springs in Iceland. That protein, called cytochrome c, normally shuttles electrons to other proteins, but the researchers found that it also happens to act like an enzyme to create silicon-carbon bonds at low levels. The scientists then mutated the DNA coding for that protein within a region that specifies an iron-containing portion of the protein thought to be responsible for its silicon-carbon bond-forming activity. Next, they tested these mutant enzymes for their ability to make organosilicon compounds better than the original.
"This iron-based, genetically encoded catalyst is nontoxic, cheaper, and easier to modify compared to other catalysts used in chemical synthesis," says Kan. "The new reaction can also be done at room temperature and in water."
As to the question of whether life can evolve to use silicon on its own, Arnold says that is up to nature. "This study shows how quickly nature can adapt to new challenges," she says. "The DNA-encoded catalytic machinery of the cell can rapidly learn to promote new chemical reactions when we provide new reagents and the appropriate incentive in the form of artificial selection. Nature could have done this herself if she cared to."
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