Chemists found how cyanide may have allowed chemical processes that metabolized carbon dioxide in the absence of the sophisticated proteins that living creatures employ today.
The colorless, lethal gas cyanide is increasingly used as a quick-acting poison and a chemical weapon. However, four billion years ago, it might have been a sign of life. Scripps Research chemists have shown for the first time how cyanide might have facilitated the creation of carbon-based molecules from carbon dioxide in some of the first metabolic processes. This finding not only helps scientists better understand the development of life on Earth, but it also provides insight into the chemistry of life on other worlds.
“We base our search for indications of life, whether on early Earth or on distant worlds, on the biochemistry we know exists in life today. The fact that cyanide may drive these same metabolic events highlights how life can be extremely different “Ramanarayanan Krishnamurthy, PhD, an associate professor of chemistry at Scripps Research and main author of the new research published in the journal Nature Chemistry on February 3, 2022, says.
The reverse tricarboxylic acid cycle (r-TCA cycle) is a sequence of chemical events used by certain bacteria on Earth today to convert carbon dioxide and water into chemical molecules that are required for life. Many scientists believe that the r-TCA cycle happened on the early Earth’s surface to produce chemicals required for life. The main issue is that today’s r-TCA cycle depends on a sophisticated collection of proteins that would not have existed before life developed. Researchers have discovered that some metals may have powered the same processes without today’s proteins in the murky primordial soup of four billion years ago, but only under very acidic and hot circumstances not thought to have existed on early Earth.
Krishnamurthy and his colleagues wondered whether the same reactions might be triggered by a different chemical under more mild settings. They knew cyanide was prevalent in the early Earth’s atmosphere, so they concocted a series of scenarios in which cyanide may have been employed to build organic molecules from carbon dioxide. The responses were then tested in a test tube. It functioned because cyanide served as a substitute for proteins or metals in transporting electrons between molecules.
“It was frightening how straightforward it was,” Krishnamurthy adds. “We didn’t have to do anything extra; we just brought these chemicals together, waited, and the reaction occurred on its own.”
Unlike prior metal-based r-TCA versions, the cyanide-based cycle performed at room temperature and throughout a broad pH range, resembling what would have existed on the early Earth.
Furthermore, the researchers demonstrated that cyanide enabled an even simpler form of the r-TCA cycle, one that avoided several of the contemporary cycle’s stages and less stable intermediate intermediates. According to Krishnamurthy, this subset of reactions might have occurred before the whole r-TCA cycle in the genesis of life.
He continues, “There is no method of demonstrating without a shadow of a doubt what chemistry transpired on the early Earth.” However, the discovery of a new set of reactions opens up the possibility of a new set of hypothetical circumstances that are compatible for life. And this has ramifications for the hunt for life in the history and present of our planet.
“It frees us up from saying these metals and these severe circumstances must exist,” Krishnamurthy adds. “This cyanide-based chemistry might lead to the emergence of life.”
The Simons Foundation (32712FY19), a NASA Exobiology award (80NSSC18K1300), and a combined grant from the National Science Foundation and the NASA Astrobiology Program under the Center for Chemical Evolution all contributed to this research (CHE-1504217).
