Scientific Method / Science & Exploration

Adding iron is like giving early RNA enzymes “steroids”

The RNA world that came before life might have gotten a big boost from iron.

Iron, trapped in this rock as bands of rust, would have been dissolved in watery pools on early Earth.

Life may have started from an RNA world, where RNA both carried genetic information and catalyzed chemical reactions, jobs that are now divided between DNA and proteins. But sussing out the chemistry of the RNA world is challenging, not least because we’ll never really know what metals and molecules were present on the early Earth. Scientists have some clues from the chemistry of rocks, computer models, and lab experiments.

New research suggests that RNA on the early Earth could have interacted with different metals than it does today. Magnesium currently helps our RNA fold into the proper shapes for catalysis. Changing that metal to iron could increase the types of reactions that could be catalyzed by early RNAs.

Iron dissolved in watery pools was plentiful on the oxygen-free early Earth. Once photosynthetic organisms appeared and started pumping out oxygen, that iron turned to rust, and was trapped in rocks as bands still visible today. Since the RNA world is thought to have existed before this Great Oxidation Event, Loren Williams at Georgia Institute of Technology and his colleagues wondered if RNA on early Earth could have bound iron.

First, the researchers modeled of a snippet of the RNA backbone – just one sugar flanked by two phosphate groups. They plunked a magnesium or an iron ion in between the phosphates and calculated the most stable shape of the backbone. Both backbones had the same shape, regardless of metal ion inside.

Knowing that an iron ion (Fe2+) could fit in the same place as Mg2+ in RNA, the researchers tested modern RNA enzymes, or ribozymes, to see if they would still function with iron under oxygen-free conditions. One ribozyme, a ligase that connects RNA molecules, was actually 25 times more active using iron than magnesium. The other ribozyme snipped RNA apart three times faster when it bound iron instead magnesium.

Not only can modern RNA enzymes bind iron, they also work more efficiently using that ion. What else might iron do for the function of early ribozymes? Iron can transfer electrons more efficiently than magnesium. Ancient ribozymes holding iron could likely perform a broader range of reactions than modern ones, since they could shuffle electrons between molecules more efficiently. (Most of basic metabolism, like conversion of sugars into usable energy, involves electron transfer reactions.)

An RNA world with iron would be the RNA world on steroids, the researchers write. Over time, less-reactive magnesium ions may have replaced the iron ions because they better stabilize folded RNAs.

The researchers plan to continue to study the chemistry of iron-RNA complexes, looking for reactions not possible with magnesium.

Making molecules that resemble ones we find in life today is simple. Stanley Miller did it in 1953, when he combined ammonia, hydrogen, methane and water vapor in a jar and zapped the mixture with a lightning bolt. In that primordial soup, Miller found building blocks of proteins and simpler molecules like urea that could be used to build more complicated biochemicals. Analyzing that mixture more than 50 years later with modern methods, researchers found even more amino acids than Miller originally thought.

Now, many researchers are looking into how collections of these simple molecules started interacting. “The big challenge today is figuring how you select, concentrate, and assemble all of those molecules into a larger lifelike system, one which starts to make copies of itself,” geophysicist Robert Hazen of the Carnegie Institution of Washington told the Economist this week. “And that remains a huge mystery.”

Catalysts, whether salts, metals or nucleic acids, can help with the selection process. They guide chemical reactions away from a sticky mix of products to a group of specific molecules. So identifying early catalysts could help scientists identify possible chemical reactions that could lead to the building blocks of life.

PLoS ONE , 2012. DOI: 10.1371/journal.pone.0038024 (About DOIs).

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