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Meteorite Reveals Amino Acid Preference
13 February 1997 (All day)
One of evolution's most intriguing mysteries is why organisms prefer to assemble their proteins from just one of two types of building blocks. Amino acids--the constituents of proteins--come in two mirror-image shapes that have identical chemical compositions but differ from each other much like your left hand differs from your right. Yet organically produced proteins are made up predominantly from just one form. Now, research published in tomorrow's issue of Science* shows that the predominance of one amino acid shape isn't unique to life on Earth: It also shows up in a meteorite from space.
This surprising finding suggests that an excess of one form of amino acid did not evolve on Earth, as many scientists have believed. Instead, it may have been the result of chemical processes in the interstellar gases at the time the solar system was formed.
Two theories have been raised in the past to explain the earthly amino acid imbalance. One, dubbed the biotic theory, suggests that Earth's early atmosphere and "prebiotic soup" contained equal amounts of the two amino acid forms--labeled L and D--and that quirks of evolution eventually resulted in the dependence of most organisms on the L form. The abiotic theory, on the other hand, contends that before life began on Earth, the chemical soup already contained mostly L amino acids, and organisms evolved to use that form.
Chemists John Cronin and Sandra Pizzarello of Arizona State University reasoned that if the abiotic theory were true, amino acids present in meteorites--whose composition reflects chemical processes that occurred elsewhere in the solar system or beyond--would also show a prevalence of L amino acids. To test this idea, they studied the amino acids in a meteorite that fell to Earth in 1969. To make sure they weren't looking at contaminants from Earth, the researchers looked for unusual amino acids that are either extremely rare or unreported in terrestrial organisms. They found four amino acids that fit the bill. In each case, the L form was 2% to 9% more abundant than the D form.
Although the disparity is small compared with terrestrial ratios, Jeffrey Bada, a geochemist at the Scripps Institution of Oceanography in La Jolla, California, says he was "stunned" by the new work. If the finding holds up, "it [will be] the first demonstration that there may be natural processes that generate asymmetry in the cosmos," he says. Such asymmetry, Cronin and Pizzarello suggest, may have been created by polarized light--itself generated by a neutron star--that catalyzed the preferential synthesis of L amino acids in the interstellar clouds that became our solar system.
No matter what the source, the new results could be bad news for researchers hoping for a way to discriminate between organic compounds produced by earthly organisms and those produced by putative life forms elsewhere in the solar system. Researchers have long hoped that the preponderance of L amino acids would be a terrestrial fingerprint. But if the chemical asymmetry was laid down before the evolution of all life, that distinction wouldn't apply.