March 18, 2009 — NASA scientists analyzing the dust of meteorites have discovered new
clues to a long-standing mystery about how life works on its most
basic, molecular level.
"We
found more support for the idea that biological molecules, like amino
acids, created in space and brought to Earth by meteorite impacts help
explain why life is left-handed," said Dr. Daniel Glavin of NASA’s
Goddard Space Flight Center in Greenbelt, Md. "By that I mean why all
known life uses only left-handed versions of amino acids to build
proteins." Glavin is lead author of a paper on this research appearing
in the Proceedings of the National Academy of Sciences March 16.
Proteins are the workhorse molecules of life, used in everything from
structures like hair to enzymes, the catalysts that speed up or
regulate chemical reactions. Just as the 26 letters of the alphabet are
arranged in limitless combinations to make words, life uses 20
different amino acids in a huge variety of arrangements to build
millions of different proteins. Amino acid molecules can be built in
two ways that are mirror images of each other, like your hands.
Although life based on right-handed amino acids would presumably work
fine, "you can’t mix them," says Dr. Jason Dworkin of NASA Goddard,
co-author of the study. "If you do, life turns to something resembling
scrambled eggs — it’s a mess. Since life doesn’t work with a mixture
of left-handed and right-handed amino acids, the mystery is: how did
life decide — what made life choose left-handed amino acids over
right-handed ones?"
Over the last four years, the team carefully
analyzed samples of meteorites with an abundance of carbon, called
carbonaceous chondrites. The researchers looked for the amino acid
isovaline and discovered that three types of carbonaceous meteorites
had more of the left-handed version than the right-handed variety – as
much as a record 18 percent more in the often-studied Murchison
meteorite. "Finding more left-handed isovaline in a variety of
meteorites supports the theory that amino acids brought to the early
Earth by asteroids and comets contributed to the origin of only
left-handed based protein life on Earth," said Glavin.
All amino
acids can switch from left-handed to right, or the reverse, by chemical
reactions energized with radiation or temperature, according to the
team. The scientists looked for isovaline because it has the ability to
preserve its handedness for billions of years, and it is extremely
rarely used by life, so its presence in meteorites is unlikely to be
from contamination by terrestrial life. "The meteorites we studied are
from before Earth formed, over 4.5 billion years ago," said Glavin. "We
believe the same process that created extra left-handed isovaline would
have created more left-handed versions of the other amino acids found
in these meteorites, but the bias toward left-handed versions has been
mostly erased after all this time."
The team’s discovery
validates and extends the research first reported a decade ago by Drs.
John Cronin and Sandra Pizzarello of Arizona State University, who were
first to discover excess isovaline in the Murchison meteorite, believed
to be a piece of an asteroid. "We used a different technique to find
the excess, and discovered it for the first time in the Orgueil
meteorite, which belongs to another meteorite group believed to be from
an extinct comet," said Glavin.
The team also found a pattern to
the excess. Different types of meteorites had different amounts of
water, as determined by the clays and water-bearing minerals found in
the meteorites. The team discovered meteorites with more water also had
greater amounts of left-handed isovaline. "This gives us a hint that
the creation of extra left-handed amino acids had something to do with
alteration by water," said Dworkin. "Since there are many ways to make
extra left-handed amino acids, this discovery considerably narrows down
the search."
If the bias toward left-handedness originated in
space, it makes the search for extraterrestrial life in our solar
system more difficult, while also making its origin a bit more likely,
according to the team. "If we find life anywhere else in our solar
system, it will probably be microscopic, since microbes can survive in
extreme environments," said Dworkin. "One of the biggest problems in
determining if microscopic life is truly extra-terrestrial is making
sure the sample wasn’t contaminated by microbes brought from Earth. If
we find the life is based on right-handed amino acids, then we know for
sure it isn’t from Earth. However, if the bias toward left-handed amino
acids began in space, it likely extends across the solar system, so any
life we may find on Mars, for example, will also be left-handed. On the
other hand, if there is a mechanism to choose handedness before life
emerges, it is one less problem prebiotic chemistry has to solve before
making life. If it was solved for Earth, it probably has been solved
for the other places in our solar system where the recipe for life
might exist, such as beneath the surface of Mars, or in potential
oceans under the icy crust of Europa and Enceladus, or on Titan."
The
research was funded by the NASA Astrobiology Institute, the NASA
Cosmochemistry program, and the NASA Astrobiology: Exobiology, and
Evolutionary Biology program.
Source : NASA