[meteorite-list] NASA Scientists Find Clues to a Secret of Life

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 17 Mar 2009 18:16:40 -0700 (PDT)
Message-ID: <200903180116.SAA24091_at_zagami.jpl.nasa.gov>

http://www.nasa.gov/centers/goddard/news/topstory/2009/left_hand_life.html

NASA Scientists Find Clues to a Secret of Life
03.17.09
 
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.
 
 
Bill Steigerwald
NASA Goddard Space Flight Center
 
Received on Tue 17 Mar 2009 09:16:40 PM PDT