WASHINGTON - NASA scientists analysing meteorite dust 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 Daniel Glavin of NASA’s Goddard Space Flight Centre in Greenbelt.

‘By that I mean why all known life uses only left-handed versions of amino acids to build proteins,’ added Glavin, who co-authored the study.

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 English 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’, said 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 analysed 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 energised 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, according to a NASA release.

The study appeared in the March issue of the Proceedings of the National Academy of Sciences.