Why the building blocks in our cells turned to the left
Amino acids could have become “left-handed” on a journey through space. Viviane Richter explains.
Deep down, all right handers are lefties – at least when it comes to their amino acids. These are the building blocks of life on Earth and like most molecules, they can exist in right- or left-handed mirror image orientations. But life's chemsitry only works with lefties. Yasuhiro Oba and colleagues at Japan’s Hokkaido University now offer a novel reason for why that is: left-handed biology originated in space. They published their findings in Chemical Physics Letters in July.
“This research is a piece in a very big puzzle, but I think it’s going to be a key piece,” says Elizabeth Tasker, a physicist at Hokkaido University
When amino acids are cooked up in the lab, they come in left or right-handed mirror image forms. And just as your left and right hand are functionally the same, they can never be superimposed on the same space. And that matters when two amino acids try to hook up to make proteins and conduct the chemistry of life. Just as you can’t pull your right glove on to your left hand, a left and right amino acid protein simply can’t get together.
It’s never a problem because living systems always exclude right-handed amino acids and make exclusively left-handed proteins. But it didn’t have to be that way. When life first emerged, why did it choose left and not right? “That question has a lot of answers,” says Steven Benner, a chemist at the Foundation for Applied Molecular Evolution in Florida – but none of these ‘answers’ has yet been proved.
We know amino acids can hitch a ride through space on comets and meteorites, and some scientists suspect these amino-acid-laden meteorites kicked off life on Earth. The Murchison meteorite that crashed near Melbourne in 1969, for example, carried more than 15 different amino acids. But with only a few exceptions, the amino acids found on such meteorites are evenly divided between left and right.
However, four billion years ago, when life first stirred on Earth, the Universe contained a higher density of dusty star-forming galaxies. These dust grains – smaller than a thousandth of the width of a human hair – could have been a microscopic platform letting amino acids react and pick their side. Could the amino acid cocktail that smashed into Earth during this dustier era have contained more left-handed molecules than right?
Yes, says the Hokkaido team. Oba and his colleagues considered the smallest and simplest amino acid, glycine. Uniquely among amino acids, glycine is symmetrical and has no handedness. But it can be forced into left or right-handedness if one of its hydrogen atoms is knocked out and replaced with the slightly heavier deuterium atom.
With only a few exceptions, the amino acids found on meteorites are evenly divided between left and right.
Oba and his team asked whether this hydrogen-deuterium switch could take place in space – specifically, within dusty star-forming clouds. They are rich in deuterium but so cold that some scientists question whether any chemistry could take place there. To find out, Oba built a special chamber to mimic these conditions – a near vacuum cooled to about 10 ˚C above absolute zero. Inside, the researchers bombarded glycine with deuterium atoms, then weighed the sample to see if any deuterium had stuck. Sure enough, the sample was slightly heavier, confirming it is possible to make the glycine “handed”. Moreover they showed that the glycine appeared to “pick a side”. Just which side that was, they can’t yet say because the sample was too small. But the team may soon find out, as they plan to scale up the experiment.
For now, Benner isn’t convinced amino acids from space tipped life toward left-handedness. He believes biology picked left by chance. It would only take a small amount to tip the balance and give left-handed life a crucial early numbers advantage, he says. Malcolm Walter, an astrobiologist at the Australian Centre for Astrobiology at the University of New South Wales agrees. He also doubts we’ll ever come up with a definitive answer for why biology decided to be a lefty. “It’s going to remain speculative for a very long time – if not forever!”