At the molecular level, every creature on Earth is ambidextrous. Most molecules essential for life occur naturally in two mirror image forms, but for some reason, living things are made up almost entirely of left-handed proteins and use mostly right-handed sugars. A new study claims to explain how this chemical quirk came about, potentially solving one of the more perplexing puzzles of prebiotic chemistry.
Life's one-handed bias has long baffled scientists because as a general rule, left- and right-handed molecules behave similarly and form in equal proportions. Analytical chemist Graham Cooks of Purdue University in West Lafayette, Indiana, and colleagues wondered if properties of serine, thought to be one of the oldest and simplest amino acids, might have somehow stacked the decks in favor of left-handed proteins and right-handed sugars.
To test their hypothesis, they combined serine with a variety of right-handed and left-handed molecules that would have been present in the primordial soup. In the 4 August issue of Angewandte Chemie, they report that serine hooks up into very stable clusters of eight molecules--but only if all eight are the same form, either left or right. These octamers can then combine with other amino acids, but again only with amino acids of the same orientation. (Also welcome are sugars of the opposite configuration.) That suggested that serine could have acted as a template that ensured that early clusters of amino acids were made up of amino acids of all one configuration or the other. Still unanswered, however, is how the left-handed bias arose.
The team hypothesizes that if by chance the balance between left- and right-handed octamers got out of whack and left-handed octamers became more common, the lefties would recruit more and more left-handed amino acids, making left-handed octamers even more common. As left-handed serine molecules were incorporated into octamers, right-handed serine would begin transforming into its mirror image to maintain equilibrium by way of a simple, low-energy reaction identified by Cooks's team. Eventually, left-handed octamers became predominant, they propose, forming the foundation for left-handed proteins.
"The experiments are novel and interesting," says chemist Leslie Orgell of the Salk Institute for Biological Studies in San Diego, California. The next question to answer, Orgell says, is whether the octamers can actually lead to proteins, whose covalent bonds are much stronger than the bonds within the octamers.