Life’s Baby Steps

Billions of years ago, amino acids somehow linked together to form chainlike molecules. This linkage was a vital step in the development of proteins, which are found in all living systems today.

Now Robert Hazen and Timothy Filley of the Geophysical Laboratory of the Carnegie Institution of Washington, and Glenn Goodfriend of George Washington University have discovered what may be a key step in this process – a step that has baffled researchers for more than a half a century. Their work, supported by NASA’s Astrobiology Institute and the Carnegie Institution, is reported in the May 1, 2001, issue of the Proceedings of the National Academy of Sciences.

Chirality Image
Chirality. Chiral compounds are ones which exist in two forms (enantiomers) which are non-superimposable mirror images of each other, and so are asymmetrical.
Credit: Answers Online

The molecular structure of all but one amino acid is an asymmetrical arrangement of atoms grouped around carbon. This arrangement means that there are two mirror-image forms of each amino acid; these forms are designated left-handed (L) and right-handed (D). All of the chemistry of living systems is distinguished by its selective use of these L and D, or chiral, molecules. Most scientists believe the first self-replicating organisms used L-amino acids, and today all living systems have proteins with only L-amino acids.

Non-biological processes do not usually distinguish between L and D variants. For a transition to occur between the chemical and biological eras, some natural process had to separate and concentrate the left- and right-handed amino acids from each other. This step, called chiral selection, is crucial to forming chain-like molecules of pure L-amino acids.

Hazen and his collaborators performed a simple experiment. They immersed a fist-sized crystal of the common mineral calcite, which forms limestone and the hard parts of many sea animals, in a dilute solution of the amino acid aspartic acid and found that the left-and right-handed variants of the acid molecules adsorbed (attached) preferentially onto different faces of the calcite crystal.

"Aspartic acid has a negatively charged group of atoms called the ‘side group’ that binds strongly to calcite, probably to the positively charged calcium atoms on calcite’s surface," says Hazen. "This same binding is critical to the strength of many shells, such as clam and snail shells, which are complex composite materials of calcite and proteins."

Aspartic Acid
Aspartic Acid. This amino acid was used in Hazen’s experiments to discover that left-and right-handed variants of the acid molecules adsorbed (attached) preferentially onto different faces of the calcite crystal.
Credit: Burkhard Kirste

Most minerals are centric, that is their structures are not handed. However, some minerals display pairs of crystal surfaces that have mirror relationships to each other. Calcite is one such mineral. It is common today, and was prevalent during the Archaean Era some four billion years ago, when life on earth is believed first to have emerged. This study suggests a plausible process by which the mixed D- and L-amino acids in the very dilute "primordial soup" could have been both concentrated and selected on a readily available mineral surface.

"I can imagine cycles of wetting and drying in a tidal pool," says Hazen. "Each time the calcite crystals are exposed to the amino-acid-rich ocean, they adsorb D and L amino acids selectively. Each time the crystal dries out, the amino acids link up to form homochiral polymers. Eventually, one of these polymers is autocatalytic – it makes copies of itself. This idea closely parallels similar scenarios that have employed clay minerals, which don’t perform the chiral selection trick."

Hazen says that the organic synthesis of amino acids has proven to be fairly easy. The problem, he says, has been that the pre-biotic soup was a diverse array of both right- and left-handed molecules. This study points to a mechanism that could have selected and organized the left- and right-handed molecules.

"Calcite, or some other mineral surface, seems like a good candidate for the crucial steps of selecting molecules from the complex prebiotic soup, and then organizing them into larger chain-like structures – including proteins – that are crucial to life’s origin," says Hazen.

Limestone
Limestone is made from calcite. Calcite may be a good candidate for the crucial steps of selecting molecules from the complex prebiotic soup, and then organizing them into larger chain-like structures – including proteins – that are crucial to life’s origin.
Credit: Georgia Perimeter College/Don Dowling & Tameka Wimberly

This study highlights how geology could have interacted with other elements of the Earth to form life. But according to Hazen, this study also suggests that life could emerge on other worlds that have carbonate rocks and liquid water.

"Our study tries to link key steps in life’s origin – whether here or elsewhere – with plausible prebiotic planetary conditions," says Hazen. "Surface studies of Mars and the martian meteorites, for example, provide evidence for both water and carbonates, as well as organic molecules."

On Earth, proteins are only made up of L-amino acids. But on other worlds, proteins might have formed from D-amino acids. This seemingly minor difference could have led to unique and unimagined forms of life elsewhere in the Universe.

"The calcite mechanism I suggest would have formed both left-handed and right-handed chains," says Hazen. "I’m suggesting – as have many other workers in the field – that the ultimate success of left over right was pure chance. On other worlds, right-handed amino acids may prevail."

What Next?

Hazen says this initial study opens many avenues of research. Not only are there dozens of potential minerals that could have selected and organized amino acids, but there are 20 different amino acids that occur in living systems. In addition, Hazen says there are many other factors that still have to be tested.

"There’s so much to do!" says Hazen. "We have to study effects of temperature, pH, salinity, and concentration on the adsorption. Then there are all the studies of polymerization on calcite surfaces — can we simulate wetting and drying cycles to make homochiral chains of amino acids? We also have to understand this adsorption on an atomic scale. We’re beginning atomic force microscopy studies to image the adsorbed molecules, and will attempt to model this adsorption process."


Related Web Pages

Selective adsorption of L- and D-amino acids on calcite: Implications for biochemical homochirality (PNAS Online)
One-Handed Life (NASA Astrobiology Institute)
Carbonated Mars (NASA Astrobiology Institute)