Theories abound on how life first formed on Earth. These range from a meteoroid carrying a life form crashing into Earth to the first biological cell evolving in a water body that sprang up into the life form we know today.

But one thing that has been constant is that nature has chosen only the amino acid that is known as L-amino acid and not the other form that is D-amino acid, which actually is a mirror image of the first one. Since the birth of the first organism, life on earth, starting from the simple single-cellular organisms to complex ones like humans, has been represented by only the L-amino acid.

Those that believe the meteoroid theory say that the organism that it carried had only L-amino acid, while those towing the water body theory say that the D-amino acid got sucked by the soil under the water body, leaving only the L form to evolve on earth.

But since then, all organisms have been employing an active mechanism that prevents the D-amino acids from entering the protein synthesising factory (ribosomes) of the cell. Even though there has been evidence that such a mechanism exists in the cell, it has still not been cleared as to how the cell specifically removed only the D-amino acid, leaving out the L type.

Indian scientists have now cracked open a window that gives a new insight into this nature’s mystery.

Scientists at the Centre for Cellular and Molecular Biology in Hyderabad have shown the precise mechanism by which L-amino acids are allowed to form proteins in life on earth, while the D variety is removed by a particular enzyme called DTD.

“The study has significantly advanced our knowledge on a very fundamental aspect of life and explains why we do not have a mirror image biological world (or life form that is represented by D-amino acids),” explains Rajan Sankaranarayanan, who headed the team of CCMB scientists on the study.

The work was today published in the Journal eLife, which is edited by Nobel laureate Randy Schekman.

What is the implication of this finding? First, it may lead to a better understanding of the evolution of life. And second, it may lead to designing of better synthetic biology strategies for making more diverse engineered proteins that can eventually lead to new therapeutical applications.

“Also, as D-amino acids are present in neuronal cells, the study could open new doors on their role in neuro-degenerative diseases in human beings,” Sankaranarayanan said.

amitmitra@thehindu.co.in

(This article was published on December 3, 2013)
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