Life’s molecular orientation: New discovery deepens the mystery

A recent NASA-funded study has deepened the enigma of why life uses molecules with specific orientations. 

Researchers have discovered that RNA – a crucial molecule thought to have potentially carried life’s instructions before DNA emerged – can favor making the building blocks of proteins in either left-handed or right-handed forms. This finding could provide valuable clues to the origin of life.

The puzzle of life’s molecular orientation

Proteins are essential molecules in all living organisms, serving functions from structural components like hair to enzymes that speed up or regulate chemical reactions. 

Similar to how the 26 letters of the alphabet combine to form countless words, life uses 20 different amino acids in a vast array of arrangements to create millions of different proteins. 

Some amino acids exist in two mirror-image forms, like left and right hands, but life predominantly uses the left-handed variety – a phenomenon known as homochirality. 

Scientists remain puzzled about why life chose left-handed amino acids over right-handed ones, especially since life based on right-handed amino acids would presumably function just as well.

Exploring the “RNA world” hypothesis

DNA holds the instructions for building and maintaining living organisms but is complex and specialized. It “subcontracts” the work of reading instructions to RNA molecules and building proteins to ribosomes. 

Due to DNA’s complexity, scientists theorize that a simpler molecule preceded it billions of years ago during early life evolution. RNA, capable of both storing genetic information and building proteins, is a leading candidate, giving rise to the “RNA world” hypothesis.

If this proposition is correct, perhaps something about RNA caused it to favor building left-handed proteins over right-handed ones. 

However, the new research did not support this idea, deepening the mystery of why life favored left-handed proteins.

The role of evolutionary pressures 

The experiment tested RNA molecules that act like enzymes to build proteins, called ribozymes. 

“The experiment demonstrated that ribozymes can favor either left- or right-handed amino acids, indicating that RNA worlds, in general, would not necessarily have a strong bias for the form of amino acids we observe in biology now,” said corresponding author Irene Chen, a scientist at the University of California, Los Angeles (UCLA) Samueli School of Engineering.

In simulating potential early-Earth conditions of the RNA world, researchers incubated a solution containing ribozymes and amino acid precursors to observe the relative percentages of right-handed and left-handed phenylalanine amino acids produced. 

They tested 15 different ribozyme combinations and found that ribozymes can favor either left-handed or right-handed amino acids. This lack of inherent preference challenges the notion that early life was predisposed to select left-handed amino acids, which dominate in modern proteins.

“The findings suggest that life’s eventual homochirality might not be a result of chemical determinism but could have emerged through later evolutionary pressures,” said co-author Alberto Vázquez-Salazar, a UCLA postdoctoral scholar and member of Chen’s research group.

Implications for understanding life’s origins

Earth’s prebiotic history predates the oldest fossil records, which have been erased by plate tectonics – the slow movement of Earth’s crust. 

During that time, the planet was likely bombarded by asteroids, which may have delivered some of life’s building blocks, such as amino acids. Alongside chemical experiments, other origin-of-life researchers are examining molecular evidence from meteorites and asteroids.

Study co-author Jason Dworkin is a senior scientist for astrobiology at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and director of Goddard’s Astrobiology Analytical Laboratory.

Dworkin is also the project scientist on NASA’s OSIRIS-REx mission – which extracted samples from the asteroid Bennu and delivered them to Earth.

“Understanding the chemical properties of life helps us know what to look for in our search for life across the solar system,” said Dworkin. 

“We are analyzing OSIRIS-REx samples for the chirality (handedness) of individual amino acids, and in the future, samples from Mars will also be tested in laboratories for evidence of life including ribozymes and proteins.”

The search for life’s molecular secrets

This discovery adds complexity to our understanding of life’s molecular orientations, suggesting that homochirality might not have resulted from initial chemical conditions but could have developed due to evolutionary factors. 

The findings prompt further research into the origins of life and may influence how scientists search for life beyond Earth.

The study is published in the journal Nature Communications.

Image Credit: NASA

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