NASA’s latest findings reveal that RNA’s role in life’s preference for left-handed proteins may be less about chemical bias and more about evolutionary pressures.
This discovery is part of wider efforts to trace life’s origins and characteristics, including studies on extraterrestrial amino acids.
Unraveling Molecular Orientations in Life’s Origins
The mystery of why life relies on molecules with specific orientations has deepened following a NASA-funded discovery about RNA. RNA, a molecule thought to have carried the instructions for life before DNA, has been found to support the creation of protein building blocks in both left-handed and right-handed forms. This surprising finding, published in Nature Communications, challenges long-held assumptions and may provide new insights into the origins of life.
Homochirality in Biological Molecules
Proteins, the essential molecules of life, perform a wide range of functions, from forming structures like hair to acting as enzymes that regulate chemical reactions. Life assembles these proteins from 20 amino acid building blocks, combining them in countless ways to create millions of distinct proteins.
Interestingly, some amino acids can exist in two mirror-image forms, similar to how your hands are mirror opposites. Yet, life exclusively uses the left-handed versions of these amino acids. This phenomenon, known as homochirality, has puzzled scientists for years. While life built with right-handed amino acids would likely function just as well, the reason behind life’s exclusive preference for the left-handed variety remains a mystery.
The Role of RNA in Early Life Forms
DNA (deoxyribonucleic acid) is the molecule that holds the instructions for building and running a living organism. However, DNA is complex and specialized; it “subcontracts” the work of reading the instructions to RNA (ribonucleic acid) molecules and building proteins to ribosome molecules. DNA’s specialization and complexity lead scientists to think that something simpler should have preceded it billions of years ago during the early evolution of life. A leading candidate for this is RNA, which can both store genetic information and build proteins. The hypothesis that RNA may have preceded DNA is called the “RNA world” hypothesis.
If the RNA world proposition is correct, then perhaps something about RNA caused it to favor building left-handed proteins over right-handed ones. However, the new work did not support this idea, deepening the mystery of why life went with left-handed proteins.
Exploratory Experiment on RNA’s Preferences
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 Irene Chen, of the University of California, Los Angeles (UCLA) Samueli School of Engineering, corresponding author of the Nature Communications paper.
In the experiment, the researchers simulated what could have been early-Earth conditions of the RNA world. They incubated a solution containing ribozymes and amino acid precursors to see the relative percentages of the right-handed and left-handed amino acid, phenylalanine, that it would help produce. They tested 15 different ribozyme combinations and found that ribozymes can favor either left-handed or right-handed amino acids. This suggested that RNA did not initially have a predisposed chemical bias for one form of amino acids. This lack of preference challenges the notion that early life was predisposed to select left-handed-amino acids, which dominate in modern proteins.
Implications of Findings on Life’s Chirality
“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.
Earth’s prebiotic history lies beyond the oldest part of the fossil record, which has been erased by plate tectonics, the slow churning 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. In parallel to chemical experiments, other origin-of-life researchers have been looking at molecular evidence from meteorites and asteroids.
NASA’s Pursuit of Extraterrestrial Amino Acids
“Understanding the chemical properties of life helps us know what to look for in our search for life across the solar system,” said co-author Jason Dworkin, senior scientist for astrobiology at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and director of Goddard’s Astrobiology Analytical Laboratory.
Dworkin is the project scientist on NASA’s OSIRIS-REx mission, which extracted samples from the asteroid Bennu and delivered them to Earth last year for further study.
“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,” said Dworkin.
For more on this study, see NASA Uncovers RNA Twist That Could Redefine Life’s Origin Story.
Reference: “Prebiotic chiral transfer from self-aminoacylating ribozymes may favor either handedness” by Josh Kenchel, Alberto Vázquez-Salazar, Reno Wells, Krishna Brunton, Evan Janzen, Kyle M. Schultz, Ziwei Liu, Weiwei Li, Eric T. Parker, Jason P. Dworkin and Irene A. Chen, 12 September 2024, Nature Communications.
DOI: 10.1038/s41467-024-52362-x
The research was supported by grants from NASA, the Simons Foundation Collaboration on the Origin of Life, and the National Science Foundation. Vázquez-Salazar acknowledges support through the NASA Postdoctoral Program, which is administered by Oak Ridge Associated Universities under contract with NASA.
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