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Mirror life (also called mirror-image life, chiral life, or enantiomeric life) is a hypothetical form of life with mirror-reflected molecular building blocks.[1][2][3][4][5] The possibility of mirror life was first discussed by Louis Pasteur.[6] Although this alternative life form has not been discovered in nature, efforts to build a mirror-image version of biology's molecular machinery are already underway.[7]

Homochirality

Many of the essential molecules for life on Earth can exist in two mirror-image forms, referred to as "left-handed" and "right-handed", but living organisms do not use both. Proteins are exclusively composed of left-handed amino acids; RNA and DNA contain only right-handed sugars. This phenomenon is known as homochirality.[8] It is not known whether homochirality emerged before or after life, whether the building blocks of life must have this particular chirality, or indeed whether life needs to be homochiral.[9] Protein chains built from amino acids of mixed chirality tend not to fold or function as catalysts, but mirror-image proteins have been constructed that work the same but on substrates of opposite handedness.[8]

The concept

It is thought that such mirror organisms can be highly incompatible with existing microbes (viruses, bacteria, protozoa, etc.). Hypothetically, it is possible to recreate an entire ecosystem from the bottom up, in chiral form. In this way, the creation of an Earth ecosystem without microbial diseases might be possible. In some distant future, mirror life could be employed to create robust, effective and disease-free ecosystems for use on other planets.[10]

Advances in synthetic biology, like synthesizing viruses since 2002, partially synthetic bacteria in 2010, or synthetic ribosomes in 2013, may lead to the possibility of fully synthesizing a living cell from small molecules, where we could use mirror-image versions (enantiomers) of life's building-block molecules, in place of the standard ones. Some proteins have been synthesized in mirror-image versions, including polymerase in 2016.[11]

Reconstructing regular lifeforms in mirror-image form, using the mirror-image (chiral) reflection of their cellular components, could be achieved by substituting left-handed amino acids with right-handed ones, in order to create mirror reflections of all regular proteins. Analogously, we could get reflected sugars, DNA, etc., on which reflected enzymes would work perfectly. Finally we would get a normally functioning mirror reflection of a natural organism - a chiral counterpart organism - with which natural viruses and bacteria couldn't interact. Electromagnetic force (chemistry) is unchanged under such molecular reflection transformation (P-symmetry). There is a small alteration of weak interactions under reflection, which can produce very small corrections, but these corrections are many orders of magnitude lower than thermal noise - almost certainly too tiny to alter any biochemistry.[citation needed] However, there are also theories that weak interactions can have a greater effect on longer nucleic acids or protein chains, resulting in much less efficient conversion of mirror ribozymes or enzymes than normal ribozymes or enzymes.[12]

Chiral animals would need to feed on reflected food, produced by reflected plants. The great advantage, though, is that such chiral organisms should enjoy a disease-free life, completely immune to all viruses and microbes (which virologists are now beginning to understand underlie a huge number of diseases).

Viruses would be completely incompatible with the reflected cellular structures; and bacteria, protozoa, and fungi could not function because they would not be able to find normal sugars inside reflected organisms. The reverse sugars circulating in the chiral organism's body would be indigestible as far as normal bacteria are concerned, so any bacterium entering a chiral organism would simply starve to death. The chiral environment is hostile for normal viruses, protozoa, bacteria, etc.

Mirror life presents potential dangers. For example, a chiral-mirror version of cyanobacteria, which only needs achiral nutrients and light for photosynthesis, could take over Earth's ecosystem due to lack of natural enemies, disturbing the bottom of the food chain by producing mirror versions of the required sugars. Some bacteria can digest L-glucose; exceptions like this would give some rare lifeforms an unanticipated advantage.

Direct applications

Direct application of mirror-chiral organisms can be mass production of enantiomers (mirror-image) of molecules produced by normal life.

  • Enantiopure drugs - some pharmaceuticals have known different activity depending on enantiomeric form,
  • Aptamers (L-ribonucleic acid aptamers): "That makes mirror-image biochemistry a potentially lucrative business. One company that hopes so is Noxxon Pharma in Berlin. It uses laborious chemical synthesis to make mirror-image forms of short strands of DNA or RNA called aptamers, which bind to therapeutic targets such as proteins in the body to block their activity. The firm has several mirror-aptamer candidates in human trials for diseases including cancer; the idea is that their efficacy might be improved because they aren't degraded by the body's enzymes. A process to replicate mirror-image DNA could offer a much easier route to making the aptamers, says Sven Klussmann, Noxxon Pharma's chief scientific officer."[13]
  • L-Glucose, enantiomer of standard glucose, for which tests showed that it tastes likes standard sugar, not being metabolized the same way. However, it was never marketed due to excessive manufacturing costs.[14]

In fiction

The creation of one chiral human is the basis of 1950 Arthur C. Clarke's story "Technical Error", from The Collected Stories. In this story, a physical accident transforms a person into his mirror image, speculatively explained by travel through a fourth physical dimension.

An alien machine that reverses chirality, and a blood-symbiote that functions properly only when in one chirality, were central to Roger Zelazny's 1976 novel Doorways in the Sand.

In the 2017 Daniel Suarez novel "Change Agent", an antagonist, Otto, nicknamed the "Mirror Man", is revealed to be a genetically-engineered enantiomeric human. He views other humans with disdain and causes them to feel an inexplicable repulsion by his very presence.

See also

References

  1. ^ Singer, Emily (26 November 2014). "New twist found in the story of life's start". Quanta Magazine. Retrieved 8 May 2018.
  2. ^ Markus, Schmidt (2010). "Xenobiology: A new form of life as the ultimate biosafety tool". BioEssays. 32 (4): 322–331. doi:10.1002/bies.200900147. PMC 2909387. PMID 20217844.
  3. ^ Church, George M. (2014). Regenesis how synthetic biology will reinvent nature and ourselves. New York: Basic Books. ISBN 9780465038657.
  4. ^ Sawyer, Eric (11 January 2013). "The one and only popular synthetic biology book". Scitable. Nature Education. Retrieved 8 May 2018.
  5. ^ Acevedo-Rocha, Carlos G. (2015). "The synthetic nature of biology". In Hagen, Kristin; Engelhard, Margret; Toepfer, Georg (eds.). Ambivalences of Creating Life: Societal and Philosophical Dimensions of Synthetic Biology. Springer. pp. 9–54. ISBN 9783319210889.
  6. ^ Siegel, J. S. (1992-11-20). "Left-handed comments". Science. 258 (5086): 1290. Bibcode:1992Sci...258.1289B. doi:10.1126/science.1455216. ISSN 0036-8075. PMID 1455218.
  7. ^ Peplow, Mark (2018-07-25). "A Conversation with Ting Zhu". ACS Central Science. 4 (7): 783–784. doi:10.1021/acscentsci.8b00432. ISSN 2374-7943. PMC 6062833. PMID 30062104.
  8. ^ a b Plaxco, Kevin W.; Michael, Michael (2011). Astrobiology: A Brief Introduction. JHU Press. pp. 140–141. ISBN 9781421401942.
  9. ^ Sedbrook, Danielle (28 July 2016). "Must the Molecules of Life Always be Left-Handed or Right-Handed?". Smithsonian.com. Retrieved 8 May 2018.
  10. ^ Bohannon, John (2010). "Mirror-image cells could transform science - or kill us all". Wired. 18 (12).
  11. ^ Wang, Zimou; Xu, Weiliang; Liu, Lei; Zhu, Ting F. (2016). "A synthetic molecular system capable of mirror-image genetic replication and transcription". Nature Chemistry. 8 (7): 698–704. Bibcode:2016NatCh...8..698W. doi:10.1038/nchem.2517. ISSN 1755-4330. PMID 27325097.
  12. ^ Pitkänen, M. "Could the replication of mirror DNA teach something about chiral …" (PDF). Topological Geometrodynamics. Retrieved 27 July 2018.
  13. ^ Peplow, Mark (16 May 2016). "Mirror-image enzyme copies looking-glass DNA". Nature. 533 (7603): 303–304. Bibcode:2016Natur.533..303P. doi:10.1038/nature.2016.19918. PMID 27193657.
  14. ^ A natural way to stay sweet, NASA
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