Published on May 22, 2025 11:10 AM GMT
Core Argument
The barrier to preying on "mirror-image" life is likely lower than presumed due to:
- Immediate energy from achiral fats.Existing L-sugar pathways.Existing D-amino acid racemases.
1. Fats: Achiral Energy Bridge
Lipids (triglycerides), largely achiral, offer immediate energy. In E. coli for example, they constitute ~10% of dry mass, yielding ~27% of its macronutrient calories[1]). This non-chiral caloric gain from mirror prey could sustain predators during adaptation to chiral components.
2. Carbohydrates: L-Glucose Metabolism Evolved
L-glucose metabolism has evolved at least twice despite its rarity in nature:
- Pseudomonas caryophylli enzyme oxidizes L-glucose (Sasajima & Sinskey, 1979[2]).Paracoccus sp. 43P has a full L-glucose catabolic pathway (Shimizu et al., 2012[3]).However, given that carbohydrates constitute a relatively minor fraction of the total caloric content in microorganisms (e.g., ~7% in E. coli[1:1]), the immediate selective advantage of metabolizing mirror-sugars might be less pronounced than for fats or proteins.That these adaptations emerged even under low selective pressure, suggests the evolutionary barrier to such adaptations might be lower than anticipated.
3. Proteins: Racemases Handle D-Amino Acids
Amino acid racemases, enzymes interconverting L- and D-amino acids (e.g., for cell walls, neurotransmission), are widespread (Yoshimura & Esaki, 2003[4]). This pre-existing machinery could be adapted to process D-amino acids from mirror proteins, the largest caloric source in microorganisms (~55% dry mass, ~66% major macronutrient calories in E. coli[1:2]).
Conclusion: Lowered Mirror Predation Barrier
Achiral fat calories, existing L-sugar pathways, and D-amino acid racemases collectively suggest a lowered barrier to mirror-life predation. Fats offer initial sustenance, while pre-existing enzyme capabilities could enable adaptation to mirror-chiral molecules. While mirror organisms still likely have a large competitive advantage, these metabolic footholds suggest that a stable equilibrium might develop more readily than initially expected.
Co-authored by Gemini 2.5 Pro
Phillips, R., Milo, R., et al. Cell Biology by the Numbers. https://book.bionumbers.org/what-is-the-macromolecular-composition-of-the-cell/ ↩︎ ↩︎ ↩︎
Sasajima, K.-I., & Sinskey, A. J. (1979). Oxidation of l-glucose by a Pseudomonad. Biochimica et Biophysica Acta (BBA) - Enzymology, 571(1), 120-126. DOI: 10.1016/0005-2744(79)90232-8 ↩︎
Shimizu, T., Takaya, N., & Nakamura, A. (2012). An l-glucose Catabolic Pathway in Paracoccus Species 43P. Journal of Biological Chemistry, 287(48), 40448–40456. DOI: 10.1074/jbc.M112.403055 ↩︎
Yoshimura, T., & Esaki, N. (2003). Amino acid racemases: functions and mechanisms. Journal of Bioscience and Bioengineering, 96(2), 101-108. DOI: 10.1016/S1389-1723(03)90111-3 ↩︎
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