Chapter 4: Biochemistry of Life
"Our life is written in six letters. Our proteins are a mirror reflection. Our membranes sing at a silicon frequency. And yet — we live. Is this not a miracle?" — Xenobiochemist Thash-Ven, "Comparative Biochemistry: Thexar and Hypothetical Worlds," Era of Unity
Related sections: see [Ch.03, "Planet Thexar"], see [Ch.05, "Xha'Ruun Species"]
Introduction
Biochemistry is the language of life. All life on Thexar is built on a single biochemical standard — but this standard is unique.
Life on Thexar uses six nucleotides, 24 amino acids, D-chirality, and incorporates silicon into key structural components of the cell. The genetic code has approximately 29.25% greater information capacity per nucleotide. Metabolic networks are more branched. And hydrogen sulfide (H₂S) is not a poison but a key metabolic signal.
This chapter is not just a description of alien biochemistry. It is the foundation for understanding how the Xha'Ruun are built, how their ecosystems function, and how they interact with the planet.
4.1 Molecular Basis
4.1.1 Solvent
The water-based branch of life on Théxar, including Xha’Ruun, uses H₂O as its primary solvent. Ammonia and ammonium participate in its nitrogen metabolism and buffering systems. Kel’vash belong to a different biochemical branch with an acidic internal medium.
TERM: Ammonia Modification
The presence of NH₃ in the cytoplasm:
- Lowers the freezing point to −4.4°X (versus 45°X for pure water)
- Increases the solubility of nonpolar compounds
- Modifies pH buffer capacity (optimal cytoplasmic pH: 7.6–8.4)
- Participates in nitrogen metabolism as a nitrogen donor
| Parameter | Thexar Cell |
|---|---|
| Primary solvent | H₂O + 3% NH₃ |
| pH | 7.6–8.4 |
| Freezing point | −4.4°X |
| Ion content (mM) | Na⁺ 15, K⁺ 140, Cl⁻ 12 |
| Buffer system | HCO₃⁻/CO₂ + NH₃/NH₄⁺ |
4.1.2 Six-Letter Genetic Code
KEY FACT
All life on Thexar uses six nucleotides for storing hereditary information. Two additional nucleotides — pyrimidine-N (P) and xanthosine (X) — form triple hydrogen bonds, making the double helix more stable and informationally denser.
Canonical nucleotides of Thexar life:
| Letter | Name | Type | Pairs | H-bonds | Derivative |
|---|---|---|---|---|---|
| G | Guanine | Purine | G≡C | 3 | — |
| C | Cytosine | Pyrimidine | C≡G | 3 | — |
| A | Adenine | Purine | A=T | 2 | — |
| T | Thymine | Pyrimidine | T=A | 2 | — |
| P | Pyrimidine-N | Pyrimidine | P≡X | 3 | 6-amino-2-oxo-4-NH₂-pyrimidine |
| X | Xanthosine | Purine | X≡P | 3 | 2-oxo-6-oxopurine |
Théxar DNA — segment
| Position | Upper strand | Lower strand | Bond notation |
|---|---|---|---|
| 1 | G | C | triple |
| 2 | C | G | triple |
| 3 | A | T | double |
| 4 | P | X | triple |
| 5 | T | A | double |
| 6 | X | P | triple |
| 7 | G | C | triple |
| 8 | C | G | triple |
| Position | Upper strand | Lower strand | Bond notation |
|---|---|---|---|
| 1 | A | T | double |
| 2 | P | X | triple |
| 3 | G | C | triple |
| 4 | X | P | triple |
| 5 | C | G | triple |
| 6 | T | A | double |
| 7 | A | T | double |
| 8 | P | X | triple |
- Triple bond
- G≡C, P≡X
- Double bond
- A=T
Fig. 4.1. Diagram of a Thexar DNA double helix segment. Pairs G≡C and P≡X form three hydrogen bonds (more stable), A=T — two.
Statistical consequences:
| Parameter | Value |
|---|---|
| Number of nucleotides | 6 |
| Number of triplets (codons) | 216 |
| Number of encoded amino acids | 24 (+3 stop codons) |
| Stop codons | 3 |
| Code redundancy (average) | ~9 codons/AA |
| Information density per nucleotide | 2.58 bits |
The redundancy of the six-letter code means:
- High resistance to mutations (most substitutions do not change the amino acid)
- Possibility of fine regulation of gene expression through codon frequency
- Additional levels of epigenetic regulation
4.1.3 Amino Acids
Life on Thexar uses 24 proteinogenic amino acids, of which 20 are standard (in D-form) and 4 are unique:
| Amino Acid | Property | Function |
|---|---|---|
| Selenomethionine (SeMet) | Selenium-containing | Antioxidant, radioprotection |
| Tyrocyanine (Tcn) | Aromatic + CN | Electron transfer |
| Silicoasparagine (SiAsn) | Silicon-based | Structural, membrane channels |
| Hydroxyvaline (hVal) | Polar | Thermostability |
TERM: D-Chirality
The chiral proteinogenic amino acids of this branch have D-configuration; glycine is achiral. Sugars predominantly have L-configuration. This limits enzyme and food-substrate compatibility with life of opposite chirality.
4.1.4 Cell Structure
Thexar cells are eukaryote-like, with a nucleus and organelles, but with a number of unique differences:
| Structure | Thexar Cell |
|---|---|
| Plasma membrane | Lipid bilayer + silicon-carbon channels |
| Cell wall | Glycocalyx + silicate inclusions |
| Nucleus | Yes (1) |
| Mitochondria | Yes (but with a different cycle) |
| Chloroplasts | Chromoplasts (absorption peak 570 nm) |
| Vacuoles | Silicon-accumulating |
| ER | Rough + smooth |
| Ribosomes | 75S |
| Cytoskeleton | Based on silica filaments (+ microtubules) |
Silicon-carbon channels (Si-channels):
These are unique transmembrane protein structures containing silicon atoms in the active center. They perform the functions of:
- Transport of ions and small molecules with high selectivity
- Thermoregulation (expel excess heat)
- Electrochemical signaling through selective ion transport
- Participation in biomineralization (SiO₂ deposition)
4.2 Metabolism
4.2.1 Energy Cycle
The main energy currency of Thexar cells is xanthosine triphosphate (XTP) — based on xanthosine. XTP carries ~15% more energy per molecule than the standard ATP-like analog.
Energy conversion cycle — simplified
- Nutrientsleads toBreakdown (hydrolysis)
- Breakdown (hydrolysis)leads toXanthose-NAD cycle
- Xanthose-NAD cycleleads toRespiratory chain
- Breakdown (hydrolysis)produces by-products forExcretion of waste
- Respiratory chainprovides energy through XTP forSynthesis of macromolecules
Nutrient energy supports cellular synthesis; metabolic end products are excreted.
Fig. 4.2. Simplified scheme of energy metabolism. Respiratory chain efficiency is ~10% higher thanks to silicon-channel conductivity.
Théxar genetic code
- Nucleotides
- 6: A, T, G, C, P, X
- Codons and amino acids
- 216 codons → 24 amino acids
- Information per nucleotide
- 2.58 bits
- Double hydrogen bond
- A=T
- Triple hydrogen bond
- G≡C, P≡X
- P
- Pyrimidine-N (unique)
- X
- Xanthosine (unique)
Fig. 4.3. Structure of the Thexar genetic code. Additional nucleotides P and X form triple bonds, increasing the stability and information capacity of DNA.
4.2.2 Photosynthesis
Due to the spectrum of Khar'Vex (see [Ch.02, "Khar'Vex"]), photosynthesis on Thexar is adapted to orange-red light (peak ~570 nm).
PROFILE: Photosynthesis on Thexar
Parameter Value Pigment Chromophyll (a+b) Absorption peak 1 520 nm Absorption peak 2 590 nm Efficiency (quantum yield) ~0.91 Light phase Photosystems I, II, III Additional pigments Carotenoids + silicolipens Type C₄-like (analog)
Unique Photosystem III — an additional protein complex. It absorbs in the UV-A region (320–380 nm) and uses this energy for additional XTP synthesis. The overall efficiency of photosynthesis on Thexar is high — an evolutionary adaptation to lower insolation.
4.2.3 Hydrogen Sulfide Metabolism
KEY FACT
H₂S serves as a signalling molecule and a substrate in particular metabolic pathways. Its effects depend on concentration and organism: a physiological role does not make excess sulfide harmless. Atmospheric H₂S is replenished by volcanic sources and anaerobic microorganisms.
Biological functions of H₂S:
- Substrate for chemosynthesis (in deep-sea vent and cave ecosystems)
- Signaling molecule (regulation of metabolism, stress response)
- Antioxidant (neutralization of free radicals)
- Substrate for synthesis of sulfur-containing amino acids (selenomethionine)
| Organism | Effect of H₂S (1 µM) | Reason |
|---|---|---|
| Xha'Ruun | Normal metabolism | Adapted respiratory chain |
| Thexar plants | Growth +15% | Chemoautotrophic supplementation |
4.2.4 Nitrogen Metabolism
Nitrogen on Thexar is fixed via an alternative pathway using ammonia (NH₃) from the intracellular environment. The key enzyme — aminase-6 — contains molybdenum and vanadium in its active center in a 1:2 ratio.
| Process | End product | Energy cost (XTP) |
|---|---|---|
| Fixation N₂ → NH₃ | Ammonia | 12 XTP |
| NH₃ → amino acids | Glutamate, glutamine | 2 XTP |
| NH₃ → bases | G, C, A, T, P, X | Variable |
4.3 Genetics and Heredity
4.3.1 Replication and Repair
Thexar DNA replicates with high precision. This is achieved through:
- Additional proofreading mechanisms (eccentric nuclease-3)
- Stabilization of the double helix by triple bonds P≡X
- Increased activity of repair systems (3× higher than standard)
4.3.2 Radiation Resistance
Due to Thexar's weak magnetosphere (see [Ch.03, "Magnetosphere"]) and Khar'Vex's more active stellar wind in the past, all life on Thexar evolved with increased radiation resistance:
| Parameter | Value |
|---|---|
| LD₅₀ (lethal dose, 50%) | ~12 Gy |
| DNA repair after 1 Gy | ~25 min |
| Antioxidant system activity | 3× |
| Tolerance to UV-C (254 nm) | 480 J/mo² |
4.4 Origin of Life
4.4.1 Abiogenesis
Life on Thexar arose ~4.0 billion cycles ago in the tidal zones of the primordial ocean, rich in sulfur and phosphorus compounds. The key prerequisites were:
- High H₂S content in volcanic emissions
- Elevated silicon concentration in the primordial ocean
- Stable conditions in the center of the habitable zone (see [Ch.02, "Habitable Zone"])
- Enriched protoplanetary disk (metallicity +0.12)
4.4.2 RNA World and Code Evolution
Early evolution on Thexar passed through an "RNA world" — an epoch when catalytic RNA (ribozymes) served simultaneously as information carrier and catalyst. The six-letter code probably became fixed due to a selective advantage: triple bonds P≡X are more resistant to UV damage than double bonds A=T.
4.4.3 Key Evolutionary Transitions
| Time (billion cycles ago) | Event | Consequence |
|---|---|---|
| ~4.0 | First self-replicating molecules | Abiogenesis |
| ~3.8–3.5 | Universal six-letter code | CODE FIXATION |
| ~3.5 | Silicon in membranes | Structural reinforcement |
| ~3.0 | Oxygen catastrophe (photosynthesis) | O₂ in atmosphere |
| ~2.5 | Eukaryote-like cells | Endosymbiosis |
| ~1.8 | Multicellularity | — |
| ~0.8 | Land colonization | — |
| ~0.3 | Complex ecosystems | — |
| ~0.015 | First intelligent beings (Xha'Ruun) | — |
4.5 Ecosystems and Food Chains
4.5.1 Trophic Levels
Thexar's food chain has a typical structure:
Théxar trophic pyramid
Levels from apex to base
- Consumers of third order
- Consumers of second order (predators)
- Consumers of first order (herbivores)
- Producers (plants)
- Decomposers (bacteria, fungi) + H₂S
A separate group below the producer level
Unique feature: decomposers on Thexar not only break down organic matter but also release H₂S, which is consumed by chemoautotrophs, closing the "sulfur cycle" — another energy pathway in addition to photosynthesis.
4.5.2 Major Biochemical Cycles
| Cycle | Key Compounds |
|---|---|
| Carbon | CO₂, organic acids |
| Nitrogen | NH₃, NO₃⁻, amino acids |
| Sulfur | H₂S, S⁰, SO₄²⁻ — key role of H₂S |
| Phosphorus | PO₄³⁻, XTP, DNA |
| Silicon | SiO₂, Si-channels — unique |
| Selenium | Se, SeMet — additional antioxidant |
4.6 Limits of Life
4.6.1 Habitability Conditions for Thexar Life
| Parameter | Minimum | Optimum | Maximum |
|---|---|---|---|
| T | 38.6°X | 51.6…66°X | 84°X |
| pH | 2.5 | 7.6–8.4 | 11.0 |
| Pressure | 0.01 tyazh | 0.5–5 tyazh | 120 tyazh |
| Humidity | 10% | 50–95% | 100% |
| Radiation | 0 mSv/year | up to 100 mSv/year | ~3 Sv/year |
4.6.2 Extremophiles
On Thexar, organisms adapted to extreme conditions exist:
| Type | Environment | Feature |
|---|---|---|
| Thermophiles | Hydrothermal vents (up to 110°X) | Lipids with silicon bridges |
| Psychrophiles | Polar regions (38.6…45°X) | Ammonia cryoprotectant |
| Acidophiles | Volcanic lakes (pH 2.5–4.0) | Silicon channels are stable |
| Radiophiles | Deserts (high UV) | Doubled repair systems |
Chapter 4 Summary
Key Takeaways
- Six-letter genetic code (G, C, A, T, P, X) — 216 possible codons, 24 amino acids, information density +29%
- D-chirality — amino acids in D-form, sugars in L-form
- Silicon — not a replacement for carbon, but an additional element: Si-channels in membranes, silica filaments in cytoskeleton
- H₂S — not a poison, but a key signaling molecule and metabolic substrate
- Radiation resistance is high — evolutionary adaptation to the weak magnetosphere
- Three photosystems instead of two — adaptation to the orange spectrum of Khar'Vex
Links to Other Chapters
- see [Ch.03, "Planet Thexar"] — geochemical prerequisites for life
- see [Ch.05, "Xha'Ruun Species"] — how the six-letter code is realized in anatomy and physiology
- see [APP:bio-ref] — catalog of biochemical compounds