Notes on LUCA physiology and the origin of the cell membrane

The origin of the cell membrane

A universal feature of life is the establishment of an electrochemical gradient across membranes that yields free energy to be stored chemically. This gradient is central to energy harvesting in all life forms.
The cell membrane origin is linked to the self-assembly of amphiphilic molecules in the early Earth environment.
Reference: Deamer et al., Astrobiology (2002).

The structure of cell membrane

Discovery in the early 1900s: all cells are bounded by a boundary that responds to osmotic gradients by swelling or shrinking.
Classic experiment (Gorter/Grendel): the average surface area of cells compared to a monolayer formed by extracted lipids; ratio ~ 1:2, indicating a lipid bilayer nature.
Liposomes (lipid vesicles) form mono- and bilayers and serve as essential models for studying self-assembly of amphiphiles and early cellular life.
Key figure: Evert Gorter (1881–1954).

Lipids and phospholipid structure (Figure 4.14 context)

General structure of phospholipids: fatty acids, glycerol, phosphate group; amphipathic (one hydrophobic tail, one hydrophilic head).
Bilayer formation: phospholipids spontaneously aggregate in water; hydrophobic tails face inward, hydrophilic heads face water.
Liposomes form when the lipid bilayer folds to enclose an internal aqueous compartment.
Illustration reference: Fig. 4.14 from the source material (A: general structure; B: bilayer; C: liposome).

Are phospholipids ancestral membrane components?

Hypotheses about LUCA’s membrane and lipid biosynthesis:
- Koga (1998): LUCA was acellular (no membrane); phospholipid biosynthesis emerged late and independently in bacteria and archaea.
- Martin & Russell (2003): LUCA had mineral (FeS) compartments rather than lipid membranes.
- Wächtershäuser (2003): early cellularization via membranes composed of simple lipids synthesized non-enzymatically.
- Lombard (2012): LUCA might have used ancestral enzymes to synthesize a mix of G1P and G3P lipids.

Lipid diversity: archaeal vs bacterial/eukaryotic membranes

Isoprenoids and lipid types differ between domains:
- Archaeal lipids often use ether linkages and branched isoprenoid tails; can have L-glycerol backbones and ether bonds.
- Bacterial and eukaryotic lipids use ester linkages and typically unbranched, straight-chain fatty acids with D-glycerol backbones.
Structural schematic (from slide):
- Archaeal phospholipid: ether linkage; G1P backbone; branched isoprenoid tails; sometimes methyl-branched
- Bacterial/eukaryotic phospholipid: ester linkage; G3P backbone; linear fatty acid tails
Two major bond types: ether (archaea) vs ester (bacteria/eukarya); tail architecture and stereochemistry differ (sn-glycerol-1-phosphate vs sn-glycerol-3-phosphate).

Lipid biosynthesis: domain-specific features

Are there universal lipid biosynthesis pathways?
- Acetyl-CoA and MVA pathway are involved in isoprenoid synthesis in many domains; details vary by organism.
- Archaea: sn-glycerol-1-phosphate (G1P); ether-linked, branched isoprenoid tails; distinct enzymes (e.g., G1PDH, G3PDH, GGPS, GAT, DGGGPS) and potential universal superfamily relationships.
- Bacteria/eukaryotes: sn-glycerol-3-phosphate (G3P); ester-linked lipids; fatty acids synthesized via FAS pathways (FAS I/II).
Polarity and head-group linkage differences influence membrane stability and permeability in different environments.

Are phospholipids ancestral? (Continued)

The debate about LUCA’s membrane composition remains unresolved, with competing models ranging from acellular LUCA without a true lipid membrane to LUCA with early lipid membranes or mineral compartments.

Evolutionary models for early phospholipid biosynthesis

A spectrum of models for how early membranes and phospholipid biosynthesis evolved:
- Cenancestral state: a membrane-containing ancestor with archaeal/bacterial lipid diversification later on.
- Surface metabolism on pyrite: mineral surfaces acting as a scaffold for metabolism prior to cellular membranes.
- Chemical evolution: non-enzymatic lipid formation preceding enzymatic lipid biosynthesis.
The slide presents a lattice of hypotheses (bacteria/archaea branches, pre-cell stem with heterochiral membranes, non-enzymatic glycerol phosphate synthesis, etc.).

Journey from RNA world to modern cells: a plausible sequence

A schematic progression from protocell to DNA-based cells:
1) Evolution starts: first protocell is a sac of water and RNA; external cycling (heat/cold) drives reproduction.
2) RNA catalysts (ribozymes) speed up replication and help stabilize the protocell's membrane; protocells begin to reproduce.
3) RNA-based metabolism expands: ribozymes catalyze metabolisms, enabling uptake of environmental nutrients.
4) Proteins appear: translation from RNA to amino acids via ribosomes; enzyme-like proteins augment catalysis.
5) Proteins take over: protein enzymes replace most ribozymes in cellular functions.
6) Birth of DNA: DNA replication enzymes emerge; DNA becomes the primary genetic molecule; RNA shifts to bridging roles between DNA and proteins.
7) Bacterial world: organisms resembling modern bacteria become dominant in diverse environments.
This sequence portrays a gradual handover of information storage from RNA to DNA and catalysis from RNA to proteins.

Timeline to LUCA and the early Earth environment

Earth age: ~ $4.6$ Ga (billion years ago).
Moon-forming impact: ~ $4.4$ Ga.
Post-impact magma oceans: water driven into gas phase; CO2 enriched atmosphere/ocean chemistry.
Ocean formation: ~ $4.2$ Ga, enabling liquid water and global oceans.
Hydrothermal activity: convection currents sequestered water into the crust, enabling formation of organic molecules and progenotes.
First cells: ~ $3.8$ Ga.

Early genome-minimalism attempts to predict LUCA's genome (historical context)

Mushegian & Koonin (PNAS, 1996): attempt to identify minimal gene set by comparing two parasitic bacteria:
- Mycoplasma genitalium: $468$ protein-coding genes
- Haemophilus influenzae: $1703$ protein-coding genes
Approach: genes conserved between the two species were presumed essential for cellular life; found $256$ conserved proteins, many with homologs in eukaryotes/archaea.
Inference: these 256 genes might approximate a minimal set for a modern-type cell.
Key caveat: M. genitalium and H. influenzae are highly streamlined parasitic genomes; secondary gene losses bias the inference.

LUCA genome predictions with broader sampling

Kyrpides et al. (1999): include complete archaeal genomes along with bacterial/eukaryotic data for the first time to reconstruct LUCA.
- Identified 324 proteins in Methanococcus jannaschii with at least one homolog in bacteria and eukaryotes.
- Interpreted LUCA as possibly complex, with metabolism similar to modern cells but transcription more like archaea.

Phylogeny and the two-domain model

Phylogenetic analyses using ribosomal RNA support LUCA as the common ancestor of Bacteria, Archaea, and Eukaryotes.
Two-domain view (Archaea + Bacteria) with Eukaryotes emerging from within Archaea; implies a deep evolutionary split with late divergence of eukaryotes.

Weiss et al. (Nature Microbiology, 2016): physiology and habitat of LUCA

Data set: 6,103,411 protein-coding genes across Bacteria and Archaea; analyzed using Markov clustering (MCL).
Clusters: 286,514 protein clusters; 11,093 clusters contain homologues from both Bacteria and Archaea.
After alignment and maximum-likelihood (ML) construction, only 355 protein clusters preserve domain monophyly and have homologues in ≥2 bacterial and archaeal phyla.
These 355 protein-coding genes were probably present in LUCA’s genome and provide a glimpse of its physiology.

LUCA gene families and functional categories

The 355 LUCA-related gene clusters span multiple functional categories, including but not limited to:
- Ribosome biogenesis
- Translation
- RNA modification
- DNA binding
- Nucleic acid handling
- Energy metabolism
- Carbon assimilation
- Nitrogen assimilation
- Cofactor biosynthesis
- Nucleotide metabolism
- Amino acid metabolism
- Redox chemistry
- Protein modification
- Lipid metabolism
- Sugar-related pathways
- Cellular processes
- Other/unknown categories
These gene sets imply LUCA possessed a broad metabolic toolkit and translation/biochemical machinery similar to early cellular life.

Specific features of LUCA's RNA and methylation chemistry

LUCA’s RNA nucleoside modification machinery indicates SAM-dependent methylation of RNA (tRNA and rRNA).
Methyl groups likely derived from methane produced via serpentinization or the WL pathway in methanogens.
Methylation at wobble position contributes to codon–anticodon interactions, enabling translation fidelity in LUCA.
Concept: hydrothermal vents supplied geochemical methyl sources enabling SAM-dependent methylation.

LUCA’s metabolism and energy strategy

Autotrophic vs. heterotrophic nature; thermophile vs mesophile question resolved through genomic reconstruction.
LUCA is reconstructed as an anaerobic autotroph using the Wood–Ljungdahl (WL) pathway for CO2 fixation; existed in a hydrothermal (thermophilic) setting, geochemistry-driven energy.
Core energy enzyme: ATP synthase; PTA (phosphotransacetylase) and acetate kinase (Ack) form the acetyl-phosphate/ATP generation route.
Key feature: The WL pathway reduces CO2 to acetate (acetyl-CoA) via a methyl branch and carbonyl branch; acetyl-CoA is produced by combining CO and a methyl group with CO2-derived carbon in CODH/ACS chemistry.
Overall CO2 reduction route (simplified): $CO2 + 4\ H2 \rightarrow CH3COOH + 2\ H2O$
- This represents the reductive acetyl-CoA (WL) pathway as the only exergonic CO2-fixation route present in both Archaea and Bacteria.
CODH/ACS complex catalyzes the formation of acetyl-CoA from CO and a methyl group; the methyl group originates from a series of transfers in the WL pathway.
Electron bifurcation: Hydrogenases provide reduced ferredoxin necessary for WL and other redox steps; flavin-based electron bifurcation allows coupling of exergonic and endergonic electron transfers to drive energetically unfavorable reductions.
A key energy conservation step is via the Rnf complex, generating a Na+ motive force used for ATP synthesis.

Electron bifurcation and redox biology in LUCA

Flavin-based electron bifurcation mechanism (general): two electrons from an electron donor (e.g., H2) are funneled through a flavin (FAD) so that one electron goes to a favorable acceptor (e.g., NAD+) and the other to an unfavored acceptor (e.g., ferredoxin).
Conceptual representation:
- $H2 + FAD \rightarrow FADH2$
- $FADH_2 + NAD^+ \rightarrow FAD + NADH$
- $FADH2 + Fd{ox} \rightarrow FAD + Fd_{red}$
This mechanism enables energetically challenging redox chemistry essential for CO2 reduction and other biosynthetic steps in ancient metabolism.

LUCA’s energy-harvesting and membrane features

Availability of rotor–stator ATP synthase suggests LUCA could exploit ion gradients to synthesize ATP (chemiosmotic principle).
However, LUCA likely lacked proton-pump-driven electron transport chains; instead, energy could be harnessed from geochemically derived ion gradients via H+/Na+ antiporters.
Anoxic conditions: LUCA and its immediate relatives were likely oxygen-sensitive; energy and biosynthesis relied on gases available in hydrothermal environments (e.g., H2, CO2, N2).

Reverse gyrase and thermal adaptation

Reverse gyrase is a topoisomerase that introduces positive supercoils, stabilizing DNA at high temperatures.
Its presence in the inferred LUCA genome supports a thermophilic lifestyle for LUCA and adaptation to hot environments.

CO2 fixation, nitrogen, and electron donors in LUCA

LUCA possessed enzymes for CO2 fixation via WL pathway; hydrogenases provide electrons; nitrogenase and glutamine synthetase indicate LUCA could process nitrogen sources.
Oxygen sensitivity of WL enzymes, hydrogenases, and nitrogenase reinforces an anaerobic lifestyle.

FeS clusters and the ancient metabolic milieu

FeS clusters are abundant cofactors in LUCA proteins, second only to ATP in frequency.
Their abundance points to an FeS-rich, hydrothermal setting for LUCA and ancient metabolism.
Sugar metabolism enzymes in LUCA are largely glycosylases, hydrolases, and non-oxidative sugar pathways, possibly reflecting primitive cell wall synthesis.
Many LUCA enzymes are SAM-dependent, linking methylation chemistry to core metabolism.

LUCA’s closest relatives and the two-domain picture

Phylogenetic analyses suggest close relationships between LUCA and H2-based acetogenic bacteria (e.g., Clostridia) and H2-based methanogenic archaea.
These groups rely on WL pathway and hydrogen as an energy source, consistent with a hydrothermal-type niche for LUCA.
Serpentinization is proposed as a source of H2 for LUCA metabolism, aligning with a geochemistry-driven origin.
Methylation of RNA/tRNA by SAM, possibly powered by methane from serpentinization, is a hallmark of LUCA’s RNA-modifying toolkit.

The Weiss et al. 2016 perspective (summary of the metabolic portrait)

LUCA inhabited a hydrothermal setting with geochemical energy sources (H2, CO2, N2).
LUCA possessed core components for translation and metabolism, including SAM-dependent methylation and FeS cluster–containing enzymes.
The metabolic wiring centered on WL CO2 fixation, hydrogenases for electron donation, and energy conservation via Rnf and ATP synthase mechanisms.
The two-domain view positions LUCA as the ancestor of both Bacteria and Archaea, with eukaryotes arising in the archaeal lineage.

Final synthesis: LUCA’s physiology and its legacy

LUCA was likely an anaerobic autotroph living in a hydrothermal/geochemically rich environment, using the WL pathway to fix CO2 into acetyl-CoA, with energy conservation via ATP synthase and the Rnf Na+ gradient system.
Its genome probably contained a conserved core of ~355 protein-coding gene clusters found across multiple bacterial and archaeal phyla, providing a window into LUCA’s physiology (translation, energy metabolism, cofactor biosynthesis, and SAM-dependent modifications).
The combination of FeS clusters, SAM-dependent chemistry, hydrogenases, and serpentinization-associated methane hints at a deep, geochemically driven biology at the roots of cellular life.

Quick reference: key numerical and factual anchors

Estimated Earth timeline: $4.6\,\text{Ga}$ (Earth formation) → $4.4\,\text{Ga}$ (moon-forming impact) → $4.2\,\text{Ga}$ (oceans form) → $3.8\,\text{Ga}$ (first cells).
Early LUCA genome-minimalism attempts (1996):
- Mycoplasma genitalium: $468$ protein-coding genes
- Haemophilus influenzae: $1703$ protein-coding genes
- Conserved proteins: $256$ (across both species) with many homologs in eukaryotes/archaea
Later reconstruction (1999):
- Methanococcus jannaschii: $324$ proteins with homologs in bacteria/eukaryotes
- Implication: LUCA possibly complex with metabolism akin to modern cells but archaea-like transcription
Large-scale phylogenomics (Weiss et al., 2016):
- Total bacterial/archaeal genes analyzed: $6{,}103{,}411$
- Protein clusters: $286{,}514$ ; clusters with homologs in both domains: $11{,}093$
- LUCA-preserved clusters with domain monophyly and across ≥2 bacterial/archaeal phyla: $355$
WL (Wood–Ljungdahl) pathway critical points:
- Overall fixation: $CO2 + 4 H2 ightarrow CH3COOH + 2 H2O$
- Enzymes: CODH/ACS (for acetyl-CoA formation), hydrogenases, electron-bifurcating complexes, Rnf complex for Na+ gradient
- PTA and Ack enzymes enable ATP generation from acetyl phosphate
LUCA’s features linked to a thermophilic, hydrothermal habitat: rotary ATP synthase, FeS clusters, serpentinization-derived methane as potential methyl donor via SAM