L2 : Origins and Evolution of Life Flashcards
2 or 3 domains of life?
Recent work suggests only two primary domains of life, where eukaryotes are derived through endosymbiosis
Suggests that to reconstruct properties of LUCA, need to compare bacteria and archaea
What are some paradoxical traits of bacteria and archaea?
Membrane bioenergetics are universally conserved but cell membranes are not
Transcription and translation processes similar but appears DNA replication arose independently
Respiratory chains also differ
Where was life predicted to begin?
Alkaline hydrothermal vents (Lost City vent field) with geological proton gradients (could resolve paradox)
Inorganic vent pores appear analogous to autotrophic cells
What is the hypothesis on the origin of life?
Inorganic pores may have evolved into organic cells
Electrical charge (proton motive force) drives reaction between H2 and CO2 to form protocells in alkaline pores, eventually leading to metabolism, growth, and emergence of genes
What are electrochemical flow reactors?
High conc of H2 and CO2
H+ gradients across catalytic Fe(Ni)S walls
Predicted as location of origin of life before existence was known (Lost City)
Is abiotic organic synthesis from H2 + CO2 possible? Step 1?
Synthesising cell biomass from H2 + CO2 is exergonic under alkaline hydrothermal conditions
Thermodynamically possible but kinetic barriers prevent it
Why did archaea and bacteria first evolve a proton gradient?
Methanogens and acetogens are generally considered some of the most ancient groups
Acetyl CoA pathway is only CO2 fixation pathway found in both archaea and bacteria
Uses proton gradient to lower energy barrier for reaction and drive CO2 fixation between H2 and CO2
What is Ech and how does it work?
Energy converting hydrogenase is membrane protein that contains 4Fe4S clusters (crystal structures affine to mineral greigite - evolutionary path to Fd)
Uses proton motive to drive work and reduce ferredoxin, Fd can then reduce CO2 and power carbon metabolism
Redox potential depends on pH - more favourable reaction in lower pH environment
What are the hypothetical conditions in vents?
pH = 5-12
Temp = 50-100
Salinity = NaCl (600mM)
Divalent cations = Mg2+, Ca2+
Is hydrothermal synthesis of long-chain lipids possible? Step 2?
FTT (Fischer-Tropsch type) synthesis reactions on mineral surfaces possible under these conditions
Starting from H2 and CO2, chain lengths are similar to modern membrane lipids
Further investigation shows that protocells with bilayer membrane enclosing aqueous spaces are stable in extremophile conditions (70 deg, pH12, 1:1 FA:FOH mix)
Are FeS clusters able to form under these conditions?
Low potential FeS clusters shown to form spontaneously from cysteine, Fe3+, S2- under alkaline conditions
FeS clusters can fix CO2 on electrode
Can CO2 fixation drive spontaneous metabolism?
Sequential reactions of H2 + CO2 in biology give rise to acetyl CoA and Kebrs cycle intermediates
AAs, FAs and sugars all derive from Krebs cycle intermediates (precursors seen can form nucleotides)
Reaction network topology of autotrophic metabolism is universally conserved across life and seems to occur spontaneously (in absence of genes)
How does direction of flux depend on environment?
In terms of free energy change, metabolism is quite close to equilibrium - metabolic intermediates interchange easily
CO2 + H2 are in environmental disequilibrium
Concentration differences will tend to drive flux in one direction
If environmental disequilibrium dissipates, direction of flux reverses (likely emergence of heterotrophy - autotrophic pathway reversal)
Amino acid synthesis and interconversion
In presence of pyridoxamine, transamination of oxaloacetate yield (aspartate or alanine) depends mostly on specific metal ions
Reactions starting from O-phosphoserine also
GC-MS and HPLC detection show Asp, Ala, Gly, Ser, Cys all formed in presence of pyridoxamine + metal ions
Is uracil synthesis possible?
One pot synthesis of uracil from ammonia, carbonate, and aspartate (optimised at 90 deg, pH 9, 1M NaOH)
Shows all these intermediates are able to spontaneously interchange
Is ATP synthesis possible?
Only need water, ferric iron and 2C phosphate donor (acetyl phosphate)
Does not work for other nucleotide triphosphates
What does computer simulation show about rudimentary membrane heredity in protocells?
AAs chelate FeS crystals, hindering growth and giving more smaller crystals (larger SA)
Chelation by amino acids partitions FeS crystals to membrane, giving ‘proto-Ech’
Natural proton gradients drive formation of new organucs through proto-Ech (positive feedback)
What was the original role of nucleotide cofactors?
Involvement in CO2 fixation, driving flux and possible catalysing other pathways
Autocatalysis which would increase ATP synthesis
Theoretically may have led to accumulation of nucleotides, though not observed in prebiotic experiments
Emergence of genetic code from polymerising nucleotides?
Biophysical interactions mean random RNA will template non-random peptides so information has meaning in protocells
Do biophysical interactions enable evolution?
Simple mathermatcal model introducing random RNA into autotrophically growing protocells shows:
Random peptides produced may feedback somehow
Hydrophobic peptides go to membrane to drive CO2 fixation
Hydrophilic peptides facilitate RNA coping
Evolution depends on low free polymerisation rate
Higher error rates undermine selection
What are the 3 steps in the origins of life?
- Proton gradient drives CO2 fixation
- CO2 fixation drives spontaneous metabolism
- Metabolism gives rise to genetic information
What does this hypothesis predict of the first cells?
Autotrophic, chemiosmotic, thermophiles
