L3 : Energy Flow for Life in Extremes Flashcards
What is the basic energetic structure of cells?
System centres around respiratory chain and generated proton motive force, which drives energy-requiring cellular processes, esp ATP synthesis
Couples electron transfer with proton pumping
What is the problem with an impermeable membrane?
If impermeable to H+ and OH-, transfer of H+ through ATPase without a pump dissipates proton motive force and builds electric charge on membrane, opposing further transfer
How could the problem of membrane permeability be solved?
If combined a continuous hydrothermal flow with a ‘proton leaky’ membrane, could neutralise proton flux, maintain gradient and retain useful larger products (eg. ATP)
Without leaky membrane, useful products lost
Why do modern membranes collapse electrical potential?
Modern membranes in bacteria and archaea have proton permeability of 10^-5 cm/sec
Given this and 1% ATPase in membrane, proton flux collapses Gibbs free energy (deltaG0 in secs regardless of size of pH gradient
Can geochemical proton gradients drive work in leaky membranes?
Geochemically sustained proton gradients CAN power membrane bioenergetics if membrane leaky
Leaky (FA) vesicles can retain good steady-state deltaG
Modern membrane properties should be selected against in absence of proton pumps in vents
How do natural proton gradients drive growth?
CO2 fixation can be driven via Ech and ATP synthesis via ATPase but only if membrane is leaky
Consistent with LUCA model living in vents without ion pumps and may explain why bacterial and archaeal membranes differ
Note: didn’t consider proton transfer via FA shuttling
Why does this hypothesis still not work?
Pumping protons across leaky membrane does not increase deltaG as membrane permeability decreased over 1000-fold
No selective advantage to drive evolution of more proton-tight membrane
Can conclude that protocells should not evolve membrane pumps or modern phospholipid membranes
What is the benefit of SPAP (sodium proton antiporter)?
Adds biochemical Na+ gradient to natural H+ gradient, increasing deltaG by ~60%
Membranes ~6 orders of magnitude more permeable to H+ than Na+
For each H+ entering, Na+ leaves and less likely to re-enter though membrane
SPAP could drive Na+ energetics from natural pmf
SPAP universally conserved across bacteria and archaea
Can SPAP power work though Na+/H+ promiscuity?
Some methagonen ATP synthases have dual affinity for H+ and Na+ (also sodium motive)
Ion pumps Ech and Rnf also promiscuous for both
Possible that first ion harvesting proteins utilise H+ and first pumping protein utilise Na+, accounting for promiscuity
SPAP antiporter could have optimised ion balance if life arose in oceans
What could SPAP facilitate?
Divergence in vents:
SPAP increases deltaG by ~60%
Cells can survive on smaller gradients allowing colonisation of wider regions in the vent
Evolution of active pumping:
With SPAP, selective advantage to actively pumping protons across membrane that continuously increases with increasing membrane tightness
Can this account for differences between bacteria and archaea?
Origin of pumping hypothesis could explain why membrane energetics are universal but membranes are not
DNA replication also fundamentally different and may relate to binding of replicon to membrane
Flavin-based electron bifurcation apparently arose independently, linked to divergence of pumping mechanisms
Active pumping and proton flux occurs in different directions through Ech for archaea (methanogens pump inward) and bacteria (acetogens pump outwards)
What is the link between Ech/SPAP and respiratory complex I?
Ech made from SPAP
Central to core of complex I, formed of Ech and 3 other SPAPs
Complex I still operates as Na+/H+ antiporter, generating Na+ gradient accounting for ~50% mitochondrial membrane potential and doesn’t pass through but drives faster spinning of ATP synthase
How could introduction of quinones change bioenergetics?
Quinones acquire H+ (not Na+) when reduced so generate H+ gradient
H+/Na+ promiscuity of chemiosmotic circuits pushed towards H+ as coupling ion when quinones involved in respiratory chain
Promiscuity of Ech, complex I, ATP synthase suggest they arose earlier than quinone
Helps explain evolutionary primacy of H+ over Na+ energetics
Could quinones have been introduced as soluble extracellular electron shuttles?
May have dumped electrons onto rust (ferric hydroxide) by reducing Fe3+ to soluble Fe2+ and dissolving rust
May be done by unknown quinones related to menaquinone
What is the earliest quinone?
Deep split in quinone and haem synthesis?
How does quinone transfer electrons to cytochrome?
How is flavin based electron bifurcation and Q cycle similar?
What is the Q cycle?
Bifurcating redox loop
