L3 : Energy Flow for Life in Extremes Flashcards
(32 cards)
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 main function of the respiratory chain in cells?
To generate proton motive force (pmf) for ATP synthesis and autotrophic growth
What is the order of electron flow in the respiratory chain?
Quinone (Q)
Cytochrome C
Oxygen (O2)
- final electron accpetor
What problem do impermeable membranes pose for early cells without pumps?
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 are modern phospholipid membranes not compatible with natural proton gradients?
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
Low permeability causes rapid collapse of pmf
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
Why couldn’t early cells evolve proton pumps immediately?
Pumps offered no advantage without membrane tightness or control
Will not drive evolution of more proton-tight membrane
What is the benefit of sodium ions for energy coupling in hydrothermal vents?
Early cell membranes are ‘leaky’ to protons, resulting in collapsed pmf
Na+ ions much less permeable allowing stable gradients that
could persist longer
- More useful for energy storage and ATP generation
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
What evidence for promiscuity is there?
Some methanogen ATP synthases have dual affinity for H+ and Na+
- Utilise either gradient for ATP synthesis
Ion pumps Ech and Rnf also promiscuous for both
What evolutionary scenario explains ion promiscuity?
First ATP synthases may have used H⁺, while the first ion-pumping proteins (e.g. SPAP) evolved to pump Na⁺, creating mixed specificity
How does SPAP generate energy?
By using a proton gradient to power sodium export via an antiporter, building a sodium gradient that drives ATP synthesis
How could SPAP facilitate evolution of active pumping?
SPAP systems create selective pressure for evolving active proton pumping, especially as membranes became tighter and less leaky
Why does membrane tightness matter for SPAP evolution?
As membranes became less permeable, maintaining ion gradients required active transport, which SPAP systems could support more efficiently
How 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
What is flavin-based electron bifurcation?
Splitting of electron pairs so each goes to high and low potential acceptor, coupling energy yielding and energy consuming reactions
What energy mechansim is flavin-based energy bifurcation linked to?
Divergence of active proton pumping mechanisms in cells
Did flavin based electron bifurcation arise indepdently?
Yes, in bacteria and archaea
Active pumping and proton flux occurs in different directions through Ech for archaea (methanogens pump inward) and bacteria (acetogens pump outwards)
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
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
What do quinones do in bioenergetics?
Act as electron shuttles
Acquire H+ (not Na+) and contribute to H+ gradient
